Binuclear Cyclopentadienylmetal Carbonyl Derivatives of the

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Organometallics 2009, 28, 6410–6424 DOI: 10.1021/om900332k

Binuclear Cyclopentadienylmetal Carbonyl Derivatives of the Oxophilic Metal Niobium Xiuhui Zhang,† Qian-shu Li,*,†,‡ Yaoming Xie,§ R. Bruce King,*,‡,§ and Henry F. Schaefer III§ †

Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, People’s Republic of China, Center for Computational Quantum Chemistry, School of Chemistry and Environment, South China Normal University, Guangzhou 510631, People’s Republic of China, and §Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602

‡

Received April 28, 2009

Comparison of the lowest energy Cp2Nb2(CO)n structures (n = 7, 6, 5, 4, 3, 2, 1) with the corresponding Cp2V2(CO)n structures using density functional theory predicts structures having fourelectron donor η2-μ-CO groups and short M-O distances to be more prevalent for niobium than for vanadium, in accord with the greater oxophilicity of niobium relative to vanadium. The lowest energy Cp2Nb2(CO)7 structure consists of CpNb(CO)4 and CpNb(CO)3 units joined by a relatively long ∼3.4 A˚ Nb-Nb bond and is marginally stable by ∼9 kcal/mol with respect to dissociation into mononuclear fragments. The lowest energy Cp2Nb2(CO)6 structure contains a η2-μ-CO group with a Nb-Nb single bond of length ∼3.2 A˚. The lowest energy Cp2Nb2(CO)5 structure is similar to that of the known Cp2V2(CO)5 with a predicted NbtNb triple bond length of ∼2.7 A˚. Two η2-μ-CO groups are found in the lowest energy Cp2Nb2(CO)4 structure with a predicted NbdNb double bond length of ∼3.0 A˚. The lowest energy Cp2Nb2(CO)3 structure is predicted to have two η2-μ-CO groups and a NbtNb triple bond of length ∼2.8 A˚. The lowest energy Cp2Nb2(CO)2 structure is also predicted to have two η2-μ-CO groups but a significantly shorter Nb-Nb quadruple bond of length ∼2.6 A˚. All of these structures are singlets with 18-electron configurations for both niobium atoms. The lowest energy structure for Cp2Nb2(CO) is a triplet with a η2-μ-CO group and a Nb-Nb quadruple bond of length ∼2.6 A˚. The lowest energy structures for Cp2Nb2(CO)5 and Cp2Nb2(CO)3 are both stable with respect to disproportionation into Cp2Nb2(CO)nþ1 þ Cp2Nb2(CO)n-1, whereas Cp2Nb2(CO)6 and Cp2Nb2(CO)4 are unstable with respect to such disproportionation. The reaction of Cp2Nb2(CO)5 with a CpNb(CO)3 fragment to give Cp3Nb3(CO)8 is predicted to be exothermic by ∼13 kcal/mol, thereby suggesting a possible mechanism for the experimentally observed formation of the trinuclear derivative Cp3Nb3(CO)6(η2-μ3-CO) in the photolysis of CpNb(CO)4. 1. Introduction Niobium is of interest in metal carbonyl chemistry because of its oxophilicity relative to other metals forming stable carbonyls, even its lighter congener vanadium. This is manifested in the very different products obtained by the photolysis of the corresponding cyclopentadienylmetal tetracarbonyls, CpM(CO)4 (Cp=η5-C5H5; M=V, Nb). Thus the photolysis of CpV(CO)4 gives the binuclear derivative1-3 Cp2V2(CO)5 (Figure 1a), in which all of the carbonyl groups are the usual two-electron donors, and the short VtV distance is indicative of the formal triple bond required to give both metal atoms the favored 18-electron configuration. However, the photolysis of CpNb(CO)4 follows a completely different course, giving the trinuclear derivative4 Cp3Nb3(CO)6(η2μ3-CO) (Figure 1b), in which the unique carbonyl group *Corresponding author. E-mail: [email protected]. (1) Fischer, E. O.; Schneider, R. J. J. Angew. Chem. 1967, 79, 537. (2) Cotton, F. A.; Kruczynski, L. J. Organomet. Chem. 1978, 160, 93. (3) Huffman, J. C.; Lewis, L. N.; Caulton, K. G. Inorg. Chem. 1980, 19, 2755. (4) Herrmann, W. A.; Biersack, H.; Ziegler, M. L.; Weidenhammer, K.; Siegel, R.; Rehder, D. J. Am. Chem. Soc. 1981, 103, 1692. pubs.acs.org/Organometallics

Published on Web 10/19/2009

functions as a formal six-electron donor bridging all three metal atoms by using both its carbon and oxygen atom to bond to the Nb3 triangle. This difference in the photolysis products of CpV(CO)4 and CpNb(CO)4 can be ascribed to the higher oxophility of niobium, since the niobium photolysis product exhibits niobium-oxygen bonding to the unique bridging η2-μ3-CO carbonyl group as well as niobiumcarbon bonding. This difference between vanadium and niobium carbonyl chemistry as exemplified by the different photolysis products of CpM(CO)4 (M = V, Nb) raises the question whether analogous differences would be found in the chemistry of the binuclear cyclopentadienylmetal carbonyls Cp2M2(CO)n (M=V, Nb; n=7, 6, 5, 4, 3, 2, 1). All of these vanadium derivatives have been previously investigated by density functional theory.5,6 The analogous niobium derivatives might be expected to differ from the vanadium derivatives in the following two ways: (1) a greater tendency for the niobium (5) Li, Q.-S.; Zhang, X.; Xie, Y.; King, R. B.; Schaefer, H. F. J. Am. Chem. Soc. 2007, 129, 3433. (6) Zhang, X.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. Eur. J. Inorg. Chem. 2007, 1599. r 2009 American Chemical Society

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Organometallics, Vol. 28, No. 22, 2009

Figure 1. Products obtained from the photolysis of CpM(CO)4 (M = V, Nb): (a) Cp2V2(CO)5; (b) Cp3Nb3(CO)6(η2-μ3-CO). The unique six-electron-donor bridging carbonyl group in the latter compound is shown in red.

derivatives to have structures containing four-electrondonor η2-μ-CO groups, owing to the higher oxophilicity of niobium relative to vanadium; (2) greater stability of the binuclear derivatives Cp2Nb2(CO)7 and Cp2Nb2(CO)6 toward dissociation into mononuclear fragments, because of the stability of higher coordination numbers for the secondrow transition metal niobium relative to the first-row transition metal vanadium. This paper reports our density functional theory studies on the complete series of binuclear cyclopentadienylniobium carbonyls Cp2Nb2(CO)n (n = 7, 6, 5, 4, 3, 2, 1), which were undertaken to explore these and related points. Because of the large number of structures encountered in this work, only those structures with energies within ∼30 kcal/mol of the global minima are considered. Higher energy structures not considered in this paper include the highly unsaturated Cp2Nb2(CO)n (n = 3, 2) derivatives, in which both cyclopentadienyl rings are attached to the same metal atom.

2. Theoretical Methods Electron correlation effects were considered by employing density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds.7-21 In this work, two DFT methods were used. The BP86 method is a pure DFT method combining Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradient-corrected correlation functional (P86).22,23 The MPW1PW91 method is a so-called second-generation functional, namely, a combination of the modified Perdew-Wang

(7) Ehlers, A. W.; Frenking, G. J. Am. Chem. Soc. 1994, 116, 1514. (8) Delley, B.; Wrinn, M.; L€ uthi, H. P. J. Chem. Phys. 1994, 100, 5785. (9) Li, J.; Schreckenbach, G.; Ziegler, T. J. Am. Chem. Soc. 1995, 117, 486. (10) Jonas, V.; Thiel, W. J. Chem. Phys. 1995, 102, 8474. (11) Barckholtz, T. A.; Bursten, B. E. J. Am. Chem. Soc. 1998, 120, 1926. (12) Niu, S.; Hall, M. B. Chem. Rev. 2000, 100, 353. (13) Macchi, P.; Sironi, A. Coord. Chem. Rev. 2003, 238, 383. (14) Carreon, J.-L.; Harvey, J. N. Phys. Chem. Chem. Phys. 2006, 8, 93. (15) B€ uhl, M.; Kabrede, H. J. Chem. Theory Comput. 2006, 2, 1282. (16) Ziegler, T.; Autschbach, J. J. Chem. Rev. 2005, 105, 2695. (17) Waller, M. P.; B€ uhl, M.; Geethanakshmi, K. R.; Wang, D.; Thiel, W. Chem.;Eur. J. 2007, 13, 4723. (18) Hayes, P. G.; Beddie, C.; Hall, M. B.; Waterman, R.; Tilley, T. D. J. Am. Chem. Soc. 2006, 128, 428. (19) B€ uhl, M.; Reimann, C.; Pantazis, D. A.; Bredow, T.; Neese, F. J. Chem. Theory Comput. 2008, 4, 1449. (20) Besora, M.; Carreon-Macedo, J.-L.; Cowan, J.; George, M. W.; Harvey, J. N.; Portius, P.; Ronayne, K. L.; Sun, X.-Z.; Towrie, M. J. Am. Chem. Soc. 2009, 131, 3583. (21) Ye, S.; Tuttle, T.; Bill, E.; Simkhorich, L.; Gross, Z.; Thiel, W.; Neese, F. Chem.;Eur. J. 2008, 14, 10839. (22) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (23) Perdew, J. P. Phys. Rev. B 1986, 33, 8822.

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exchange functional with the Perdew-Wang 91 gradient-correlation functional.24 The MPW1PW91 functional has been found to be more suitable than the first-generation functionals for second- and third-row transition metal systems, while the BP86 method usually provides better vibrational frequencies.25,26 For the second-row transition metals, the large numbers of electrons increase exponentially the computational efforts. In order to reduce the cost, relativistic effective core potential (ECP) basis sets were used. In this study the SDD (StuttgartDresden ECP plus DZ)27,28 basis set was used for the niobium atoms. For the C and O atoms, the double-ζ plus polarization (DZP) basis sets were used. They are Huzinaga-Dunning’s contracted double-ζ contraction sets plus a set of spherical harmonic d polarization functions with orbital exponents Rd(C) = 0.75 and Rd(O) = 0.85 designated as (9s5p1d/4s2p1d).29,30 For H, a set of p polarization functions, Rp(H) = 0.75, was added to the Huzinaga-Dunning DZ set. The geometries of the structures were fully optimized using the two selected DFT methods with the SDD ECP basis set. The vibrational frequencies were determined by evaluating analytically the second derivatives of the energy with respect to the nuclear coordinates at the same levels. The corresponding infrared intensities were also evaluated analytically. All of the computations were carried out with the Gaussian 03 program.31 The fine grid (75, 302) was the default for evaluating integrals numerically, and the tight (10-8 hartree) designation was the default for the energy convergence. The finer grid (120, 974) was used for investigating the small imaginary vibrational frequencies.32 All of the predicted triplet structures were found to have negligible spin contamination; that is, the values of S(S þ 1) are very close to the ideal outcome of 2.0.

3. Results 3.1. Cp2Nb2(CO)7. For Cp2Nb2(CO)7 three energetically low-lying structures were found with the niobium-niobium distances short enough to suggest significant interaction between the CpNb(CO)4 and CpNb(CO)3 fragments (Figure 2 and Tables 1 and 2). The global minimum 7S-1 is a C1 structure with one semibridging carbonyl and six terminal carbonyls. This structure is predicted to be a genuine minimum with all real vibrational frequencies. For the semibridging CO group, the longer Nb-C distance is 2.760 A˚ (MPW1PW91) or 2.581 A˚ (BP86) and the shorter Nb-C distance is 2.163 A˚ (MPW1PW91) or 2.217 A˚ (BP86). The theoretical vibrational frequency ν(CO) of 7S-1 at 1805 cm-1 (BP86) corresponds to this semibridging CO group. The Nb-C distances for the terminal CO groups are in the range 2.043 to 2.127 A˚ (MPW1PW91) or 2.053 to 2.120 A˚ (BP86). The predicted Nb-Nb distance, namely, 3.382 A˚ (MPW1PW91) or 3.411 A˚ (BP86), is consistent with the Nb-Nb single bond required to give each niobium atom the favored 18-electron configuration. The second singlet stationary point of Cp2Nb2(CO)7, namely, 7S-2, is a C2 structure with one bridging carbonyl, (24) Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664. (25) Feng, X.; Gu, J.; Xie, Y.; King, R. B.; Schaefer, H. F. J. Chem. Theor. Comput. 2007, 3, 1580. (26) Zhao, S.; Wang, W.; Li, Z.; Liu, Z. P.; Fan, K.; Xie, Y.; Schaefer, H. F. J. Chem. Phys. 2006, 124, 184102. (27) Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1993, 85, 441. (28) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431. (29) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823. (30) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293. (31) Frisch, M. J.; et al. Gaussian 03, Revision C 02; Gaussian, Inc.: Wallingford, CT, 2004 (see Supporting Information for full reference ). (32) Papas, B. N.; Schaefer, H. F. J. Mol. Struct. 2006, 768, 275.

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Figure 2. Optimized geometries of Cp2Nb2(CO)7. The distances in Figure 1 are given in A˚. For the bond distances listed in Figures 2 to 9 the upper numbers were determined by the MPW1PW91 method and the lower numbers by the BP86 method. Table 1. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), and Nb-Nb Bond Distances (A˚) for the Three Singlet Cp2Nb2(CO)7 Structures

MPW1PW91

BP86

7S-1 (C1)

7S-2 (C2)

7S-3 (C1)

E

-1294.44854

-1294.44581

-1294.39874

ΔE Nimag Nb-Nb E ΔE Nimag Nb-Nb

0.0 0 3.382 -1294.90813 0.0 0 3.411

1.7 1(15i) 3.406 -1294.90758 0.3 1(12i) 3.473

31.2 0 3.247 -1294.86180 29.1 0 3.253

Table 2. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted for the Three Lowest Energy Structures of Cp2Nb2(CO)7a BP86 7S-1 (C1) 7S-2 (C2) 7S-3 (C1) a

1805(430), 1888(50), 1900(387), 1908(230), 1931(2032), 1943(1493), 1987(205) 1734(473), 1900(35), 1906(616), 1919(184), 1924(991), 1952(1860), 1995(502) 1638(258), 1875(314), 1918(839), 1930(192), 1951(1741), 1974(778), 2013(307)

Infrared intensities in parentheses are in km/mol.

two very weakly semibridging carbonyls, and four terminal carbonyls. Structure 7S-2 lies only 1.7 kcal/mol (MPW1PW91) or 0.3 kcal/mol (BP86) above 7S-1 (Figure 2 and Table 1). The Nb-C distances to the symmetrical bridging carbonyl in 7S-2 are 2.274 A˚ (MPW1PW91) or 2.293 A˚ (BP86). The ν(CO) frequency at 1734 cm-1 (BP86) can be assigned to the bridging CO group. For the two equivalent semibridging carbonyl groups, the Nb-C distances are 3.265 and 2.118 A˚ (MPW1PW91) or 3.276 and 2.121 A˚ (BP86). The long Nb 3 3 3 C distance to these semibridging carbonyl groups suggests that the latter can almost be regarded as terminal carbonyl groups. The Nb-C distances to the four terminal carbonyls in 7S-2 are in the range 2.068 to 2.117 A˚ (MPW1PW91) or 2.071 to 2.113 A˚ (BP86). The Nb-Nb distance in 7S-2 is 3.406 A˚ (MPW1PW91) or 3.473 A˚ (BP86), close to that in 7S-1, and can also be interpreted as the formal Nb-Nb single bond required to give each niobium atom the favored 18-electron configuration. The third located singlet structure of Cp2Nb2(CO)7, namely, 7S-3 (Figure 2 and Tables 1 and 2), is a relatively

Figure 3. Optimized geometries of the four Cp2Nb2(CO)6 structures with distances in A˚.

high energy structure lying 31.2 kcal/mol (MPW1PW91) or 29.1 kcal/mol (BP86) above 7S-1 in energy with all real harmonic vibrational frequencies. Structure 7S-3 has one four-electron-donor η2-μ-CO bridging carbonyl group, two semibridging carbonyls, and four terminal carbonyls. The four-electron-donor carbonyl group in 7S-3 is characterized by a short Nb-O distance of 2.356 A˚ (MPW1PW91) or 2.397 A˚ (BP86), a relatively long C-O distance of 1.212 A˚ (MPW1PW91) or 1.228 A˚ (BP86), indicating a relatively low C-O bond order for a carbonyl group, and a correspondingly very low ν(CO) frequency of 1638 cm-1 (BP86). The Nb-Nb distance in 7S-3 is predicted to be 3.247 A˚ (MPW1PW91) or 3.253 A˚ (BP86). The 18-electron rule

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Table 3. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Number of Imaginary Vibrational Frequencies (Nimag), Nb-Nb Bond Distances (A˚), and Spin Contamination ÆS2æ for the Four Lowest Energy Cp2Nb2(CO)6 Structures

MPW1PW91

BP86

E ΔE Nimag Nb-Nb ÆS2æ E ΔE Nimag Nb-Nb ÆS2æ

6S-1 (C1)

6S-2 (Ci)

6T-1 (C2)

6T-2 (Ci)

-1181.12569 0.0 0 3.206 0.0 -1181.54838 0.0 0 3.216 0.0

-1181.11568 6.3 1 (9i) 2.972 0.0 -1181.54375 2.9 1 (20i) 3.017 0.0

-1181.11332 7.8 0 3.131 2.01 -1181.53861 6.1 0 3.156 2.00

-1181.10811 11.0 0 3.094 2.01 -1181.53636 7.5 0 3.134 2.00

Table 4. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted for the Four Lowest Energy Structures of Cp2Nb2(CO)6a BP86 6S-1 (C1) 6S-2 (Ci) 6T-1 (C2) 6T-2 (Ci) a

1643(336), 1847(339), 1900(850), 1931(1025), 1935(1006), 1996(435) 1802(0), 1807(616), 1912(1738), 1913(0), 1942(2332), 1975(0) 1798(596), 1807(419), 1908(1), 1921(877), 1923(1553), 1982(1086) 1791(560), 1791(0), 1905(0), 1909(1415), 1929(3017), 1966(0)


suggests that this Nb-Nb distance in 7S-3 should be considered a nonbonding distance because of the four-electrondonor carbonyl group, although it is similar to the Nb-Nb single-bond distances in the lower energy structures 7S-1 and 7S-2. The geometry of the bridging four-electron-donor group can force the two niobium atoms to remain at a ∼3.2 A˚ distance in the absence of a direct niobium-niobium interaction in 7S-3. 3.2. Cp2Nb2(CO)6. Two singlet structures and two triplet structures (Figure 3 and Tables 3 and 4) were found for Cp2Nb2(CO)6. The global minimum 6S-1 is predicted to be a singlet C1 cis structure with one four-electron-donor η2-μCO group, one weakly semibridging two-electron donor carbonyl group, and four terminal carbonyl groups. Structure 6S-1 is predicted to be a genuine minimum with all real vibrational frequencies. The four-electron-donor carbonyl group in 6S-1 is characterized by a very short Nb-O distance of 2.392 A˚ (MPW1PW91) or 2.436 A˚ (BP86), a relatively long C-O distance of 1.211 A˚ (MPW1PW91) or 1.228 A˚ (BP86), and a correspondingly low ν(CO) frequency of 1643 cm-1. For the other semibridging carbonyl, the shorter Nb-C distance is 2.063 A˚ (MPW1PW91) or 2.074 A˚ (BP86) and the longer one is 2.893 A˚ (MPW1PW91) or 2.872 A˚ (BP86), indicating only weakly semibridging character. The terminal CO groups in 6S-1 have Nb-C distances in the range 2.052 to 2.172 A˚ (MPW1PW91) or 2.057 to 2.169 A˚ (BP86). The Nb-Nb distance of 3.206 A˚ (MPW1PW91) or 3.216 A˚ (BP86) is consistent with the bridged Nb-Nb single bond required to give each niobium atom the favored 18-electron configuration with the single four-electron-donor bridging carbonyl group. The second energetically low-lying singlet structure, 6S-2, with Ci symmetry (Figure 3 and Tables 3 and 4) is a trans structure lying above 6S-1 by 6.3 kcal/mol (MPW1PW91) or 2.9 kcal/mol (BP86) with a very small imaginary vibrational frequency at 9i (MPW1PW91) or 20i (BP86). The MPW1PW91 imaginary vibrational frequency becomes real when a

finer integration grid (120, 974) is used. Structure 6S-2 has two semibridging carbonyls and four terminal carbonyls. For the two semibridging carbonyls, the shorter Nb-C distance is 2.072 A˚ (MPW1PW91) or 2.091 A˚ (BP86) and the longer Nb-C distance is 2.604 A˚ (MPW1PW91) or 2.580 A˚ (BP86). The predicted ν(CO) frequencies of 1802 and 1807 cm-1 (BP86) correspond to these two semibridging carbonyl groups. The Nb-C distances for the terminal carbonyl groups in 6S-2 are in the range 2.045 to 2.098 A˚ (MPW1PW91) or 2.048 to 2.096 A˚ (BP86). The NbdNb bond distance in 6S-2 is 2.972 A˚ (MPW1PW91) or 3.017 A˚ (BP86), which is shorter than that in 6S-1 by ∼0.2 A˚ and thus is consistent with the formal double bond required to give each niobium atom an 18-electron configuration in a Cp2Nb2(CO)6 structure with all two-electron-donor carbonyl groups. The lowest lying Cp2Nb2(CO)6 triplet structure 6T-1 (Figure 3 and Tables 3 and 4) is a C2 structure with two semibridging carbonyls and four terminal carbonyls. It has all real harmonic vibrational frequencies by both MPW1PW91 and BP86 methods and lies 7.8 kcal/mol (MPW1PW91) or 6.1 kcal/mol (BP86) above the global minimum 6S-1. The shorter Nb-C distances to the two equivalent semibridging carbonyls in 6T-1 are 2.079 A˚ (MPW1PW91) or 2.096 A˚ (BP86) and the longer Nb-C distances are 2.684 A˚ (MPW1PW91) or 2.574 A˚ (BP86). The ν(CO) frequencies at 1798 and 1807 cm-1 (BP86) can be assigned to these two semibridging CO groups. The Nb-Nb distance in 6T-1 is 3.131 A˚ (MPW1PW91) or 3.156 A˚ (BP86), which is very close to the single bond in 6S-1 and consistent with the Nb-Nb single bond required to give each niobium atom the 17-electron configuration in the binuclear triplet structure. The second energetically low-lying Cp2Nb2(CO)6 triplet structure, 6T-2, lies above 6S-1 by 11.0 kcal/mol (MPW1PW91) or 7.5 kcal/mol (BP86) with all real vibrational frequencies by both methods. The structure 6T-2 is a Ci structure with a trans orientation of the two Cp rings, two semibridging carbonyls, and four terminal carbonyls. The Nb-C bond distances to the two semibridging CO groups are 2.091 A˚ (MPW1PW91) or 2.106 A˚ (BP86) and 2.554 A˚ (MPW1PW91) or 2.537 A˚ (BP86). The predicted ν(CO) frequency of 1791 cm-1 (BP86) in 6T-2 can be assigned to these two semibridging CO groups. The Nb-Nb distance in 6T-2 is 3.094 A˚ (MPW1PW91) or 3.134 A˚ (BP86), corresponding to the formal single bond required to give both niobium atoms the 17-electron configurations for a binuclear triplet structure. 3.3. Cp2Nb2(CO)5. Three singlet structures and three triplet structures (Figure 4 and Tables 5, 6, and 7) were found for Cp2Nb2(CO)5. The global minimum 5S-1 is predicted to be a Cs trans singlet structure with two semibridging

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Figure 4. Optimized geometries of the six Cp2Nb2(CO)5 structures with distances in A˚. Table 5. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Number of Imaginary Vibrational Frequencies (Nimag), and Nb-Nb Bond Distances (A˚) for the Three Lowest Energy Singlet Cp2Nb2(CO)5 Structures

MPW1PW91

BP86

5S-1 (Cs)

5S-2 (C1)

5S-3 (Cs)

E

-1067.80668

-1067.80290

-1067.77050

ΔE Nimag Nb-Nb E ΔE Nimag Nb-Nb

0.0 0 2.730 -1068.19505 0.0 0 2.756

2.4 0 2.764 -1068.19551 -0.3 0 2.798

22.7 0 2.800 -1068.15836 23.0 1 (36i) 2.804

and three terminal CO groups and all real harmonic vibrational frequencies by MPW1PW91 and BP86. The Nb-C distances to the semibridging carbonyls in 5S-1 are 2.065 and 2.606 A˚ (MPW1PW91) or 2.075 and 2.596 A˚ (BP86). The ν(CO) frequencies of 1822 and 1851 cm-1 (BP86) correspond to these two semibridging CO groups. The Nb-C distances for the terminal CO groups are in the range 2.037 to 2.094 A˚ (MPW1PW91) or 2.043 to 2.092 A˚ (BP86). The NbtNb distance in 5S-1 is 2.730 A˚ (MPW1PW91) or 2.756 A˚ (BP86), consistent with the formal triple bond required to give each niobium atom the favored 18-electron configuration. This NbtΝb triple-bond distance is appreciably longer than the NbtNb triple-bond distances of 2.62 ( 0.01 A˚ in the Nb(II) anions [Nb2X6(SC4H8)3]2- (X=Cl, Br) found by X-ray crystallography.33 However, this difference can be a consequence of the very different ligand sets in Cp2Nb2(CO)5 and (33) Cotton, F. A.; Diebold, M. P.; Roth, W. J. J. Am. Chem. Soc. 1987, 109, 5506.

Table 6. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), Nb-Nb Bond Distances (A˚), and Spin Contamination ÆS2æ for the Three Lowest Energy Triplet Structures of Cp2Nb2(CO)5

MPW1PW91

BP86

5T-1 (C1)

5T-2 (C1)

5T-3 (C1)

E

-1067.77360

-1067.77351

-1067.76795

ΔE Nimag Nb-Nb ÆS2æ E ΔE Nimag Nb-Nb ÆS2æ

20.8 0 2.845 2.03 -1068.16092 21.4 0 2.875 2.01

20.8 0 2.897 2.05 -1068.15965 22.2 0 2.938 2.02

24.3 0 3.034 2.03 -1068.15037 28.0 0 3.014 2.01

[Nb2X6(SC4H8)3]2-. Furthermore, structure 5S-1 for Cp2Nb2(CO)5 is analogous to the experimentally reported structure2 for the vanadium analogue Cp2V2(CO)5. The second singlet stationary point of Cp2Nb2(CO)5, namely, 5S-2 (Figure 4 and Tables 5 and 7), is a C1 symmetry cis structure with two semibridging carbonyls and three terminal carbonyls. Structure 5S-2 lies only 2.4 kcal/mol higher (MPW1PW91) or 0.3 kcal/mol (BP86) lower than 5S-1. The Nb-C distances to the semibridging carbonyl groups are about 2.06 and 2.53 A˚ by either method. The ν(CO) frequencies at 1799 and 1831 cm-1 (BP86) can be assigned to these two semibridging CO groups. The Nb-C distances to the three terminal carbonyls are in the range 2.063 to 2.070 A˚ (MPW1PW91) or 2.066 to 2.080 A˚ (BP86). The NbtNb distance is 2.764 A˚ (MPW1PW91) or 2.798 A˚ (BP86), which is close to that of 5S-1, and can also be

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Table 7. Infrared Active ν(CO) Vibrational Frequencies (cm-1) Predicted for the Five Lowest Energy Structures of Cp2Nb2(CO)5a BP86 5S-1 (Cs) 5S-2 (C1) 5S-3 (Cs) 5T-1 (C1) 5T-2 (C1) 5T-3 (C1) a

1822(547), 1851(172), 1882(1056), 1923(1157), 1968(600) 1799(725), 1831(599), 1888(344), 1921(944), 1975(833) 1845(1048), 1847(521), 1926(945), 1942(724), 1995(1551) 1743(372), 1843(364), 1881(1030), 1923(1961), 1952(134) 1715(419), 1815(621), 1891(438), 1922(885), 1972(953) 1824(719), 1864(969), 1913(1026), 1938(636), 1990(1552)


interpreted as a formal NbtNb triple bond required to give each niobium atom the favored 18-electron configuration. The third energetically low-lying singlet Cp2Nb2(CO)5 structure, 5S-3 (Figure 4 and Tables 5 and 7), has the two Cp rings bonded to one niobium atom and only carbonyl groups bonded to the second niobium atom. Structure 5S-3 is a Cs structure, lying 22.7 kcal/mol (MPW1PW91) or 23.0 kcal/ mol (BP86) above the global minimum 5S-1. It is a genuine minimum with all real vibrational frequencies by MPW1PW91 and has a small imaginary vibrational frequency at 36i cm-1 with the BP86 method. Structure 5S-3 has two semibridging carbonyls and three terminal carbonyls. For the two equivalent semibridging carbonyls, the shorter Nb-C bond distances are 2.114 A˚ (MPW1PW91) or 2.142 A˚ (BP86) and the longer Nb-C bond distances are 2.642 A˚ (MPW1PW91) or 2.590 A˚ (BP86). The ν(CO) vibrational frequencies of 1845 and 1847 cm-1 (BP86) correspond to these two semibridging carbonyl groups. The Nb-Nb distance in 5S-3 is predicted to be 2.800 A˚ (MPW1PW91) or 2.804 A˚ (BP86). The lowest lying triplet structure of Cp2Nb2(CO)5, namely, 5T-1 (Figure 4 and Tables 6 and 7), is a C1 structure with two semibridging carbonyls and three terminal carbonyls. It has all real harmonic vibrational frequencies and lies 20.8 kcal/mol (MPW1PW91) or 21.4 kcal/mol (BP86) above 5S-1. One of the semibridging carbonyls in 5T-1 is nearly symmetrical, with the shorter (∼2.2 A˚) and longer (∼2.3 A˚) Nb-C bond distances differing by only ∼0.1 A˚. The corresponding ν(CO) frequency is 1743 cm-1 (BP86), in a typical region for symmetrical bridging carbonyl groups. For another semibridging carbonyl, the short Nb-C distance is 2.065 A˚ (MPW1PW91) or 2.065 A˚ (BP86) and the long Nb-C distance is 2.809 A˚ (MPW1PW91) or 2.786 A˚ (BP86). The predicted ν(CO) frequency for this semibridging carbonyl group is 1843 cm-1 (BP86), which is close to the terminal ν(CO) frequency region. The predicted NbdNb distance in 5T-1 is 2.845 A˚ (MPW1PW91) or 2.875 A˚ (BP86), which is ∼0.1 A˚ longer than the predicted NbtNb triplebond distances in the singlet structures 5S-1 and 5S-2 and therefore consistent with the formal double bond required to give each niobium atom the 17-electron configuration for a binuclear triplet. The next triplet Cp2Nb2(CO)5 structure, 5T-2 (Figure 4 and Tables 6 and 7), is a C1 structure with two semibridging carbonyls. Structure 5T-2 is a genuine minimum with all real vibrational frequencies by MPW1PW91 and BP86. One of the nominally semibridging carbonyls in 5T-2 is close to being symmetrical, with the shorter and longer Nb-C distances

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differing by only 0.1 A˚ (about 2.2 A˚ for the shorter Nb-C distance and about 2.3 A˚ for the longer one) by either method. The predicted ν(CO) frequency of 1715 cm-1 (BP86) can be assigned to this semibridging carbonyl. The other semibridging carbonyl in 5T-2 is much less symmetrical, with a short Nb-C distance of 2.061 A˚ (MPW1PW91) or 2.070 A˚ (BP86) and a long Nb-C distance of 2.739 A˚ (MPW1PW91) or 2.704 A˚ (BP86). The predicted ν(CO) frequency of 1815 cm-1 (BP86) corresponds to this semibridging carbonyl. The predicted NbdNb distance in 5T-2 of 2.897 A˚ (MPW1PW91) or 2.938 A˚ (BP86) is ∼0.15 A˚ longer than the predicted NbtNb triple-bond distances in the singlet structures 5S-1 and 5S-2 and therefore consistent with the formal double bond required to give each niobium atom the 17-electron configuration for a binuclear triplet. Another Cp2Nb2(CO)5 triplet structure, namely, the C1 structure 5T-3 (Figure 4 and Tables 6 and 7), has the two Cp rings bonded to one niobium atom and only carbonyl groups bonded to the second niobium atom, similar to structure 5S-3. Structure 5T-3 lies 24.3 kcal/mol (MPW1PW91) or 28.0 kcal/mol (BP86) above the global minimum 5S-1 with all real vibrational frequencies. Structure 5T-3 has two weakly semibridging CO groups with shorter Nb-C distances of 2.087 to 2.115 A˚ (MPW1PW91) or 2.115 to 2.120 A˚ (BP86) and longer Nb-C distances of 2.648 to 3.056 A˚ (MPW1PW91) or 2.619 to 2.908 A˚ (BP86). The predicted ν(CO) frequencies of 1824 and 1864 cm-1 (BP86) correspond to these two semibridging carbonyl groups. The Nb-Nb distance in 5T-3 is 3.034 A˚ (MPW1PW91) or 3.014 A˚ (BP86). 3.4. Cp2Nb2(CO)4. Two singlet structures and four triplet structures (Figure 5 and Tables 8 and 9) were found for Cp2Nb2(CO)4. The global minimum 4S-1 is predicted to be a singlet cis structure (C1 symmetry) with two four-electrondonor η2-μ-CO bridging carbonyl groups and two terminal carbonyl groups. This structure is predicted to be a genuine minimum with all real vibrational frequencies. The Nb-O distances to the η2-μ-CO bridging carbonyl groups of 2.44 to 2.50 A˚ in 4S-1 and their very low ν(CO) frequencies of 1629 and 1641 cm-1 are consistent with these bridging carbonyl groups being formal four-electron donors. The predicted NbdNb distance in 4S-1 of 2.955 A˚ (MPW1PW91) or 2.986 A˚ (BP86) is consistent with the NbdNb double bond required to give each niobium atom the favored 18-electron configuration in a structure with two four-electron-donor carbonyl groups. The second energetically low-lying singlet structure of Cp2Nb2(CO)4, namely, 4S-2 (Figure 5 and Tables 8 and 9), has C2h symmetry and lies above 4S-1 by 6.3 kcal/mol (MPW1PW91) or 3.7 kcal/mol (BP86) in energy. Structure 4S-2 has all real vibrational frequencies by MPW1PW91 and two very small imaginary vibrational frequencies (19i and 8i) by BP86. These imaginary vibrational frequencies become real when a finer integration grid (120, 974) is used, indicating that they originate from numerical integration error. Structure 4S-2 has four equivalent semibridging carbonyls with short Nb-C distances of 2.064 A˚ (MPW1PW91) or 2.073 A˚ (BP86) and long Nb-C distances of 2.638 A˚ (MPW1PW91) or 2.642 A˚ (BP86). The Nb-Nb quadruplebond distance in 4S-2 is 2.586 A˚ (MPW1PW91) or 2.620 A˚ (BP86), which is 0.4 A˚ shorter than that of the NbdNb double bond in 4S-1 and thus consistent with the formal quadruple bond required to give each niobium atom the favored 18-electron configuration in 4S-2.

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Figure 5. Optimized geometries of the six Cp2Nb2(CO)4 structures with distances in A˚. Table 8. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), Nb-Nb Bond Distances (A˚), and Spin Expectation Values ÆS2æ for the Six Lowest Energy Cp2Nb2(CO)4 Structures

MPW1PW91

BP86


4S-1 (C1)

4S-2 (C2h)

4T-1 (C2)

4T-2 (C2h)

4T-3 (Cs)

4T-4 (Cs)

-954.44763 0.0 0 2.955 0.0 -954.80442 0.0 0 2.986 0.0

-954.43759 6.3 0 2.586 0.0 -954.79851 3.7 2(19i,8i) 2.620 0.0

-954.44226 3.4 0 3.120 2.02 -954.79367 6.7 1(19i) 3.103 2.01

-954.43176 10.0 3(17i,17i,12i) 2.654 2.01 -954.78379 12.9 0 2.628 2.01

-954.43149 10.1 0 3.073 2.02 -954.78546 11.9 0 3.091 2.01

-954.40894 24.3 0 2.998 2.02 -954.76246 26.3 1(9i) 2.977 2.01

The lowest lying Cp2Nb2(CO)4 triplet structure, 4T-1 (Figure 5 and Tables 8 and 9), has a cis arrangement of the Cp rings, two four-electron-donor η2-μ-CO bridging carbonyls, two terminal carbonyls, and C2 symmetry. It has all real harmonic vibrational frequencies by MPW1PW91, but has a small imaginary vibrational frequency (19i cm-1) by BP86. This imaginary vibrational frequency becomes real when a finer integration grid (120, 974) is used. Structure 4T-1 lies 3.4 kcal/mol (MPW1PW91) or 6.7 kcal/mol (BP86) above 4S-1. The two η2-μ-CO groups in 4T-1 have the expected short Nb-O distances of 2.320 A˚ (MPW1PW91) or 2.347 A˚ (BP86) and the expected relatively long C-O distances of 1.216 A˚ (MPW1PW91) or 1.237 A˚ (BP86) and exhibit the expected low ν(CO) frequencies of 1585 and 1610 cm-1. The Nb-Nb distance in 4T-1 is 3.120 A˚ (MPW1PW91) or 3.103 A˚ (BP86), consistent with the formal single bond required to give each niobium atom the 17-electron configuration for a binuclear triplet structure with two four-electron-donor carbonyl groups.

Table 9. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted for the Six Lowest Energy Structures of Cp2Nb2(CO)4a BP86 4S-1 (C1) 4S-2 (C2h) 4T-1 (C2) 4T-2 (C2h) 4T-3 (Cs) 4T-4 (Cs) a

1629(522), 1641(156), 1896(530), 1941(1323) 1815(0), 1846(1280), 1848(1434), 1892(0) 1585(515), 1610(4), 1897(1130), 1928(1320) 1835(0), 1854(1747), 1862(1178), 1904(0) 1566(591), 1597(31), 1885(2350), 1912(190) 1564(459), 1875(1070), 1882(1182), 1937(1661)


The second energetically low-lying triplet Cp2Nb2(CO)4 structure, 4T-2 (Figure 5 and Tables 8 and 9), lies 10.0 kcal/mol (MPW1PW91) or 12.9 kcal/mol (BP86) above 4S-1. Structure 4T-2 is a trans structure (C2h symmetry) with four equivalent semibridging carbonyls. It is predicted to be a genuine minimum by BP86, but it has three small imaginary vibrational frequencies (17i, 17i, and 12i cm-1) by MPW1PW91. The Nb-C

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Figure 6. Optimized geometries of the seven Cp2Nb2(CO)3 structures with distances in A˚.

distances to the semibridging carbonyls in 4T-2 are 2.061 A˚ (MPW1PW91) or 2.069 A˚ (BP86) and 2.810 A˚ (MPW1PW91) or 2.841 A˚ (BP86). The NbtNb distance in 4T-2 is 2.654 A˚ (MPW1PW91) or 2.628 A˚ (BP86) and can be interpreted as the formal NbtNb triple bond required to give each niobium atom the 17-electron configuration for a binuclear triplet. The triplet Cp2Nb2(CO)4 structure 4T-2, with four equivalent semibridging carbonyls and a short NbtNb distance, is very similar to the experimental structure of the singlet Cp2Mo2(CO)4, which has an MotMo distance of 2.448 A˚, determined by X-ray diffraction.34 (34) Huang, J.-S.; Dahl, L. F. J. Organomet. Chem. 1983, 243, 57.

The third energetically low-lying triplet Cp2Nb2(CO)4 structure, 4T-3, lies 10.1 kcal/mol (MPW1PW91) or 11.9 kcal/mol (BP86) in energy above the global minimum 4S-1 with all real vibrational frequencies. Structure 4T-3 is a Cs structure with two four-electron-donor η2-μ-CO bridging carbonyls and two terminal carbonyls. Both four-electron-donor carbonyl groups are characterized by short Nb-O distances of 2.327 A˚ (MPW1PW91) or 2.358 A˚ (BP86) and very low ν(CO) frequencies of 1566 and 1597 cm-1 (BP86). The predicted Nb-Nb distance in 4T-3 of 3.073 A˚ (MPW1PW91) or 3.091 A˚ (BP86) is consistent with the formal single bond required to give each niobium atom the 17-electron configuration for a binuclear triplet structure with two four-electron-donor bridging carbonyl groups.

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Table 10. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), and Nb-Nb Bond Distances (A˚) for the Five Singlet Cp2Nb2(CO)3 Structures

MPW1PW91

BP86

E ΔE Nimag Nb-Nb E ΔE Nimag Nb-Nb

3S-1 (Cs)

3S-2 (Cs)

3S-3 (C2)

3S-4 (C1)

3S-5 (Cs)

-841.11255 0.0 1 (17i) 2.826 -841.43038 0.0 1 (15i) 2.849

-841.10133 7.0 1 (21i) 2.817 -841.42047 6.2 1 (19i) 2.835

-841.10009 7.8 0 2.815 -841.42220 5.1 0 2.824

-841.09110 13.5 0 2.818 -841.41521 9.5 0 2.847

-841.06542 29.6 0 2.706 -841.37869 32.4 0 2.710

Table 11. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), Spin Expectation Values ÆS2æ, and Nb-Nb Bond Distances (A˚) for the Four Triplet Cp2Nb2(CO)3 Structures

MPW1PW91

BP86


3T-1 (Cs)

3T-2 (Cs)

-841.10368 5.6 1(5i) 2.765 2.10 -841.41786 7.9 1(8i) 2.763 2.03

-841.09788 9.2 0 3.139 2.02 -841.40754 14.3 0 3.068 2.01

Another triplet Cp2Nb2(CO)4 structure, 4T-4 (Figure 5 and Tables 8 and 9), has the two Cp rings bonded to one niobium atom and only carbonyl groups bonded to the second niobium atom. 4T-4 is predicted to be a genuine minimum by MPW1PW91, but has a small imaginary vibrational frequency (9i cm-1) by BP86. This imaginary vibrational frequency becomes real when a finer integration grid (120, 974) is used, indicating that it originates from numerical integration error. Structure 4T-4 has Cs symmetry and lies 24.3 kcal/mol (MPW1PW91) or 26.3 kcal/mol (BP86) above the global minimum 4S-1. One of the carbonyl groups in 4T-4 is a four-electron-donor η2-μ-CO group, as indicated by a short Nb-O distance of 2.282 A˚ (MPW1PW91) or 2.300 A˚ (BP86), a relatively long C-O distance of 1.220 A˚ (MPW1PW91) or 1.241 A˚ (BP86), and a very low ν(CO) frequency of 1564 cm-1 (BP86). The Nb-Nb distance in 4T-4 is 2.998 A˚ (MPW1PW91) or 2.977 A˚ (BP86). 3.5. Cp2Nb2(CO)3. Five singlet structures and two triplet structures (Figure 6 and Tables 10 to 12) are predicted for Cp2Nb2(CO)3 within 30 kcal/mol of the global minimum. The global minimum 3S-1 is predicted to be a singlet Cs structure with two four-electron-donor η2-μ-CO carbonyl groups and one terminal carbonyl group. Structure 3S-1 has a very small imaginary vibrational frequency at 17i (MPW1PW91) or 15i cm-1 (BP86). This imaginary vibrational frequency becomes real when a finer integration grid (120, 974) is used, indicating that this small imaginary frequency arises from numerical integration error. The four-electrondonor carbonyl groups in 3S-1 are characterized by short Nb-O distances of 2.151 A˚ (MPW1PW91) or 2.158 A˚ (BP86), very long C-O distances of 1.261 A˚ (MPW1PW91) or 1.286 A˚ (BP86), and extremely low ν(CO) frequencies of 1390 and 1404 cm-1 (BP86). The predicted NbtNb distance of 2.826 A˚ (MPW1PW91) or 2.849 A˚ (BP86) is somewhat long for the formal triple bond required to give each niobium atom in Cp2Nb2(CO)3 the favored 18-electron configuration with two four-electron-donor bridging carbonyl

Table 12. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted for the Lowest Energy Structures of Cp2Nb2(CO)3a BP86 3S-1 (Cs) 3S-2 (Cs) 3S-3 (C2) 3S-4 (C1) 3S-5 (Cs) 3T-1 (Cs) 3T-2 (Cs) a

1390(263), 1404(203), 1911(719) 1474(351), 1477(21), 1698(392) 1559(538), 1603(310), 1710(420) 1508(371), 1561(217), 1693(462) 1804(1006), 1828(117), 1926(1395) 1555(407), 1771(1065), 1805(147) 1493(411), 1518(80), 1920(1193)


groups. However, the geometry of the two four-electron-donor bridging carbonyl groups for optimum bonding may prevent the niobium atoms from coming too close to each other. The second energetically low-lying singlet Cp2Nb2(CO)3 structure, 3S-2 (Figure 6 and Tables 10 and 12), is a Cs structure lying 7.0 kcal/mol (MPW1PW91) or 6.2 kcal/mol (BP86) in energy above 3S-1, with a small imaginary vibrational frequency at 21i cm-1 (MPW1PW91) or 19i cm-1 (BP86). This frequency becomes real when a finer integration grid (120, 974) is used, indicating that it arises from numerical integration error. Two of the three carbonyl groups in 3S-2 are clearly four-electron-donor η2-μ-CO carbonyls, as indicated by short Nb-O distances of 2.228 A˚ (MPW1PW91) or 2.248 A˚ (BP86) and very low ν(CO) frequencies of 1474 and 1477 cm-1. The third bridging carbonyl group in 3S-2 has a somewhat short Nb-O distance of 2.534 A˚ (MPW1PW91) or 2.560 A˚ (BP86) but a fairly normal bridging ν(CO) frequency of 1698 cm-1, leading to some uncertainty as to whether this third carbonyl group is a formal two- or four-electron donor. The predicted NbdNb bond distance in 3S-2 of 2.817 A˚ (MPW1PW91) or 2.835 A˚ (BP86) adds to this ambiguity. The fact that this distance is shorter than the niobium-niobium distance in 3S-1 by only ∼0.01 A˚ suggests a formal triple bond, implying that the questionable carbonyl group is only a formal two-electron donor. However, since these niobium-niobium distances are rather long for a formal triple bond, the possibility cannot be excluded that the NbdNb bond in 3S-2 is a formal double bond with the questionable carbonyl group being a formal four-electron donor. The third energetically low-lying singlet Cp2Nb2(CO)3 structure, 3S-3 (Figure 6 and Tables 10 and 12), lies 7.8 kcal/ mol (MPW1PW91) or 5.1 kcal/mol (BP86) in energy above 3S-1, with all real harmonic vibrational frequencies. Structure 3S-3 is a C2 structure with two four-electron-donor η2-μ-CO bridging carbonyls and one normal two-electron bridging carbonyl. The two equivalent four-electron-donor bridging carbonyls are characterized by short Nb-O distances of 2.364 A˚ (MPW1PW91) or 2.398 A˚ (BP86) and very low ν(CO) frequencies of 1559 and 1603 cm-1 (BP86). By contrast the ν(CO) frequency of the two-electron-donor bridging carbonyl

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Figure 7. Optimized geometries of the five Cp2Nb2(CO)2 structures with distances in A˚.

group in 3S-3 is predicted to be significantly higher at 1710 cm-1. The NbtNb bond distance in 3S-3 is predicted to be 2.815 A˚ (MPW1PW91) or 2.824 A˚ (BP86), which is very similar to that in 3S-1, which likewise has two four-electron-donor carbonyls and one two-electron-donor bridging carbonyl. Structures 3S-3 and 3S-1 differ in the locations of the short Nb-O distances that are diagnostic of the four-electron carbonyl groups. In structure 3S-1 the short Nb-O distances are to the same niobium atom, whereas in structure 3S-3 the short Nb-O distances are to different niobium atoms. The fourth energetically low-lying singlet Cp2Nb2(CO)3 structure, 3S-4, lies above 3S-1 by 13.5 kcal/mol (MPW1PW91) or 9.5 kcal/mol (BP86) with all real harmonic vibrational frequencies. Structure 3S-4 has two four-electron-donor η2-μ-CO bridging carbonyls and one normal two-electron-donor bridging carbonyl, similar to structure 3S-3. The two four-electron-donor bridging carbonyls are characterized by short Nb-O distances of ∼2.26 A˚ (MPW1PW91) or ∼2.29 A˚ (BP86) and very low ν(CO) frequencies of 1508 and 1561 cm-1 as compared with the 1693 cm-1 (BP86) frequency of the two-electron bridging carbonyl (BP86). The Nb-Nb bond distance in 3S-4 is 2.818 A˚ (MPW1PW91) or 2.847 A˚ (BP86), which, like in 3S-1, can be interpreted as the formal triple bond required to give both niobium atoms the favored 18-electron configuration in a Cp2Nb2(CO)3 structure with two fourelectron-donor bridging carbonyl groups. Structure 3S-5 is a relatively high-energy singlet structure, lying 29.6 kcal/mol (MPW1PW91) or 32.4 kcal/mol (BP86)

above 3S-1 with no imaginary vibrational frequencies. Structure 3S-5 is a Cs structure with two Cp rings bonded to one niobium atom and only carbonyl groups bonded to the other niobium atom. Structure 3S-5 has two semibridging carbonyls and one terminal carbonyl. For the two equivalent semibridging carbonyls, the shorter Nb-C bond distances are 2.003 A˚ (MPW1PW91) or 2.016 A˚ (BP86) and the longer Nb-C bond distances are 2.701 A˚ (MPW1PW91) or 2.680 A˚ (BP86). The Nb-Nb bond distance in 3S-5 is 2.706 A˚ (MPW1PW91) or 2.710 A˚ (BP86). The lowest lying Cp2Nb2(CO)3 triplet structure, 3T-1 (Figure 6 and Tables 11 and 12), is a Cs structure with a very small imaginary vibrational frequency at 5i (MPW1PW91) or 8i cm-1 (BP86). This imaginary vibrational frequency becomes real when a finer integration grid (120, 974) is used, indicating that this very small imaginary frequency arises from numerical integration error. One of the carbonyl groups in 3T-1 is a four-electron-donor η2-μ-CO bridging carbonyl group, as indicated by a short Nb-O distance of 2.327 A˚ (MPW1PW91) or 2.329 A˚ (BP86), a long C-O bond distance of 1.224 A˚ (MPW1PW91) or 1.247 A˚(BP86), and a low predicted ν(CO) frequency of 1555 cm-1 (BP86). For the two equivalent semibridging carbonyls, the Nb-C distances are 2.074 and 2.503 A˚ (MPW1PW91) or 2.087 and 2.459 A˚ (BP86). The predicted ν(CO) frequencies of 1771 and 1805 cm-1 (BP86) correspond to these two semibridging carbonyl groups. The NbtNb bond distance in 3T-1 is predicted to be 2.765 A˚ (MPW1PW91) or 2.763 A˚ (BP86), consistent with the formal Nb-Nb triple bond required to give each

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Table 13. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), and Nb-Nb Bond Distances (A˚) for the Two Singlet Cp2Nb2(CO)2 Structures

MPW1PW91

BP86

E ΔE Nimag Nb-Nb E ΔE Nimag Nb-Nb

2S-1 (Cs)

2S-2 (C2)

-727.73341 0.0 0 2.608 -728.01859 0.0 0 2.641

-727.69722 22.7 0 2.543 -727.98443 21.4 0 2.565

niobium atom in Cp2Nb2(CO)3 the 17-electron configuration for a binuclear triplet with one four-electron-donor bridging carbonyl group. The second energetically low-lying triplet structure, 3T-2 (Figure 6 and Tables 11 and 12), lies above 3S-1 by 9.2 kcal/ mol (MPW1PW91) or 14.3 kcal/mol (BP86). Structure 3T-2 is a Cs structure (with two semibridging carbonyls and one terminal carbonyl) and has all real harmonic vibrational frequencies by BP86 and MPW1PW91. For the semibridging carbonyls in 3T-2, the shorter Nb-C distances are 1.984 A˚ (MPW1PW91) or 2.009 A˚ (BP86) and the longer Nb-C distances are 2.328 A˚ (MPW1PW91) or 2.279 A˚ (BP86). The related Nb-O distances of these two equivalent semibridging carbonyl groups are very short, namely, 2.230 A˚ (MPW1PW91) or 2.222 A˚ (BP86), indicating two fourelectron-donor η2-μ-CO groups. This is consistent with their extremely low ν(CO) frequencies at 1493 and 1518 cm-1 (BP86). The NbdNb bond distance is 3.139 A˚ (MPW1PW91) or 3.068 A˚ (BP86), consistent with the formal double bond required to give each niobium atom in Cp2Nb2(CO)3 the 17-electron configuration for a binuclear triplet. 3.6. Cp2Nb2(CO)2. A total of five energetically low-lying Cp2Nb2(CO)2 structures were found (Figure 7 and Tables 13, 14, and 15), including three triplet and two singlet structures. The global minimum 2S-1 is predicted to be a singlet Cs structure. The two carbonyl groups are both predicted to be four-electron-donor η2-μ-CO bridging carbonyl groups, as indicated by their Nb-O distances of 2.105 A˚ (MPW1PW91) or 2.116 A˚ (BP86) and extremely low ν(CO) frequencies of 1321 and 1392 cm-1 (BP86). The relatively short Nb-Nb quadruple-bond distance in 2S-1 of 2.608 A˚ (MPW1PW91) or 2.641 A˚ (BP86) can be interpreted as the formal quadruple bond required to give each niobium atom an 18-electron configuration. The second singlet Cp2Nb2(CO)2 structure, 2S-2 (Figure 7 and Tables 13 and 15), also has two semibridging CO groups and all real vibrational frequencies by MPW1PW91 and BP86. Structure 2S-2 is a C2 structure and lies 22.7 kcal/ mol (MPW1PW91) or 21.4 kcal/mol (BP86) above 2S-1. The Nb-C distances to the semibridging carbonyls are 2.012 A˚ (MPW1PW91) or 2.032 A˚ (BP86) and 2.397 A˚ (MPW1PW91) or 2.390 A˚ (BP86). The ν(CO) frequencies of 2S-2 at 1720 cm-1 (BP86) and the long Nb 3 3 3 O distances indicate that these two semibridging CO groups are the normal twoelectron donors rather than four-electron-donor η2-μ-CO carbonyl groups. The very short Nb-Nb distance of 2.543 A˚ (MPW1PW91) or 2.565 A˚ (BP86) in 2S-2 clearly indicates a high formal metal-metal bond order. The lowest lying Cp2Nb2(CO)2 triplet structure, 2T-1 (Figure 7 and Tables 14 and 15), lies 2.2 kcal/mol (MPW1PW91) or 8.2 kcal/mol (BP86) above 2S-1 in energy

Table 14. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), Nb-Nb Bond Distances (A˚), and Spin Expectation Values ÆS2æ for the Three Triplet Cp2Nb2(CO)2 Optimized Structures

MPW1PW91

BP86


2T-1 (C2)

2T-2 (C1)

2T-3 (Cs)

-727.72992 2.2 0 2.998 2.03 -728.00548 8.2 0 2.981 2.01

-727.72175 7.3 0 2.760 2.16 -727.99796 12.9 0 2.646 2.02

-727.71906 9.0 0 3.058 2.02 -728.01138 4.5 0 2.859 2.01

Table 15. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted for the Five Lowest Energy Structures of Cp2Nb2(CO)2a BP86 2S-1 (Cs) 2S-2 (C2) 2T-1 (C2) 2T-2 (C1) 2T-3 (Cs) a

1321(270), 1392(206) 1720(1198), 1720(73) 1456(406), 1498(79) 1383(309), 1787(599) 1388(342), 1415(96)


and has all real vibrational frequencies by MPW1PW91 and BP86. The Nb-O distances of 2.214 A˚ (MPW1PW91) or 2.229 A˚ (BP86) to the bridging carbonyl groups are very short, indicating that they are four-electron-donor η2-μ-CO groups, consistent with their very low ν(CO) frequencies of 1456 and 1498 cm-1 (BP86). The Nb-Nb distance in 2T-1 is 2.998 A˚ (MPW1PW91) or 2.981 A˚ (BP86). The second triplet Cp2Nb2(CO)2 structure, namely, 2T-2 (Figure 7 and Tables 14 and 15), is a C1 structure and has one four-electron-donor η2-μ-CO carbonyl group and one normal two-electron-donor semibridging carbonyl group. Structure 2T-2 is a genuine minimum with all real harmonic vibrational frequencies and lies 7.3 kcal/mol kcal/mol (MPW1PW91) or 12.9 kcal/mol (BP86) above 2S-1. The four-electron-donor bridging carbonyl group in 2T-2 is characterized by a short Nb-O distance of 2.243 A˚ (MPW1PW91) or 2.155 A˚ (BP86) and a very low ν(CO) frequency of 1383 cm-1 (BP86). For the two-electron-donor semibridging carbonyl group in 2T-2 the shorter Nb-C distance is 2.067 A˚ (MPW1PW91) or 2.069 A˚ (BP86) and the longer Nb-C distance is 2.522 A˚ (MPW1PW91) or 2.577 A˚ (BP86). This two-electron-donor carbonyl group is predicted to exhibit a ν(CO) frequency at 1787 cm-1. The predicted Nb-Nb bond distance in 2T-2 is 2.760 A˚ (MPW1PW91) or 2.646 A˚ (BP86). The third triplet Cp2Nb2(CO)2 structure, namely, 2T-3 (Figure 7 and Tables 14 and 15), is a Cs structure with two equivalent four-electron-donor η2-μ-CO bridging carbonyl groups. It lis 9.0 kcal/mol (MPW1PW91) or 4.5 kcal/mol (BP86) above the global minimum 2S-1 with no imaginary vibrational frequencies. These four-electron-donor bridging carbonyl groups are characterized by very short Nb-O distances of 2.232 A˚ (MPW1PW91) or 2.154 A˚ (BP86) and very low ν(CO) frequencies of 1388 and 1415 cm-1 (BP86). The Nb-Nb bond distance in 2T-3 is 3.058 A˚ (MPW1PW91) or 2.859 A˚ (BP86).

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Figure 8. Optimized geometries of the six Cp2Nb2(CO) structures with distances in A˚. Table 16. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), Nb-Nb Bond Distances (A˚), ν(CO) Frequencies in cm-1 (infrared intensities in km/mol in parentheses), and Spin Expectation Values ÆS2æ for Each of the Triplet Cp2Nb2(CO) Optimized Structures

MPW1PW91

BP86

E ΔE Nimag Nb-Nb ν(CO) ÆS2æ E ΔE Nimag Nb-Nb ν(CO) ÆS2æ

1T-1 (C1)

1T-2 (Cs)

-614.34385 0.0 0 2.619 1386(349) 2.07 -614.58470 0.0 0 2.627 1311(278) 2.02

-614.30251 25.9 0 2.324 1903(890) 2.03 -614.55463 18.9 0 2.331 1808(698) 2.01

3.7. Cp2Nb2(CO). A total of six energetically low-lying Cp2Nb2(CO) structures were found (Figure 8 and Tables 16 and 17), including two triplet and four singlet structures. The Cp2Nb2(CO) global minimum 1T-1 is predicted to be a triplet C1 structure with all real vibrational frequencies by MPW1PW91 and BP86. The unsymmetrically bridging carbonyl group in 1T-1 is a four-electron-donor η2-μ-CO group with a short Nb-O distance of 2.086 A˚ (MPW1PW91) or 2.111 A˚ (BP86), a long C-O distance of 1.290 A˚ (MPW1PW91) or 1.309 A˚ (BP86), and an extremely low ν(CO) frequency of 1311 cm-1 (BP86). The Nb-Nb distance of 2.619 A˚ (MPW1PW91) or 2.627 A˚ (BP86) corresponds to a formal quadruple bond to give both niobium atoms 16-electron configurations.

Table 17. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), Nb-Nb Bond Distances (A˚), and ν(CO) Frequencies in cm-1 (infrared intensities in km/mol in parentheses) for Each of the Singlet Cp2Nb2(CO) Optimized Structures 1S-1 (C1) MPW1- E PW91 ΔE Nimag Nb-Nb ν(CO) BP86

E ΔE Nimag Nb-Nb ν(CO)

1S-2 (C1)

1S-3 (C1)

1S-4 (C2v)

-614.33460 -614.32673 -614.32172 -614.31253 5.8 0 2.650 1411(398)

10.7 0 2.340 1953(1182)

13.9 0 2.565 1354(292)

19.7 0 2.357 1779(692)

-614.58380 0.6 0 2.639 1321(220)

-614.58040 2.7 0 2.370 1845(922)

-614.57249 7.7 0 2.584 1270(211)

-614.56649 11.4 0 2.372 1683(569)

The other triplet structure of Cp2Nb2(CO), namely, 1T-2 (Figure 8 and Table 16), lies 25.9 kcal/mol (MPW1PW91) or 18.9 kcal/mol (BP86) above the global minimum 1T-1 with Cs symmetry and all real harmonic vibrational frequencies. Structure 1T-2 is of a very different type than 1T-1, since it has the metal-metal bond axis perpendicular to the original C5 axes of the Cp rings with a semibridging CO ligand bonded to the two niobium atoms. In structure 1T-2, each niobium atom is bonded to a portion of each Cp ring, in contrast to structure 1T-1, in which each metal atom is bound to only one of the Cp rings. The semibridging carbonyl in 1T-2 has a short Nb-C distance of 2.015 A˚ (MPW1PW91) or 2.031 A˚ (BP86) and a long Nb-C distance of 2.604 A˚ (MPW1PW91) or 2.680 A˚ (BP86) and is predicted to exhibit a ν(CO) frequency of 1808 cm-1 (BP86, Table 16). The predicted Nb-Nb distance is uniquely short at 2.324 A˚ (MPW1PW91) or 2.331 A˚ (BP86).

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The lowest lying singlet structure of Cp2Nb2(CO), 1S-1, lies above the global minimum 1T-1 by 5.8 kcal/mol (MPW1PW91) or 0.6 kcal/mol (BP86) with all real harmonic vibrational frequencies (Figure 8 and Table 17). The carbonyl group in 1S-1 is a four-electron-donor η2-μ-CO bridging carbonyl group, as indicated by a short Nb-O distance of 2.088 A˚ (MPW1PW91) or 2.112 A˚ (BP86) and an extremely low ν(CO) frequency of 1321 cm-1 (BP86). The Nb-Nb distance in 1S-1 is 2.650 A˚ (MPW1PW91) or 2.639 A˚ (BP86). The second energetically low-lying singlet Cp2Nb2(CO) structure, 1S-2 (Figure 8 and Table 17), lies above the global minimum 1T-1 by 10.7 kcal/mol (MPW1PW91) or 2.7 kcal/ mol (BP86), with all real harmonic vibrational frequencies. Structure 1S-2 has the metal-metal bond axis perpendicular to the original C5 axes of one of the Cp rings, as well as a semibridging CO ligand bonded to the two niobium atoms. In structure 1S-2, one Cp ring bridges both niobium atoms and the other Cp ring is bonded exclusively to a single niobium atom. The Nb-C distances to the weakly semibridging carbonyl in 1S-2 are 2.066 and 2.898 A˚ (MPW1PW91) or 2.065 A˚ or 2.959 A˚ (BP86), and the corresponding predicted ν(CO) frequency is 1845 cm-1 (BP86). The Nb-Nb distance in 1S-2 is very short at 2.340 A˚ (MPW1PW91) or 2.370 A˚ (BP86), again possibly because of the geometrical constraint of the bridging Cp ring. The third energetically low-lying singlet Cp2Nb2(CO) structure, 1S-3 (Figure 8 and Table 17), lies above 1T-1 by 13.9 kcal/mol (MPW1PW91) or 7.7 kcal/mol (BP86) with all real harmonic vibrational frequencies. The carbonyl group in 1S-3 is a four-electron-donor η2-μ-CO bridging carbonyl group, as indicated by a short Nb-O distance of 2.054 A˚ (MPW1PW91) or 2.059 A˚ (BP86) and an extremely low ν(CO) frequency of 1270 cm-1. The Nb-Nb distance is 2.565 A˚ (MPW1PW91) or 2.584 A˚ (BP86). Another energetically low-lying singlet structure of Cp2Nb2(CO), namely, 1S-4 (Figure 8 and Table 17), lies above 1T-1 by 19.7 kcal/mol (MPW1PW91) or 11.4 kcal/mol (BP86), with C2v symmetry and all real harmonic vibrational frequencies. In 1S-4 the metal-metal bond axis is perpendicular to the original C5 axes of the Cp rings, with a symmetrical bridging CO ligand. In structure 1S-4, each niobium atom is bonded to a portion of each Cp ring. The bridging carbonyl group in 1S-4 has Nb-C distances of 2.148 A˚ (MPW1PW91) or 2.160 A˚ (BP86) and a predicted ν(CO) frequency of 1683 cm-1 (BP86). The predicted Nb-Nb distance in 1S-4 is a very short 2.357 A˚ (MPW1PW91) or 2.372 A˚ (BP86). 3.8. Dissociation and Disproportionation Reactions. Table 18 lists the dissociation energies for the reactions Cp2Nb2(CO)n f Cp2Nb2(CO)n-1 þ CO based on the lowest energy structures. The bond dissociation energy (BDE) values for the losses of one carbonyl group from Cp2Nb2(CO)7 and Cp2Nb2(CO)6 are relatively low at 53 kcal/mol, indicating their stabilities. Table 19 lists the energies of the disproportionation reactions 2Cp2Nb2(CO)n f Cp2Nb2(CO)nþ1 þ Cp2Nb2(CO)n-1 based on the lowest energy structures. These results indicate that the structures Cp2Nb2(CO)n (n=5 and 3) are stable with respect to such disproportionations. Cp2Nb2(CO)2 is also

Zhang et al. Table 18. Dissociation Energies (kcal/mol) for the Successive Removal of Carbonyl Groups from Cp2Nb2(CO)n (n = 7, 6, 5, 4, 3, 2) MPW1PW91/ DZP

BP86/ DZP

17.6

20.4

15.2

16.4

40.3

39.8

25.3

29.4

52.9

53.1

59.5

66.9

Cp2Nb2(CO)7 (7S-1) f Cp2Nb2(CO)5 (6S-1) þ CO Cp2Nb2(CO)6 (6S-1) f Cp2Nb2(CO)5 (5S-1) þ CO Cp2Nb2(CO)5 (5S-1) f Cp2Nb2(CO)4 (4S-1) þ CO Cp2Nb2(CO)4 (4S-1) f Cp2Nb2(CO)3 (3S-1) þ CO Cp2Nb2(CO)3 (3S-1) f Cp2Nb2(CO)2 (2S-1) þ CO Cp2Nb2(CO)2 (2S-1) f Cp2Nb2(CO) (1T-1) þ CO

Table 19. Disproportionation Energies (kcal/mol) of the Cp2Nb2(CO)n (n = 7, 6, 5, 4, 3, 2) Species Based on the Lowest Energy Structures MPW1PW91/DZP

BP86/DZP

-2.4

-4.0

25.1

23.4

-15.0

-10.4

27.6

23.7

6.5

13.9

2Cp2Nb2(CO)6 f Cp2Nb2(CO)5 þ Cp2Nb2(CO)7 2Cp2Nb2(CO)5 f Cp2Nb2(CO)6 þ Cp2Nb2(CO)4 2Cp2Nb2(CO)4 f Cp2Nb2(CO)5 þ Cp2Nb2(CO)3 2Cp2Nb2(CO)3 f Cp2Nb2(CO)2 þ Cp2Nb2(CO)4 2Cp2Nb2(CO)2 f Cp2Nb2(CO) þ Cp2Nb2(CO)3

Table 20. Total Energies (E, in hartree) for the Global Minima of CpNb(CO)m (m = 4, 3, 2, 1)a CpNb(CO)4 (Cs)

CpNb(CO)3 (Cs)

CpNb(CO)2 (Cs)

CpNb(CO) (Cs)

MPW1- E -703.90229 PW91 BP86 E -704.15034

-590.53172

-477.16050

-363.77337

-590.74398

-477.33464

-363.90792

a

None of these structures had any imaginary vibrational frequencies.

relatively stable, whereas Cp2Nb2(CO)4 and Cp2Nb2(CO)6 are energetically unfavorable and will go to the more stable systems Cp2Nb2(CO)5 þ Cp2Nb2(CO)3 or Cp2Nb2(CO)5 þ Cp2Nb2(CO)7, respectively. The dissociation of Cp2Nb2(CO)n into mononuclear fragments can result from fragmentation into CpNb(CO)p þ CpNb(CO)q, where p þ q = n and each fragment has a cyclopentadienyl ring bonded to the niobium atom. To obtain the energetic data for the dissociation of the Cp2Nb2(CO)n derivatives into mononuclear CpNb(CO)m fragments, the structures of the mononuclear CpNb(CO)m (m = 4, 3, 2, 1) were optimized by the same DFT methods used to study the binuclear derivatives. The structures and total energies of the global minima for the CpNb(CO)m are shown in Table 20 and Figure 9. The infrared ν(CO) frequencies predicted for the mononuclear fragments are listed in Table 21 including a comparison with experimental data35 for CpNb(CO)n (n = 4, 3, 2), using data obtained in a polyethylene matrix at 100 K. (35) Childs, G. I.; Gallagher, S.; Bitterwolf, T. E.; George, M. W. J. Chem. Soc., Dalton Trans. 2000, 4534.

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Figure 9. Optimized structures of the mononuclear fragments CpNb(CO)n (n = 4, 3, 2, 1) with the distances in A˚. Table 21. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted for CpNb(CO)n (n = 4, 3, 2, 1)a CpNb(CO)4 CpNb(CO)3 CpNb(CO)2 CpNb(CO) a

BP86

experiment35

1929(573), 1930(1445), 1930(870), 2012(458) 1886(1549), 1889(574), 1967(624) 1832(822), 1900(971) 1803(1271)

1927, 1943, 2035 1870, 1878, 1981 1804, 1898


The dissociation reactions for the lowest energy Cp2Nb2(CO)n structures are all predicted to be highly endothermic (Table 22). The dissociation energies of Cp2Nb2(CO)n increase monotonically with decreasing n, in accord with the corresponding increase in formal metal-metal bond order. The dissociation energy of the saturated dimer Cp2Nb2(CO)7 into the mononuclear CpNb(CO)3 and CpNb(CO)4 fragments is found to be 9.1 kcal/mol (MPW1PW91) or 8.7 kcal/ mol (BP86), indicating that Cp2Nb2(CO)7 is marginal with respect to dissociation into CpNb(CO)3 and CpNb(CO)4 and thus unlikely to be prepared as an isolable molecule. However, the dissociation energy of the Cp2Nb2(CO)6 into two mononuclear CpNb(CO)3 fragments or CpNb(CO)2 þ CpNb(CO)4 is found to be >37 kcal/mol, indicating considerable energy for generation of mononuclear fragments from Cp2Nb2(CO)6. The dissociation energies of the more unsaturated Cp2Nb2(CO)n derivatives are higher yet, in excess of 70 kcal/mol in all cases, indicating that the dissociation of binuclear Cp2Nb2(CO)n into mononuclear fragments is not likely to be a significant pathway for their chemical reactions.

4. Discussion The analogous lowest energy Cp2M2(CO)n (n=7, 6, 5, 4, 3, 2, 1) structures of vanadium and niobium are compared in Table 23 with respect to the metal-metal bond distances, the formal metal-metal bond orders (in brackets), the spin states, and the presence of four-electron-donor η2-μ-CO bridging carbonyl groups. The BP86 data for the vanadium compounds are taken from previous research.5,6 The following general observations can be made from the data in Table 23: (1) The greater oxophilicity of niobium relative to vanadium leads to a larger number of low-energy structures with four-electron-donor bridging η2-μ-CO groups. Thus fourelectron-donor bridging carbonyl groups are not found in any of the lowest energy structures for Cp2V2(CO)n (n=7, 6,

Table 22. Dissociation Energies of the Binuclear Cp2Nb2(CO)n (n = 7, 6, 5, 4, 3, 2) into Mononuclear Fragments (kcal/mol) Based on the Lowest Energy Structures

Cp2Nb2(CO)2 f 2 CpNb(CO) Cp2Nb2(CO)4 f 2 CpNb(CO)2 Cp2Nb2(CO)6 f 2 CpNb(CO)3 Cp2Nb2(CO)3 f CpNb(CO)2 þ CpNb(CO) Cp2Nb2(CO)4 f CpNb(CO)3 þ CpNb(CO) Cp2Nb2(CO)5 f CpNb(CO)4 þ CpNb(CO) Cp2Nb2(CO)5 f CpNb(CO)3 þ CpNb(CO)2 Cp2Nb2(CO)6 f CpNb(CO)2 þ CpNb(CO)4 Cp2Nb2(CO)7 f CpNb(CO)4 þ CpNb(CO)3

MPW1PW91/DZP

BP86/DZP

117.1 79.5 39.1 112.1

127.2 84.8 37.9 117.9

89.4

95.7

82.2

85.8

71.8

73.1

39.5

39.8

9.1

8.7

5, 4) and only begin to appear for Cp2V2(CO)3. However, structures with a single four-electron-donor bridging η2-μCO group are the lowest energy structures for Cp2Nb2(CO)6 and Cp2Nb2(CO)4, namely, 6S-1 (Figure 3) and 4S-1 (Figure 5), respectively. Similarly, the lowest energy structure for Cp2V2(CO)3 has one η2-μ-CO group, whereas that for Cp2Nb2(CO)3 has two η2-μ-CO groups. (2) The lowest energy structures for the niobium derivatives in general have the same or lower spin states than the corresponding vanadium derivatives. This is the expected effect from the stronger ligand field splitting of the secondrow metal niobium relative to the first-row metal vanadium. Thus the lowest energy structures of Cp2Nb2(CO)n (n = 7, 6, 5, 4, 3, and 2) are all singlets with suitable combinations of formal metal-metal bond order and numbers of four-electron-donor carbonyl groups to give both niobium atoms the favored 18-electron configuration. Furthermore, the lowest energy structure 1T-1 (Figure 8) of Cp2Nb2(CO) is a triplet, whereas the lowest energy structure5 of Cp2V2(CO) is a quintet. The metal-metal distances (Table 23) correlate reasonably with the formal bond orders required to give the metal atoms reasonable electronic configurations for the spin state in question, with 18-electron configurations being preferred for singlet structures. However, the formal metal-metal bond order is not the only factor affecting the metal-metal distance. Thus the presence of carbonyl groups can increase the metal-metal distance for a given bond order by backbonding, in which the CtO π* antibonding orbitals withdraw

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Zhang et al.

Table 23. Comparison of the Lowest Energy Structures of Analogous Cp2M2(CO)n Derivatives for Vanadium and Niobium (BP86 method) M=V

M = Nb

n=7 n=6

3.306 A˚ [V-V], singlet 2.847 A˚ [VdV], singlet

n=5 n=4

2.452 A˚ [VtV], singlet 2.444 A˚ [VtV], triplet

n=3

2.532 A˚ [VtV], triplet, η2-μ-CO 2.414 A˚ [V-V] (quadruple bond), singlet, 2η2-μ-CO 2.528 A˚ [VdV], quintet, η2-μ-CO

3.411 A˚ [Nb-Nb], singlet 3.216 A˚ [Nb-Nb], singlet, η2-μ-CO 2.756 A˚ [NbtNb], singlet 2.986 A˚ [NbdNb], singlet, η2-μ-CO 2.849 A˚ [NbtNb], singlet, 2η2-μ-CO 2.641 A˚ [Nb-Nb] (quadruple bond), singlet, 2η2-μ-CO 2.627 A˚ [Nb-Nb] (quadruple bond), triplet, η2-μ-CO

n=2 n=1

electron density from the metal-metal bonding orbitals. With such limitations in mind, for the low-energy structures listed in Table 23 the Nb-Nb distances are predicted to fall in the ranges 3.2 to 3.4 A˚, ∼3.0 A˚, 2.76 to 2.85 A˚, and ∼2.6 A˚ for formal bond orders of 1, 2, 3, and 4, respectively. These distances are generally longer than the reported NbdNb double-bond distances of ∼2.7 A˚ in binuclear Nb(III) halide derivatives36 and NbtNb triple bond distances of ∼2.6 A˚ in binuclear Nb(II) halide derivatives33 obtained by X-ray crystallography. This may relate to the stronger π-acceptor properties of the Cp and CO ligands relative to the halide ligands. The previous study6 on Cp2V2(CO)7 predicts it to be essentially thermoneutral by BP86 with respect to dissociation into CpV(CO)4 and CpV(CO)3 fragments, indicating that Cp2V2(CO)7 is likely to be too unstable to be synthesized. However, the corresponding dissociation of Cp2Nb2(CO)7 into CpNb(CO)4 þ CpNb(CO)3 is predicted to be endothermic by ∼9 kcal/mol (Table 22). This suggests that Cp2Nb2(CO)7 might be synthesized. However, it is likely to be a source of reactive CpNb(CO)3 fragments under relatively mild conditions. The greater stability of Cp2Nb2(CO)7 relative to Cp2V2(CO)7 can be related to the larger size of niobium, making it more able to form bonds to the other niobium atom, to three or four carbonyl groups, and to the cyclopentadienyl ring. A study of the energetics of the disproportionation reactions Cp2Nb2(CO)n f Cp2Nb2(CO)nþ1 þ Cp2Nb2(CO)n-1 (Table 19) suggests that Cp2Nb2(CO)5 and Cp2Nb2(CO)3 are likely to be the most stable unsaturated binuclear cyclopentadienylniobium carbonyls. Thus both Cp2Nb2(CO)5 and Cp2Nb2(CO)3 are predicted to be energetically stable to disproportionation by >20 kcal/mol. By contrast the disproportionations of Cp2Nb2(CO)6 into Cp2Nb2(CO)7 þ Cp2Nb2(CO)5 and of Cp2Nb2(CO)4 into Cp2Nb2(CO)5 þ Cp2Nb2(CO)3 are exothermic (Table 19). Note that Cp2Nb2(CO)5 and Cp2Nb2(CO)3 are the two unsaturated cyclopentadienylniobium carbonyls predicted to have formal triple bonds. The thermodynamics of these unsaturated Cp2Nb2(CO)n derivatives suggest that the derivatives with formal NbtNb triple bonds are more viable than those with formal (36) Templeton, J. L.; Dorman, W. C.; Clardy, J. C.; McCarley, R. E. Inorg. Chem. 1978, 17, 1263.

metal-metal single or, particularly, double bonds. Similar disproportionation reactions in binuclear cyclopentadienylmetal carbonyl chemistry of MdM double-bonded derivatives into equimolar mixtures of M-M single-bonded and MtM triple-bonded derivatives have been observed experimentally for chromium37 and are predicted by DFT for vanadium5 and manganese.38 The synthesis of the binuclear cyclopentadienylniobium carbonyls by the photolysis of CpNb(CO)4 is complicated by the formation of the stable trinuclear derivative4 Cp3Nb3(CO)6(η2-μ3-CO) (Figure 1). Our energies for Cp2Nb2(CO)5 and CpNb(CO)3 by BP86/SDD combined with previously reported39 BP86/SDD results for the lowest energy structure of Cp3Nb3(CO)8 predict the reaction Cp2Nb2(CO)5 þCpNb(CO)3 f Cp3Nb3(CO)8 to be exothermic by ∼13 kcal/mol. This suggests the following pathway from CpNb(CO)4 to the known4 trinuclear derivative Cp3Nb3(CO)6(η2-μ3-CO): (1) photolysis of CpNb(CO)4 to give initially a reactive CpNb(CO)3 fragment; (2) further generation of Cp2Nb2(CO)5 by the photolysis of CpNb(CO)4 analogous to the known photolysis of CpV(CO)4 to give Cp2V2(CO)5; (3) the exothermic reaction by ∼13 kcal/mol of CpNb(CO)3 with Cp2Nb2(CO)5 to give Cp3Nb3(CO)8; (4) further decarbonylation of Cp3Nb3(CO)8 to give the final product Cp3Nb3(CO)6(η2-μ3CO). This should be a relatively easy process, since the CO dissociation energy from the lowest lying Cp3Nb3(CO)8 structure is predicted by DFT to be only ∼14 kcal/mol.39 The experimentally observed tendency for the thermodynamically favorable formation of the trinuclear Cp3Nb3(CO)6(η2-μ3-CO) from binuclear derivatives suggests that the successful synthesis of the binuclear Cp2Nb2(CO)n requires a method to suppress the formation of the trinuclear derivatives. One possibility might be to introduce substituents in the cyclopentadienyl ring in order to inhibit sterically the combination of a binuclear Cp2Nb2(CO)n fragment with a mononuclear CpNb(CO)m fragment to generate a trinuclear fragment.

Acknowledgment. We are indebted to the Chinese National Natural Science Foundation (20873045), Research Fund for the Doctoral Program of Higher Education (200800071019), Excellent Young Scholars Research Fund of BIT (2008Y0713) in China, as well as the U.S. National Science Foundation (Grants CHE-0749868 and CHE0716718) for support of this research. Supporting Information Available: Tables S1-S12: Theoretical harmonic vibrational frequencies for the 37 structures of Cp2Nb2(CO)n (n = 7 to 1) using the BP86 method; Tables S13-S49: Theoretical Cartesian coordinates for the 37 structures of Cp2Nb2(CO)n (n = 7 to 1) using the MPW1PW91/SDD method; complete Gaussian 03 reference (ref 31). This material is available free of charge via the Internet at http://pubs.acs.org. (37) Fortman, G. C.; Kegl, T.; Li, Q.-S.; Zhang, X.; Schaefer, H. F.; Xie, Y.; King, R. B.; Telser, J.; Hoff, C. D. J. Am. Chem. Soc. 2007, 129, 14388. (38) Zhang, X.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. Organometallics 2008, 27, 61. (39) Peng, B.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. Dalton Trans. 2009, 3748.

Binuclear Cyclopentadienylmetal Carbonyl Derivatives of the

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