Binuclear Cyclopentadienylmolybdenum Carbonyl ... - ACS Publications

2818

Organometallics 2009, 28, 2818–2829

Binuclear Cyclopentadienylmolybdenum Carbonyl Derivatives: Where is the Missing ModMo Double-Bonded Species Cp2Mo2(CO)5? Xiuhui Zhang,†,‡ Qian-shu Li,*,†,§ Maofa Ge,‡ Yaoming Xie,| R. Bruce King,*,§,| and Henry F. Schaefer| 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, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China, and Department of Chemistry and Center for Computational Chemistry, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed December 10, 2008

The cyclopentadienylmolybdenum carbonyls Cp2Mo2(CO)n (Cp ) η5-C5H5; n ) 6-1) have been studied by density functional theory. The two lowest energy structures predicted for Cp2Mo2(CO)6 lie within 4 kcal/mol of each other. Both have Mo-Mo single bonds of lengths 3.2-3.3 Å with all terminal carbonyl groups and correspond to stable compounds structurally characterized by X-ray diffraction. Similarly, the lowest energy structure predicted for Cp2Mo2(CO)4 has a formal MotMo triple bond of length ∼2.5 Å with four weakly semibridging carbonyl groups also corresponding to a stable compound structurally characterized by X-ray diffraction. The pentacarbonyl Cp2Mo2(CO)5, which is not known experimentally as a stable compound but only as a transient intermediate, is shown to have a structure with one symmetrical bridging two-electron donor and four terminal carbonyl groups as well as a formal ModMo double bond. Furthermore, Cp2Mo2(CO)5 is predicted to be thermodynamically unstable with respect to disproportionation into Cp2Mo2(CO)6 + Cp2Mo2(CO)4. The lowest energy structure for Cp2Mo2(CO)3 is a triplet with a formal MotMo triple bond. A higher energy singlet structure with one four-electrondonor bridging carbonyl group is also found for Cp2Mo2(CO)3. The MotMo bond distances in the lowest energy more highly unsaturated Cp2Mo2(CO)2 and Cp2Mo2(CO) structures suggest formal bond orders no higher than 3 in the lowest energy structures and thus metal atoms with less than the favored 18electron configurations. 1. Introduction Cyclopentadienylmolybdenum carbonyl derivatives are of historical interest in connection with the chemistry of stable metal carbonyl derivatives with metal-metal multiple bonding. Thus, in 1962 while at the Mellon Institute (now CarnegieMellon University) one of the authors of this paper (R.B.K.) prepared a supply of pentamethylcyclopentadiene by the multistep Devries procedure1 in order to compare its organometallic chemistry with the then already well-known organometallic chemistry of unsubstituted cyclopentadiene. In the initial work2 the thermal reactions of pentamethylcyclopentadiene with Fe(CO)5 and (CH3CN)3W(CO)3 proceeded normally to give (η5Me5C5)2Fe2(CO)2(µ-CO)2 and (η5-Me5C5)W(CO)3H, respectively, which were completely analogous to unsubstituted cyclopentadienyl derivatives that were already known in 1962. The anomalous reaction in this original series of experiments2 was the thermal reaction of pentamethylcyclopentadiene with Mo(CO)6, which might have been expected to give the product (η5-Me5C5)2Mo2(CO)6, analogous to an already known unsubstituted cyclopentadienyl derivative with a Mo-Mo single bond. †

Beijing Institute of Technology. Chinese Academy of Sciences. § South China Normal University. | University of Georgia. (1) DeVries, L. J. Org. Chem. 1960, 25, 1838. (2) King, R. B.; Bisnette, M. B. J. Organomet. Chem. 1967, 8, 129. ‡

However, direct oxygen analyses on the red product suggested formulation not as a hexacarbonyl species but instead as the tetracarbonyl (η5-Me5C5)2Mo2(CO)4. This stoichiometry, combined with the already known strong tendency of transition metals to form compounds preferentially with 18-electron configurations, indirectly suggested a MotMo triple bond in this complex. At that time structure determinations by X-ray diffraction were much more difficult than they are now, and so it was not immediately possible to confirm this suggestion by such a structure determination. This was potentially a very delicate matter, since the unsubstituted (η5-C5H5)2Mo2(CO)6 obtained from the thermal reaction of Mo(CO)6 with cyclopentadiene3 had originally been misidentified as (η5-C5H5)2Mo2(CO)5. Therefore, there was some reluctance to trust the identification of (η5-Me5C5)2Mo2(CO)4 as a tetracarbonyl rather than a hexacarbonyl or possibly even a pentacarbonyl solely on the basis of direct oxygen analysis. For this reason submission of the paper reporting the reaction between Mo(CO)6 and pentamethylcyclopentadiene to give (η5-Me5C5)2Mo2(CO)4 with a presumed MotMo triple bond was delayed until 1966, when the senior author of that paper (R.B.K.)2 moved from the Mellon Institute to the University of Georgia. By then, the postulation of a MotMo triple bond in (η5-Me5C5)2Mo2(CO)4 was made much more plausible by the discovery of the rhenium-rhenium (3) Wilkinson, G. J. Am. Chem. Soc. 1954, 76, 209.

10.1021/om801170e CCC: $40.75  2009 American Chemical Society Publication on Web 04/09/2009

Cyclopentadienylmolybdenum Carbonyl DeriVatiVes

quadruple bond in the binuclear metal halide complex Re2Cl82by Cotton and Harris4 in 1965. Formulation of the metal-metal bond in (η5-Me5C5)2Mo2(CO)4 as the MotMo triple bond suggested by the 18-electron rule was later supported by an X-ray structural determination indicating an unusually short metal-metal distance consistent with a triple bond. Thus, the metal-metal distance in (η5-Me5C5)2Mo2(CO)4 was found to be 2.488 Å as compared with 3.235 Å for (η5-C5H5)2Mo2(CO)6, which clearly has a metal-metal single bond.5 Subsequently, the analogous chromium compounds (η5-R5C5)2Cr2(CO)4 (R ) H,6 Me7,8) were also prepared and likewise found to have relatively short CrtCr distances around 2.24 Å, suggesting triple bonds. In addition, unsaturated binuclear cyclopentadienylmetal carbonyl derivatives of other metals containing formal metal-metal triple bonds such as (η5-C5R5)2V2(CO)59,10 and (η5-C5R5)2M′2(CO)3 (M′ ) Mn,11 Re12) were synthesized and structurally characterized by X-ray diffraction. In addition, (η5-C5H5)2Co2(CO)213 and (η5C5H5)2Fe2(CO)314 with formal metal-metal double bonds have been synthesized and characterized structurally. The current status of binuclear cyclopentadienylmolybdenum (CpMo) chemistry is that Cp2Mo2(CO)6 derivatives with formal Mo-Mo single bonds and Cp2Mo2(CO)4 derivatives with formal MotMo triple bonds are known compounds, which are sufficiently stable for characterization by X-ray crystallography. However, Cp2Mo2(CO)5, expected to have a ModMo double bond on the basis of the 18-electron rule, remains unknown as a stable compound. However, there is spectroscopic evidence of its generation as an unstable intermediate from the photolysis of Cp2Mo2(CO)6 based on infrared spectra in the ν(CO) region.15-18 The more highly unsaturated derivatives Cp2Mo2(CO)n (n ) 3-1) remain unknown. The 18-electron rule suggests the possibility of a molybdenum-molybdenum quadruple bond in Cp2Mo2(CO)3, similar to the known19 rheniumrhenium quadruple bond in Re2Cl82-, as well as a molybdenum-molybdenum quintuple bond in Cp2Mo2(CO)2, similar to the known19 chromium-chromium quintuple bond in a few chromium(I) aryls RCrCrR. This paper reports a density functional theory (DFT) study on the complete series of binuclear cyclopentadienylmolybdenum carbonyls Cp2Mo2(CO)n (n ) 6-1) with the following specific objectives: (4) Cotton, F. A.; Harris, C. B. Inorg. Chem. 1965, 4, 330–334. (5) Huang, J. S.; Dahl, L. F. J. Organomet. Chem. 1983, 243, 57. (6) Curtis, M. D.; Butler, W. M. J. Organomet. Chem. 1978, 155, 131. (7) King, R. B.; Efraty, A.; Douglas, W. M. J. Organomet. Chem. 1973, 60, 125. (8) Potenza, J.; Giordano, P.; Mastropaolo, D.; Efraty, A. Inorg. Chem. 1974, 13, 2540. (9) Cotton, F. A.; Kruczynski, L.; Frenz, B. A. J. Organomet. Chem. 1978, 160, 93. (10) Huffman, J. C.; Lewis, L. N.; Caulton, K. G. Inorg. Chem. 1980, 19, 2755. (11) Herrmann, W. A.; Serrano, R.; Weichmann, J. J. Organomet. Chem. 1983, 246, C57. (12) Hoyano, J. K.; Graham, W. A. G. J. Chem. Soc., Chem. Commun. 1982, 27. (13) Bailey, W. I.; Collins, D. M.; Cotton, F. A.; Baldwin, J. C. J. Organomet. Chem. 1979, 165, 373. (14) Blaha, J. P.; Bursten, B. E.; Dewan, J. C.; Frankel, R. B.; Randolph, C. L.; Wilson, B. A.; Wrighton, M. S. J. Am. Chem. Soc. 1985, 107, 4561. (15) Hooker, R. H.; Mahmoud, K. A.; Rest, A. J. J. Organomet. Chem. 1983, 254, C25. (16) Hooker, R. H.; Rest, A. J. J. Chem. Soc,. Dalton Trans. 1990, 1221. (17) Baker, M. L.; Bloyce, P. E.; Campen, A. K.; Rest, A. J.; Bitterwolf, T. E. J. Chem. Soc., Dalton Trans. 1990, 2825. (18) Peters, J.; George, M. W.; Turner, J. J. Organometallics 1995, 14, 1503. (19) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J.; Power, P. P. Science 2005, 310, 844.

Organometallics, Vol. 28, No. 9, 2009 2819

(1) What is the structure of Cp2Mo2(CO)5, which is currently unknown as a stable molecule but which has been claimed as a product from the photolysis of Cp2Mo2(CO)6? (2) Do the highly unsaturated derivatives Cp2Mo2(CO)3 and Cp2Mo2(CO)2 have low-energy structures with the very short molybdenum-molybdenum distances suggestive of the formal quadruple and quintuple bonds, respectively, predicted by the 18-electron rule?

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.20-28 Thus, two DFT methods were used in this study. The first method is the BP86 method, which uses Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradientcorrected correlation functional (P86).29,30 The other DFT method used in the present paper is MPW1PW91, which is a combination of the modified Perdew-Wang exchange functional with the Perdew-Wang 91 correlation functional.31 The results predicted by the two functionals are generally in agreement with each other. However, the MPW1PW91 functional has been found to be more suitable than the first-generation functionals for second- and thirdrow transition-metal systems, while the BP86 method usually provides better vibrational frequencies.32,33 For the second-row transition metals, the large numbers of electrons increase the computational efforts exponentially. In order to reduce the cost, relativistic effective core potential (ECP) basis sets were used. In this study the SDD (Stuttgart-Dresden ECP plus DZ)34,35 was used for the molybdenum atoms. For the C and O atoms, the all-electron 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).36,37 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.38 The fine grid (75, 302) was the default for evaluating integrals numerically, and the tight (20) Ehlers, A. W.; Frenking, G. J. Am. Chem. Soc. 1994, 116, 1514. (21) Delley, B.; Wrinn, M.; Lu¨thi, H. P. J. Chem. Phys. 1994, 100, 5785. (22) Li, J.; Schreckenbach, G.; Ziegler, T. J. Am. Chem. Soc. 1995, 117, 486. (23) Jonas, V.; Thiel, W. J. Chem. Phys. 1995, 102, 8474. (24) Barckholtz, T. A.; Bursten, B. E. J. Am. Chem. Soc. 1998, 120, 1926. (25) Niu, S.; Hall, M. B. Chem. ReV. 2000, 100, 353. (26) Macchi, P.; Sironi, A. Coord. Chem. ReV. 2003, 238, 383. (27) Carreon, J.-L.; Harvey, J. N. Phys. Chem. Chem. Phys. 2006, 8, 93. (28) Bu¨hl, M.; Kabrede, H. J. Chem. Theory Comput. 2006, 2, 1282. (29) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (30) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. (31) Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664. (32) Feng, X.; Gu, J.; Xie, Y.; King, R. B.; Schaefer, H. F. J. Chem. Theor. Comput. 2007, 3, 1580. (33) Zhao, S.; Wang, W.; Li, Z.; Liu, Z. P.; Fan, K.; Xie, Y.; Schaefer, H. F. J. Chem. Phys. 2006, 124, 184102. (34) Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1993, 85, 441. (35) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431. (36) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823. (37) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293. (38) Frisch, M. J. et al., Gaussian 03, Revision C 02; Gaussian, Inc., Wallingford, CT, 2004 (see the Supporting Information for details).

2820 Organometallics, Vol. 28, No. 9, 2009

Zhang et al.

Figure 1. Optimized geometries of the two Cp2Mo2(CO)6 structures. The distances in Figure 1 are given in Å. For the bond distances listed in Figures 1-7, 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 (∆E, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), and Mo-Mo Bond Distances (Å) for the Two Cp2Mo2(CO)6 Structures 6S-1 (C2h)

6S-2 (C2)

E ∆E Nimag Mo-Mo

MPW1PW91 -1203.643 23 0.0 0 3.241

-1203.637 46 3.6 0 3.238

E ∆E Nimag Mo-Mo

BP86 -1204.084 95 0.0 0 3.292

-1204.078 69 3.9 0 3.292

Table 2. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted by the BP86 Method for the Two Lowest Energy Isomers of Cp2Mo2(CO)6a 6S-1 (C2h) 1881 (0), 1902 (397), 1912 (1831), 1915 (0), 1955 (2266), 1986 (0) exptl 1916, 1960 6S-2 (C2) 1887 (381), 1892 (711), 1925 (1038), 1928(4), 1957 (1348), 2001 (978) a

Table 3. Total Energies (E, in hartree), Relative Energies (∆E, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), and Mo-Mo Bond Distances (Å) for Each of the Singlet Cp2Mo2(CO)5 Structures

Infrared intensities in parentheses are in km/mol.

(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.39 Unless otherwise indicated, all of the predicted triplet structures are found to have negligible spin contamination; i.e., the values of S(S + 1) are very close to the ideal outcome of 2.

3. Results 3.1. Cp2Mo2(CO)6. Two energetically low-lying structures were found for Cp2Mo2(CO)6. The global minimum 6S-1 (Figure 1 and Tables 1 and 2) is a C2h trans structure with four semibridging CO groups and is very close to the (η5C5H5)2Mo2(CO)6 structure reported experimentally.40-42 The Mo-Mo distance in 6S-1 is predicted to be 3.241 Å (MPW1PW91) or 3.292 Å (BP86), which is close to the experimentally determined value3 of 3.235 Å by X-ray diffraction and consistent with the formal Mo-Mo single bond required to give each molybdenum atom the favored 18-electron configuration. For the semibridging carbonyls in 6S-1, the Mo-C distance is 1.978 Å (MPW1PW91) or 1.985 Å (BP86), in good agreement (39) Papas, B. N.; Schaefer, H. F. J. Mol. Struct. 2006, 768, 275. (40) Wilson, F. C.; Shoemaker, D. P. Naturwissenschaften 1956, 43, 57. (41) Wilson, F. C.; Shoemaker, D. P. J. Chem. Phys. 1957, 27, 809. (42) Adams, R. D.; Collins, D. M.; Cotton, F. A. Inorg. Chem. 1974, 13, 1086.

5S-1 (C1)

5S-2 (C2)

5S-3 (C2V)

E ∆E Nimag Mo-Mo

MPW1PW91 -1090.281 55 -1090.277 86 0.0 2.3 0 0 2.837 2.913

-1090.227 95 33.6 1 (25i) 2.963

E ∆E Nimag Mo-Mo

-1090.693 31 0.0 0 2.872

BP86 -1090.689 40 2.5 0 2.966

-1090.633 30 37.7 2 (15i, 6i) 2.955

with the experimental values3 of 1.983 and 1.984 Å. For the terminal carbonyls in 6S-1, the Mo-C distance is 1.963 Å (MPW1PW91) or 1.969 Å (BP86), in close accord with the experimental value3 of 1.964 Å. The three infrared-active calculated ν(CO) frequencies of 1902, 1912, and 1955 cm-1 (BP86) for 6S-1 are in very close agreement with the experimental observation3 of 1916 and 1960 cm-1. The next lowest energy Cp2Mo2(CO)6 structure, 6S-2, is a C2 cis structure with four semibridging CO groups (Figure 1 and Tables 1 and 2), lying 3.6 kcal/mol (MPW1PW91) or 3.9 kcal/mol (BP86) in energy above the global minimum 6S-1 with all real harmonic vibrational frequencies. The structure of 6S-2 is very close to the gauche conformer of Cp2Mo2(CO)6 reported experimentally.43 The predicted Mo-Mo distance of 3.238 Å (MPW1PW91) or 3.292 Å (BP86) in 6S-2 is in agreement with the value of 3.2239 Å determined by X-ray diffraction. This is also very close to the 3.235 Å Mo-Mo distance of 6S-1 and consistent with the formal Mo-Mo single bond required to give each molybdenum atom the favored 18-electron configuration. The predicted Mo-C distances for 6S-2 of 1.976 Å (MPW1PW91) or 1.984 Å (BP86), 1.961 Å (MPW1PW91) or 1.967 Å (BP86), and 1.990 Å (MPW1PW91) or 1.996 Å (BP86) are close to the experimentally reported Mo-C distances for the gauche conformer of Cp2Mo2(CO)6 of 1.943, 1.955, and 1.971 Å. 3.2. Cp2Mo2(CO)5. Three singlet structures and three triplet structures (Figure 2 and Tables 3-5) were found for Cp2Mo2(CO)5. The global minimum 5S-1 is predicted to be a singlet C1 structure with one bridging, one semibridging, and three terminal CO groups. This structure is predicted to be a (43) Gould, R. O. Acta Crystallogr. 1988, C44, 461.



Table 4. Total Energies (E, in hartree), Energies Relative to 5S-1 (∆E in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), and Mo-Mo Bond Distances (Å) for Each of the Triplet Cp2Mo2(CO)5 Structures 5T-1 (Cs)

5T-2 (Cs)

5T-3 (C2V)

E ∆E Nimag Mo-Mo

MPW1PW91 -1090.276 26 -1090.271 88 3.3 6.1 0 0 2.887 2.907

-1090.243 25 24.0 0 3.340

E ∆E Nimag Mo-Mo

-1090.682 84 6.6 1 (18i) 2.889

BP86 -1090.680 51 8.0 1 (16i) 2.928

-1090.639 59 33.7 0 3.347

Table 5. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted by the BP86 Method for the Six Lowest Energy Structures of Cp2Mo2(CO)5a 5S-1 (C1) exptl 5S-2 (C2) 5S-3 (Cs) 5T-1 (Cs) 5T-2 (Cs) 5T-3 (C2V) a

1721 (371), 1857 (455), 1909 (1006), 1932 (964), 1982 (834) 1665, 1872, 1944, 1982 1689 (580), 1895 (158), 1910 (907), 1915 (863), 1977 (909) 1902 (1583), 1916 (1517), 1923 (194), 1934 (354), 2001 (1129) 1827 (578), 1842 (50), 1901 (1041), 1923 (2130), 1955 (11) 1791 (702), 1804 (285), 1913 (518), 1914 (864), 1971 (1200) 1909 (1609), 1916 (1658), 1919 (563), 1937 (86), 2014 (749)


genuine minimum with all real vibrational frequencies. For the bridging carbonyl, the Mo-C distances are 2.102 and 2.159 Å (MPW1PW91) or 2.117 and 2.171 Å (BP86). For the semibridging carbonyl, the longer Mo-C distance is 2.833 Å (MPW1PW91) or 2.836 Å (BP86) and the shorter one is 1.946 Å (MPW1PW91) or 1.953 Å (BP86). The calculated ν(CO) frequencies of 5S-1 at 1721 and 1857 cm-1 (BP86, Table 5) correspond to the bridging and semibridging CO groups, respectively. For the terminal CO groups in 5S-1, the Mo-C distances are in the range 1.945-1.980 Å (MPW1PW91) or 1.951-1.983 Å (BP86). The predicted ModMo distance, namely 2.837 Å (MPW1PW91) or 2.872 Å (BP86), is 0.4 Å shorter than the Mo-Mo single bond of 6S-1, consistent with the ModMo double bond required to give each molybdenum atom the favored 18-electron configuration. The other energetically low-lying singlet structure of Cp2Mo2(CO)5, namely 5S-2 (Figure 2 and Table 3), has C2 symmetry and lies above 5S-1 by only 2.3 kcal/mol (MPW1PW91) or 2.5 kcal/mol (BP86) in energy with all real harmonic vibrational frequencies. Structure 5S-2 has one bridging carbonyl and four terminal carbonyls. The Mo-C distance of the bridging carbonyl is 2.098 Å (MPW1PW91) or 2.115 Å (BP86), The predicted ν(CO) frequency of 1689 cm-1 (BP86, Table 5) corresponds to this symmetrical bridging carbonyl group. The Mo-C distances for the terminal CO groups fall in the range 1.968-1.990 Å (MPW1PW91) or 1.976-1.987 Å (BP86). The ModMo double-bond distance in 5S-2 is 2.913 Å (MPW1PW91) or 2.966 Å (BP86), which is longer than that in 5S-1 by ∼0.1 Å but still significantly shorter than the Mo-Mo single-bond distances of ∼3.2 Å in Cp2Mo2(CO)6 (Figure 1 and Table 1). The third singlet structure of Cp2Mo2(CO)5, namely 5S-3 (Figure 2 and Table 3), is of a very different type than 5S-1 or 5S-2, since it has the two Cp rings bonded to one molybdenum atom and only carbonyl groups bonded to the other molybdenum atom. Structure 5S-3 is a relatively high energy structure, lying 33.6 kcal/mol (MPW1PW91) or 37.7 kcal/mol (BP86) above 5S-1. Structure 5S-3 has a very small imaginary vibrational frequency (25i cm-1) by MPW1PW91 and two very small imaginary vibrational frequencies (15i, 6i cm-1) by BP86. The

Mo-Mo distance in 5S-3 is predicted to be 2.963 Å (MPW1PW91) or 2.955 Å (BP86), which is 0.1 Å longer than that in 5S-1. The lowest lying triplet structure of Cp2Mo2(CO)5, namely 5T-1 (Figure 2 and Table 4), is a Cs trans structure with two semibridging carbonyls and three terminal carbonyls. It has all real harmonic vibrational frequencies by the MPW1PW91 method but a very small imaginary vibrational frequency (18i cm-1) by BP86. Structure 5T-1 lies 3.3 kcal/mol (MPW1PW91) or 6.6 kcal/mol (BP86) above the Cp2Mo2(CO)5 global minimum 5S-1. For the semibridging carbonyls in 5T-1, the longer Mo-C distance is 2.620 Å (MPW1PW91) or 2.556 Å (BP86) and the shorter Mo-C distance is 1.983 Å (MPW1PW91) or 1.998 Å (BP86). The predicted ν(CO) frequencies of 1827 and 1842 cm-1 (BP86, Table 5) correspond to the semibridging carbonyl groups. For the terminal carbonyl groups, the Mo-C distances are in the range of 1.969-1.979 Å (MPW1PW91) or 1.976-1.977 Å (BP86). The ModMo distance in 5T-1 is 2.887 Å (MPW1PW91) or 2.889 Å (BP86). The other energetically low lying triplet structure of Cp2Mo2(CO)5, namely 5T-2 (Figure 2), lies 6.1 kcal/mol (MPW1PW91) or 8.0 kcal/mol (BP86) above 5S-1. Structure 5T-2 is a Cs cis structure with two semibridging carbonyls and three terminal carbonyls. It was predicted to be a genuine minimum with all real harmonic vibrational frequencies by MPW1PW91, while it has a very small imaginary vibrational frequency (16i cm-1) by BP86. This imaginary frequency becomes real when a finer (120, 974) integration grid is used, indicating that this small imaginary frequency arises from numerical integration error. The Mo-C distances to the semibridging carbonyls in 5T-2 are 1.989 Å (MPW1PW91) or 2.000 Å (BP86) for the shorter distances and 2.498 Å (MPW1PW91) or 2.475 Å (BP86) for the longer distances. The ν(CO) frequencies of 1791 and 1804 cm-1 (BP86, Table 5) correspond to these two semibridging CO groups. The Mo-C distances for the terminal CO groups are in the range of 1.980-1.981 Å (MPW1PW91) or 1.981-1.987 Å (BP86). The ModMo distance in 5T-2 is 2.907 Å (MPW1PW91) or 2.928 Å (BP86). Another triplet structure of Cp2Mo2(CO)5, namely 5T-3 (Figure 2 and Table 4), has the two Cp rings bonded to one molybdenum atom and only carbonyl groups bonded to the second molybdenum atom, as found for the singlet structure 5S-3. Structure 5T-3 is of relatively high energy, lying 24.0 kcal/mol (MPW1PW91) or 33.7 kcal/mol (BP86) above the global minimum 5S-1. Structure 5T-3 is a genuine minimum with all real vibrational frequencies. The distance between the two molybdenum atoms in 5T-3 is very long, namely 3.340 Å (MPW1PW91) or 3.347 Å (BP86), which is close to the Mo-Mo single-bond distance3 of 3.235 Å in Cp2Mo2(CO)6 determined experimentally by X-ray diffraction and consistent with the formal Mo-Mo single bond required to give each molybdenum atom the favored 17-electron configuration in the binuclear triplet structure. Although Cp2Mo2(CO)5 has not been isolated as a stable compound, the photolysis of Cp2Mo2(CO)6 was claimed by Turner and co-workers18 to give Cp2Mo2(CO)5 as an unstable intermediate at room temperature as well as a stable product in low-temperature matrices.15-17 The ν(CO) frequencies of this product are reported18 to be 1665, 1872, 1944, and 1982 cm-1. These correspond rather closely to the theoretical ν(CO) frequencies of 1689, 1895, 1910, 1915, and 1977 cm-1 for the Cp2Mo2(CO)5 structure 5S-2, assuming that the closely spaced theoretical 1915 and 1910 cm-1 bands are not resolved in the experiments (Table 5). Our theoretical studies therefore suggest


Zhang et al.

Figure 2. Optimized geometries of the six Cp2Mo2(CO)5 structures.

Figure 3. The three optimized structures of Cp2Mo2(CO)4 within 30 kcal/mol of the global minimum 4S-1.

that a normal two-electron-donor bridging carbonyl group is sufficient to account for the ν(CO) frequency below 1700 cm-1 in Cp2Mo2(CO)5 rather than the four-electron-donor carbonyl group suggested in the experimental work.18 Thus, the Cp2Mo2(CO)5 generated in the photolysis appears to have a twoelectron bridging carbonyl group and a formal ModMo double bond rather than a four-electron bridging carbonyl group and a formal Mo-Mo single bond. 3.3. Cp2Mo2(CO)4. One singlet and two triplet structures (Figure 3 and Tables 6 and 7) were found for Cp2Mo2(CO)4 within 30 kcal/mol of the global minimum 4S-1; higher energy structures of Cp2Mo2(CO)4 are not discussed in this paper. The global minimum 4S-1 is a C2 trans structure with four semibridging CO groups, which is very close to the (η5C5H5)2Mo2(CO)4 structure reported experimentally.6 The MotMo distance in 4S-1 is predicted to be 2.508 Å (MPW1PW91) or 2.529 Å (BP86), consistent with the experimentally determined

Table 6. Total Energies (E, in hartree), Relative Energies (∆E, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), and Mo-Mo Bond Distances (Å) for Each of the Cp2Mo2(CO)4 Structures within 30 kcal/mol of the Global Minimum 4S-1 4S-1 (C2)

4T-1 (Cs)

4T-2 (Ci)

E ∆E Nimag Mo-Mo

MPW1PW91 -976.961 46 -976.922 41 0.0 24.5 0 0 2.508 2.747

-976.917 80 27.4 0 2.697

E ∆E Nimag Mo-Mo

-977.335 34 0.0 0 2.529

BP86 -977.290 65 28.0 0 2.734

-977.293 46 26.3 0 2.707

X-ray diffraction value6 of 2.448 Å. This MotMo distance in 4S-1 is 0.3 Å shorter than that of the formal ModMo double bond in 5S-1 and thus may be considered to be the MotMo

Cyclopentadienylmolybdenum Carbonyl DeriVatiVes Table 7. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted by the BP86 Method for the Six Lowest Energy Isomers of Cp2Mo2(CO)4a 4S-1 (C2) 4T-1 (Cs) 4T-2 (Ci) a

1858 (208), 1870 (1266), 1900 (1143), 1938 (290) 1810 (999), 1825 (91), 1874 (1221), 1940 (803) 1795 (1003), 1796 (0), 1888 (2168), 1913 (0)


triple bond required to give each molybdenum atom the favored 18-electron configuration. For the four semibridging carbonyls in 4S-1, the longer Mo-C distances are 2.660 Å (MPW1PW91) or 2.669 Å (BP86) and 2.866 Å (MPW1PW91) or 2.889 Å (BP86), as compared with the experimental values6,44 of 2.554 ( 0.050 Å. The shorter Mo-C distances for the two semibridging carbonyls in 4S-1 are 1.950 Å (MPW1PW91) or 1.961 Å (BP86) and 1.958 Å (MPW1PW91) or 1.968 Å (BP86), as compared with the experimental values6 of 2.130 ( 0.050 Å. The predicted ν(CO) frequencies of 1858, 1870, 1900, and 1938 cm-1 (BP86, Table 7) correspond to these four semibridging CO groups. The triplet stationary point 4T-1 of Cp2Mo2(CO)4 (Figure 3 and Tables 6 and 7), exhibits a Cs trans structure with two semibridging carbonyl groups and two terminal carbonyl groups. It is a genuine minimum with all real vibrational frequencies by MPW1PW91 and BP86. Structure 4T-1 is predicted to lie above 4S-1 by 24.5 kcal/mol (MPW1PW91) or 28.0 kcal/mol (BP86). The Mo-C distances to the two semibridging carbonyls in 4T-1 are 2.486 Å (MPW1PW91) or 2.419 Å (BP86) and 1.983 Å (MPW1PW91) or 2.002 Å (BP86). The Mo-C distances to the two terminal carbonyls are 1.968 Å (MPW1PW91) or 1.975 Å (BP86) and 1.961 Å (MPW1PW91) or 1.962 Å (BP86). The two semibridging carbonyl groups in 4T-1 exhibit ν(CO) frequencies at 1810 and 1825 cm-1, and the two terminal CO groups exhibit ν(CO) frequencies at 1874 and 1940 cm-1 (BP86, Table 7). The Mo-Mo distance in 4T-1 is 2.747 Å (MPW1PW91) or 2.734 Å (BP86), which is 0.2 Å longer than the MotMo triple bond in 4S-1, in accord with the ModMo double bond required to give each molybdenum atom a 17electron configuration consistent with the binuclear triplet structure. The second triplet stationary point of Cp2Mo2(CO)4, namely 4T-2, is a Ci structure (with two semibridging carbonyls and two terminal carbonyls) and lies 27.4 kcal/mol (MPW1PW91) or 26.3 kcal/mol (BP86) above 4S-1 (Figure 3 and Tables 6 and 7). The Mo-C distances to the semibridging carbonyl groups in 4T-2 are 2.439 Å (MPW1PW91) or 2.402 Å (BP86) and 1.947 Å (MPW1PW91) or 1.962 Å (BP86). The ν(CO) frequencies at 1795 and 1796 cm-1 (BP86, Table 7) can be assigned to the two semibridging CO groups. The equivalent Mo-C distances to the two terminal carbonyls are 1.976 Å (MPW1PW91) or 1.978 Å (BP86). The ModMo distance in 4T-2 is 2.697 Å (MPW1PW91) or 2.707 Å (BP86), which can be interpreted as the formal double bond required to give each molybdenum atom a 17-electron configuration in accord with the binuclear triplet structure. 3.4. Cp2Mo2(CO)3. A total of five energetically low-lying Cp2Mo2(CO)3 structures were found (Figure 4 and Tables 8-10), including three triplet and two singlet structures. The global minimum 3T-1 of Cp2Mo2(CO)3 is predicted to be a triplet C1 structure with three semibridging CO groups and all real vibrational frequencies. For one of the semibridging carbonyl groups in 3T-1 the short and long M-C distances (44) Klingler, R. J.; Butler, W.; Curtis, M. D. J. Am. Chem. Soc. 1975, 97, 3535.


differ by only 0.12 Å at 2.06 Å for the short distance and 2.18 Å for the long distance by either method. The lowest predicted ν(CO) frequency of 1753 cm-1 (BP86) corresponds to this semibridging carbonyl group. The other two semibridging carbonyl groups in 3T-1 have M-C distances in the range 1.97 ( 0.01 Å for the short distance and 2.52 ( 0.03 Å for the long distance. The predicted ν(CO) frequencies at 1809 and 1849 cm-1 correspond to these semibridging carbonyl groups. The predictedMotModistancein3T-1,namely2.551Å(MPW1PW91) or 2.559 Å (BP86), is close to the X-ray diffraction experimental value6 of 2.448 Å for the formal triple bond in Cp2Mo2(CO)4 (4S-1 in Figure 3), suggesting also a formal triple bond in 3T1, thereby giving each molybdenum atom the 17-electron configuration consistent with a binuclear triplet structure. The second triplet Cp2Mo2(CO)3 structure, namely 3T-2, is a C2 structure with two semibridging CO groups and one symmetrical bridging CO group. Structure 3T-2 lies 5.8 kcal/ mol (MPW1PW91) or 2.0 kcal/mol (BP86) above the global minimum 3T-1 and has a small imaginary vibrational frequency at 62i (MPW1PW91) or 18i (BP86). For the symmetrical bridging carbonyl the Mo-C distance is 2.118 Å (MPW1PW91) or 2.123 Å (BP86) and the predicted ν(CO) frequency is 1748 cm-1 (BP86, Table 10). The Mo-C distances for the semibridging carbonyls are 1.976 and 2.464 Å (MPW1PW91) or 1.990 and 2.393 Å (BP86). The predicted ν(CO) frequencies of 1790 and 1812 cm-1 (BP86, Table 10) correspond to these two semibridging CO groups. The MotMo distance in 3T-2 is 2.521 Å (MPW1PW91) or 2.530 Å (BP86), corresponding to 17electron configurations for both molybdenum atoms, consistent with a binuclear triplet. The third triplet stationary point 3T-3 is a Cs structure with two Cp rings bonded to one molybdenum atom and only carbonyl groups bonded to the second molybdenum atom. Structure 3T-3 lies 21.1 kcal/mol (MPW1PW91) or 20.5 kcal/ mol (BP86) above 3T-1 and has all real vibrational frequencies by BP86 but two imaginary vibrational frequencies (82i and 25i cm-1) by MPW1PW91. The Mo-C distances for the three carbonyls are 1.911 Å (MPW1PW91) or 1.946 Å (BP86) and 1.932 Å (MPW1PW91) or 1.952 Å (BP86). The distance between the two molybdenum atoms in 3T-3 is predicted to be 2.810 Å (MPW1PW91) or 2.712 Å (BP86). The singlet structure 3S-1 (Figure 4 and Tables 9 and 10) has all real vibrational frequencies by MPW1PW91 and lies 7.4 kcal/mol above the global minimum 3T-1. The MPW1PW91 method predicts 3S-1 to have three semibridging CO groups. Optimization of 3S-1 by the BP86 method leads to 3S-2, discussed below. The second singlet Cp2Mo2(CO)3 structure, namely 3S-2 (Figure 4 and Table 9), is a Cs structure with three semibridging CO groups and lies 9.2 kcal/mol (MPW1PW91) or 1.0 kcal/ mol (BP86) above the global minimum 3T-1. Structure 3S-2 has a very small imaginary vibrational frequency, 14i cm-1, by BP86 and two imaginary vibrational frequencies, namely 70i and 2i cm-1, by MPW1PW91. The Mo-C distances for one semibridging carbonyl are 2.267 Å (MPW1PW91) or 2.262 Å (BP86) and 1.940 Å (MPW1PW91) or 1.949 Å (BP86). The Mo-O distance is very short, namely 2.600 Å (MPW1PW91) or 2.594 Å (BP86), indicating a four-electron-donor CO group. The low vibrational frequency of 1710 cm-1 (BP86) predicted for 3S-2 can be assigned to this η2-µ-CO group. The remaining two carbonyl groups in 3S-2 are slightly unsymmetrical bridging carbonyl groups with a longer Mo-C distance of 2.126 Å (MPW1PW91) or 2.138 Å (BP86) and a shorter Mo-C distance of 2.095 Å (MPW1PW91) or 2.110 Å (BP86). The calculated


Zhang et al.

Figure 4. The three triplet and two singlet optimized Cp2Mo2(CO)3 structures within 30 kcal/mol of the global minimum 3T-1. Table 8. Total Energies (E, in hartree), Relative Energies (∆E, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), and Mo-Mo Bond Distances (Å) for Each of the Triplet Cp2Mo2(CO)3 Structures 3T-1 (C1)

3T-2 (C2)

MPW1PW91 -863.583 24 -863.573 98 0.0 5.8 0 1 (62i) 2.551 2.521

-863.549 66 21.1 2 (82i, 25i) 2.810

E ∆E Nimag Mo-Mo

BP86 -863.911 55 2.0 1 (18i) 2.530

-863.882 06 20.5 0 2.712

Table 9. Total Energies (E, in hartree), Energies (∆E, in kcal/mol) Relative to 3T-1, Numbers of Imaginary Vibrational Frequencies (Nimag), and Mo-Mo Bond Distances (Å) for Each of the Singlet Cp2Mo2(CO)3 Structures

E ∆E Nimag Mo-Mo E ∆E Nimag Mo-Mo

3S-1 (C1)

3S-2 (Cs)

MPW1PW91 -863.571 50 7.4 0 2.497

-863.568 52 9.2 2 (70i, 2i) 2.533

BP86 (converges to 3S-2)

3T-1 3T-2 3T-3 3S-2

3T-3 (Cs)

E ∆E Nimag Mo-Mo

-863.914 66 0.0 0 2.559

Table 10. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted by the BP86 Method for the Lowest Energy Isomers of Cp2Mo2(CO)3a

-863.912 99 1.0 1 (14i) 2.561

ν(CO) frequencies at 1744 and 1761 cm-1 correspond to the these two bridging carbonyl groups. The MotMo distance in 3S-2 is predicted to be 2.533 Å (MPW1PW91) or 2.561 Å

a

1753 1748 1865 1710

(790), (899), (946), (613),

1809 1790 1866 1744

(913), 1849 (311) (987), 1812 (109) (1097), 1941 (1196) (1088), 1761 (40)


(BP86) corresponding to the formal triple bond, which, with the single four-electron-donor bridging carbonyl group, is required to give both metal atoms the favored 18-electron configuration. 3.5. Cp2Mo2(CO)2. Four triplet structures and two singlet structures (Figure 5 and Tables 11-13) were found for Cp2Mo2(CO)2. The global minimum 2T-1 is predicted to be a triplet Cs structure with two semibridging CO groups. Structure 2T-1 has a very small imaginary vibrational frequency, namely 20i cm-1 by MPW1PW91 and 19i cm-1 by BP86, which becomes real when the finer integration grid (120, 974) is used, indicating that this small imaginary frequency arises from numerical integration errors. The pair of semibridging carbonyl groups in 2T-1, exhibiting ν(CO) frequencies of 1793 and 1834 cm-1, has a short Mo-C distance of 1.941 Å (MPW1PW91) or 1.962 Å (BP86) to the same molybdenum atom, namely the “top” molybdenum atom, and a long Mo-C distance to the other (namely the “bottom”) molybdenum atom in Figure 5. Our theoretical results show that the two unpaired electrons are essentially on the “bottom” molybdenum atom with a spin density of 2.13 (MPW1PW91) or 1.92 (BP86). The predicted distance between the two molybdenum atoms, namely 2.465 Å (MPW1PW91) or 2.462 Å (BP86), is 0.1 Å shorter than that of



Figure 5. The six optimized structures of Cp2Mo2(CO)2. Table 11. Total Energies (E, in hartree), Relative Energies (∆E, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), and Mo-Mo Bond Distances (Å) for Each of the Triplet Cp2Mo2(CO)2 Structures 2T-1 (Cs) E ∆E Nimag Mo-Mo E ∆E Nimag Mo-Mo

2T-2 (Ci)

2T-3 (Cs)

2T-1 (C1) 2T-2 (Ci) 2T-3 (Cs) 2T-4 (Cs) 2S-1 (C2) 2S-2 (Cs)

2T-4 (Cs)

-750.185 20 0.0 1 (20i) 2.465

MPW1PW91 -750.181 53 -750.170 67 2.3 9.1 1 (24i) 0 2.554 2.530

-750.159 01 16.4 1 (12i) 2.644

-750.481 29 0.0 1 (19i) 2.462

BP86 -750.482 47 -0.7 0 2.541

-750.452 82 17.9 1 (40i) 2.644

-750.458 89 14.1 0 2.496

Table 12. Total Energies (E, in hartree), Energies (∆E, in kcal/mol) Relative to 2T-1, Numbers of Imaginary Vibrational Frequencies (Nimag), and Mo-Mo Bond Distances (Å) for Each of the Singlet Cp2Mo2(CO)2 Structures 2S-1 (C2)

Table 13. Infrared-Active ν(CO) Vibrational Frequencies (cm-1) Predicted by the BP86 Method for the Six Lowest Energy Isomers of Cp2Mo2(CO)2a

2S-2 (Cs)

E ∆E Nimag Mo-Mo

MPW1PW91 -750.171 17 8.8 0 2.413

-750.145 72 24.8 0 2.543

E ∆E Nimag Mo-Mo

BP86 -750.477 80 2.2 0 2.410

-750.449 29 20.1 1 (41i) 2.544

3T-1 and can be interpreted as a formal strong triple or weak quadruple bond. The second triplet Cp2Mo2(CO)2 structure, namely 2T-2 (Figure 5 and Table 11), also has two semibridging carbonyl groups but with one short Mo-C distance to each molybdenum atom rather than both short Mo-C distances to the same molybdenum atom as in 2T-1. Structure 2T-2 lies 2.3 kcal/mol

a

1793 1758 1813 1822 1775 1860

(906), 1834 (586) (1235), 1766 (0) (925), 1851 (455) (1324), 1876 (1043) (1156), 1781 (247) (841), 1899 (1495)


above the global minimum 2T-1 by MPW1PW91 but 0.7 kcal/ mol lower than 2T-1 by BP86. Structure 2T-2 has a very small imaginary vibrational frequency, 24i cm-1, by MPW1PW91 and all real vibrational frequencies by BP86. The Mo-C distances to the semibridging carbonyls are 1.937 and 2.362 Å (MPW1PW91) or 1.956 and 2.331 Å (BP86). The vibrational frequencies of 1758 and 1766 cm-1 (BP86) can be assigned to these two semibridging carbonyl groups. The Mo-Mo distance in 2T-2 is predicted to be 2.554 Å (MPW1PW91) or 2.541 Å (BP86). The third triplet Cp2Mo2(CO)2 structure, 2T-3 (Figure 5 and Table 11), also has two semibridging CO groups and all real vibrational frequencies by MPW1PW91 and BP86. Structure 2T-3 lies 9.1 kcal/mol (MPW1PW91) or 14.1 kcal/mol (BP86) above 2T-1. The spin contamination of 2T-3 is somewhat significant, i.e., 〈S2〉 ) 2.9 (MPW1PW91) or 〈S2〉 ) 2.6 (BP86). The distance between the two molybdenum atoms in 2T-3 is predicted to be 2.530 Å (MPW1PW91) or 2.496 Å (BP86). For the semibridging carbonyls, the Mo-C distances are 1.943 and 2.587 Å (MPW1PW91) or 1.960 and 2.606 Å (BP86). The ν(CO) frequencies of 2T-1 at 1813 and 1851 cm-1 (BP86, Table 13) correspond to the two semibridging CO groups. Another triplet stationary point, of Cp2Mo2(CO)2 2T-4 (Figure 5 and Table 11), is a Cs structure with two Cp rings bonded to one molybdenum atom and only carbonyl groups bonded to the second molybdenum atom. Structure 2T-4 lies 16.4 kcal/mol (MPW1PW91) or 17.9 kcal/mol (BP86) above 2T-1 with a very


Zhang et al.

Table 14. Total Energies (E, in hartree), Relative Energies (∆E, in kcal/mol), Numbers of Imaginary Vibrational Frequencies (Nimag), Mo-Mo Bond Distances (Å), and ν(CO) Frequencies in cm-1a for Each of the Triplet Cp2Mo2(CO) Optimized Structures 1T-1 (C1)

1T-2 (C1)

1T-3 (C1)

E ∆E Nimag Mo-Mo ν(CO)

MPW1PW91 -636.787 94 -636.785 71 0.0 1.4 0 0 2.515 2.487 1557 (370) 1656 (526)

-636.774 95 8.2 0 2.486 1862 (1980)


-637.049 77 0.0 0 2.513 1435 (257)

BP86 -637.047 84 1.2 0 2.460 1531 (362)

-637.043 94 3.7 0 2.408 1808 (1484)

a

Infrared intensities in km/mol in parentheses.

small imaginary vibrational frequency of 12i cm-1 by MPW1PW91 and 40i cm-1 by BP86. The Mo-C distance for the two carbonyls is 1.935 Å (MPW1PW91) or 1.944 Å (BP86). The vibrational frequencies of 1822 and 1876 cm-1 (BP86) can be assigned to these two CO groups. The Mo-Mo distance in 2T-4 is predicted to be 2.644 Å by MPW1PW91 and BP86. The lowest lying singlet structure of Cp2Mo2(CO)2 (2S-1 in Figure 5 and Table 12) exhibits a C2 structure with two semibridging CO groups. The Mo-C distances to the two semibridging carbonyls are 1.928 and 2.516 Å (MPW1PW91) or 1.948 and 2.416 Å (BP86). The ν(CO) frequencies of 2S-1 at 1775 and 1781 cm-1 (BP86, Table 13) correspond to these two semibridging CO groups. The Mo-Mo distance in 2S-1 is 2.413 Å (MPW1PW91) or 2.410 Å (BP86), indicating a highorder metal-metal bond, possibly a triple bond giving both metal atoms 16-electron configurations. The second singlet stationary point, 2S-2 (Figure 5 and Table 12), is another structure with two Cp rings bonded to one molybdenum atom and only carbonyl groups bonded to the second molybdenum atom. Structure 2S-2 has all real vibrational frequencies by MPW1PW91 and a very small vibrational frequency of 41i cm-1 by BP86. It lies 24.8 kcal/mol (MPW1PW91) or 20.1 kcal/mol (BP86) above 2T-1. The Mo-C distances to the carbonyls are 1.918 Å (MPW1PW91) or 1.939 Å (BP86) and 2.022 Å (MPW1PW91) or 2.036 Å (BP86). The ν(CO) frequencies of 2S-1 at 1860 and 1899 cm-1 (BP86, Table 13) correspond to the two CO groups. 3.6. Cp2Mo2(CO). A total of six energetically low-lying Cp2Mo2(CO) structures were found (Figure 6 and Tables 14 and 15), including three triplet and three singlet structures. The global minimum of Cp2Mo2(CO) can be either at the triplet structure 1T-1 or the singlet structure 1S-1, depending on the DFT method (Tables 14 and 15). Thus, MPW1PW91 predicts 1T-1 to lie below the singlet structure 1S-1 by about 3.4 kcal/ mol, whereas BP86 predicts the singlet 1S-1 to lie below 1T-1 by about 6.0 kcal/mol. The triplet global minimum is predicted to be a C1 semibridged structure, namely 1T-1 (Figure 6 and Table 14). It has all real vibrational frequencies by MPW1PW91 and BP86. The Mo-C distances to the semibridging carbonyl in 1T-1 are 1.897 and 2.073 Å (MPW1PW91) or 1.912 and 2.091 Å (BP86). The Mo-O distance to the carbonyl oxygen is very short, namely 2.231 Å (MPW1PW91) or 2.227 Å (BP86), indicating that this semibridging carbonyl is a fourelectron-donor η2-µ-CO group. This is consistent with its extremely low ν(CO) frequency of 1435 cm-1 (BP86, Table 14) and relatively long C-O distance of 1.252 Å (MPW1PW91) or 1.279 Å (BP86), indicating a very low C-O bond order for

the carbonyl group. The Mo-Mo distance is 2.515 Å (MPW1PW91) or 2.513 Å (BP86). The next energetically low-lying triplet structure of Cp2Mo2(CO), namely 1T-2 (Figure 6 and Table 14), lies above 1T-1 by only 1.4 kcal/mol (MPW1PW91) or 1.2 kcal/mol (BP86) and also has C1 symmetry with all real harmonic vibrational frequencies. The Mo-C distances for the semibridging carbonyl in 1T-2 are 2.167 and 1.896 Å (MPW1PW91) or 2.166 and 1.914 Å (BP86). Again there is a very short Mo-O distance, namely 2.368 Å (MPW1PW91) or 2.353 Å (BP86), which indicates that this semibridging carbonyl is a fourelectron-donor η2-µ-CO group, consistent with its very low ν(CO) frequency of 1531 cm-1 (BP86, Table 14). The Mo-Mo distance is 2.487 Å (MPW1PW91) or 2.460 Å (BP86), which is 0.03 Å (MPW1PW91) or 0.05 Å (BP86) shorter than that of 1T-1. The third energetically low lying triplet structure of Cp2Mo2(CO) is 1T-3 (Figure 6 and Table 14), which has both Cp rings bonded to one molybdenum atom and only the single carbonyl group bonded to the second molybdenum atom. Structure 1T-3 lies above 1T-1 by 8.2 kcal/mol (MPW1PW91) or 3.7 kcal/mol (BP86) with all real harmonic vibrational frequencies. The Mo-C distance to the carbonyl is 1.876 Å (MPW1PW91) or 1.926 Å (BP86), and the ν(CO) frequency is at 1808 cm-1 (BP86, Table 14). The Mo-Mo distance is 2.486 Å (MPW1PW91) or 2.408 Å (BP86). The lowest lying singlet structure of Cp2Mo2(CO) is 1S-1, with a semibridging carbonyl group and all real harmonic vibrational frequencies (Figure 6 and Table 15). The Mo-C distances to the semibridging carbonyl in 1S-1 are 1.900 and 2.094 Å (MPW1PW91) or 1.915 and 2.092 Å (BP86). The Mo-O distance, namely 2.328 Å (MPW1PW91) or 2.252 Å (BP86), is very short, indicating that this semibridging carbonyl is a four-electron-donor η2-µ-CO group consistent with its extremely low ν(CO) frequency of 1466 cm-1 (BP86, Table 15). The Mo-Mo distance in 1S-1 is 2.372 Å (MPW1PW91) or 2.367 Å (BP86), which is ∼0.14 Å shorter than that of 1T-1. The second energetically low-lying singlet Cp2Mo2(CO) structure, 1S-2 (Figure 6 and Table 15), lies above the global minimum 1T-1 by 6.7 kcal/mol (MPW1PW91) or 1.7 kcal/mol (BP86) with all real harmonic vibrational frequencies. The Mo-C distances to the semibridging carbonyl in 1S-2 are 2.698 Å (MPW1PW91) or 2.593 Å (BP86) and 1.950 Å (MPW1PW91) or 1.968 Å (BP86). The ν(CO) frequency for this carbonyl group of 1796 cm-1 (BP86, Table 15) is within the range expected for a semibridging carbonyl group. The Mo-Mo distance is 2.320 Å (MPW1PW91) or 2.326 Å (BP86), suggesting a Mo-Mo quadruple bond and thus 16-electron configurations for the two molybdenum atoms. Another energetically low-lying singlet isomer of Cp2Mo2(CO) is 1S-3, with two Cp rings bonded to one molybdenum atom and the single carbonyl group bonded to the second molybdenum atom as a terminal carbonyl group. Structure 1S-3 lies above 1T-1 by 12.4 kcal/mol (MPW1PW91) or 7.5 kcal/mol (BP86) with all real harmonic vibrational frequencies. The Mo-C distance to the carbonyl is 1.928 Å (MPW1PW91) or 1.933 Å (BP86), and the predicted ν(CO) frequency of 1847 cm-1 (BP86, Table 15) is consistent with a terminal carbonyl group. The Mo-Mo distance in 1S-3 is 2.432 Å (MPW1PW91) or 2.462 Å (BP86). 3.7. Dissociation and Disproportionation Reactions. Table 16 gives the carbonyl dissociation energies for Cp2Mo2(CO)n (n ) 6-2) based on the lowest energy structures. The corre-



Figure 6. The six optimized structures of Cp2Mo2(CO). Table 15. Total Energies (E, in hartree), Relative Energies (∆E, in kcal/mol), Number of Imaginary Vibrational Frequencies (Nimag), Mo-Mo Bond Distances (Å), and ν(CO) Frequencies in cm-1a for Each of the Singlet Cp2Mo2(CO) Optimized Structures 1S-1 (C1)

1S-2 (C1)

1S-3 (Cs)


MPW1PW91 -636.782 56 -636.777 32 3.4 6.7 0 0 2.372 2.320 1636 (502) 1925 (1190)

-636.768 19 12.4 0 2.432 1960 (1224)


-637.059 26 -6.0 0 2.367 1466 (307)

BP86 -637.047 13 1.7 0 2.326 1796 (847)

-637.037 88 7.5 0 2.462 1847 (927)

a

Infrared intensities in km/mol in parentheses. Table 16. Dissociation Energies (kcal/mol) for the Successive Removal of Carbonyl Groups from Cp2Mo2(CO)n MPW1PW91/ BP86/ DZP DZP

Figure 7. The lowest energy structures for the mononuclear CpMo(CO)n (n ) 4-1) fragments.

sponding dissociation enthalpies and dissociation Gibbs free energies are available in the Supporting Information. The energies for the loss of one carbonyl group from Cp2Mo2(CO)n (n ) 5, 6) fall in the broad range of 15-42 kcal/ mol but are clearly significantly below the >50 kcal/mol energies for the loss of one carbonyl group from more highly unsaturated derivatives Cp2Mo2(CO)n (n ) 2-4). This is consistent with the experimental observation that the thermal reactions of

Cp2Mo2(CO)6 Cp2Mo2(CO)5 Cp2Mo2(CO)4 Cp2Mo2(CO)3 Cp2Mo2(CO)2

(6S-1) f Cp2Mo2(CO)5 (5S-1) + CO (5S-1) f Cp2Mo2(CO)4 (4S-1) + CO (4S-1) f Cp2Mo2(CO)3 (3T-1) + CO (3T-1) f Cp2Mo2(CO)2 (2T-1) + CO (2T-1) f Cp2Mo2(CO) (1T-1) + CO

42.0 15.9 52.3 64.8 64.3

40.4 19.3 58.6 66.6 65.5

Mo(CO)6 with cyclopentadiene derivatives have been found to go as far as Cp2Mo2(CO)4 but not beyond. Table 17 gives the predicted energies of the disproportionation reactions 2Cp2Mo2(CO)n f Cp2Mo2(CO)n+1 + Cp2Mo2(CO)n-1. These data indicate that the experimentally observed Cp2Mo2(CO)4 is stable with respect to such disproportionation (Table 17). In addition, Cp2Mo2(CO)3 is also stable but with a disproportionation energy significantly smaller than that of Cp2Mo2(CO)4. However, Cp2Mo2(CO)5 is energetically unfavor-


Zhang et al.

Table 17. Disproportionation Energies (kcal/mol) of the Cp2Mo2(CO)n (n ) 5-2) Species

2Cp2Mo2(CO)5 2Cp2Mo2(CO)4 2Cp2Mo2(CO)3 2Cp2Mo2(CO)2

f f f f

Cp2Mo2(CO)6 Cp2Mo2(CO)5 Cp2Mo2(CO)4 Cp2Mo2(CO)3

+ + + +

Cp2Mo2(CO)4 Cp2Mo2(CO)3 Cp2Mo2(CO)2 Cp2Mo2(CO)

4. Discussion

MPW1PW91/ DZP

BP86/ DZP

-26.1 36.5 12.4 -0.5

-21.1 39.4 8.0 -1.1

Table 18. Total Energies (E, in hartree) and Numbers of Imaginary Vibrational Frequencies (Nimag) of the Global Minima of CpMo(CO)m (m ) 1-4) CpMo(CO)4 (Cs)

CpMo(CO)3 (Cs)

CpMo(CO)2 (Cs)

CpMo(CO) (Cs)

E Nimag

-715.109 20 0

MPW1PW91 -601.798 22 -488.415 62 0 0

-375.030 98 0

E Nimag

-715.370 732 0

BP86 -602.018 25 0

-375.173 93 0

-488.597 94 0

Table 19. Dissociation Energies of the Binuclear Cp2Mo2(CO)n into Mononuclear Fragments (kcal/mol)

Cp2Mo2(CO)2 Cp2Mo2(CO)4 Cp2Mo2(CO)6 Cp2Mo2(CO)3 Cp2Mo2(CO)4 Cp2Mo2(CO)5 Cp2Mo2(CO)5 Cp2Mo2(CO)6

f f f f f f f f

2CpMo(CO) 2CpMo(CO)2 2CpMo(CO)3 CpMo(CO)2 + CpMo(CO)3 + CpMo(CO)4 + CpMo(CO)3 + CpMo(CO)2 +

CpMo(CO) CpMo(CO) CpMo(CO) CpMo(CO)2 CpMo(CO)4

MPW1PW91/ DZP

BP86/ DZP

77.3 81.7 29.4 85.7 83.0 88.7 42.5 74.3

83.7 87.5 30.4 89.6 89.8 93.3 48.4 73.0

able with respect to disproportionation into Cp2Mo2(CO)6 + Cp2Mo2(CO)4. The predicted instability of Cp2Mo2(CO)5 toward disproportionation is similar to the prediction and experimental observation of the instability of its chromium analogue, Cp2Cr2(CO)5, toward analogous disproportionation.45 The binuclear derivatives Cp2Mo2(CO)n can dissociate into the mononuclear fragments CpMo(CO)p + CpMo(CO)q, where p + q ) n and each fragment has a cyclopentadienyl ring bonded to the molybdenum atom. In order to obtain the energetic data for the dissociation of the Cp2Mo2(CO)n derivatives into mononuclear CpMo(CO)m fragments, the structures of the relevant mononuclear CpMo(CO)m (m ) 4-1) fragments were first optimized by the same DFT methods used to study the binuclear derivatives. The structures and total energies of the global minima for the CpMo(CO)m fragments are given in Figure 7 and Table 18. The dissociation energies of the binuclear derivatives Cp2Mo2(CO)n into these mononuclear fragments are given in Table 19. The dissociation energy of the saturated dimer Cp2Mo2(CO)6 into two mononuclear CpMo(CO)3 fragments is found to be 29.4 kcal/mol (MPW1PW91) or 30.4 kcal/mol (BP86), indicating considerable energy for generation of CpMo(CO)3• radicals from Cp2Mo2(CO)6. The dissociation energy of Cp2Mo2(CO)5 into CpMo(CO)3 + CpMo(CO)2 is seen to be significantly higher at 42.5 kcal/mol (MPW1PW91) or 48.4 kcal/ mol (BP86). The dissociation energies of the more unsaturated Cp2Mo2(CO)n derivatives are higher yet: in excess of 70 kcal/ mol in all cases. These observations suggest that dissociation of binuclear Cp2Mo2(CO)n into mononuclear fragments is not likely to be a significant pathway for their chemical reactions. (45) Fortman, G. C.; Ke´gl, 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.

Our DFT studies predict an equilibrium geometry for the saturated derivative Cp2Mo2(CO)6, namely 6S-1 in Figure 1, that is very close to the experimental structure determined by X-ray diffraction. Although the predicted and experimental Mo-Mo single-bond distances are rather long at ∼3.2 Å, this dimer is stable with respect to fragmentation into monomeric CpMo(CO)3 units, as indicated by a dissociation energy above 25 kcal/mol. The thermal reaction chemistry of Cp2Mo2(CO)6 typically results in the generation of other dimeric Cp2Mo2 derivatives. Monomeric derivatives are produced from Cp2Mo2(CO)6 only by redox processes affecting the Mo-Mo bond, such as halogenation to give CpMo(CO)3X or reduction to give the anion CpMo(CO)3-. One of the laboratory methods for preparing Cp2Mo2(CO)6 is the thermal reaction of Mo(CO)6 with cyclopentadiene.3 However, this reaction can also give the unsaturated derivative Cp2Mo2(CO)4.44 In fact, the closely related reaction of Mo(CO)6 with pentamethylcyclopentadiene leads directly at about 100 °C to (η5-Me5C5)2Mo2(CO)4, which, as noted in the Introduction, was the first known example of a metal carbonyl derivative with a formal metal-metal triple bond.2 The unlikelihood of the thermal reaction of Mo(CO)6 with a simple cyclopentadiene derivative going beyond Cp2Mo2(CO)4 to give derivatives with fewer carbonyl groups is suggested by the high carbonyl dissociation energy of more than 50 kcal/mol to give Cp2Mo2(CO)3. The very first report in 19543 of the thermal reaction of Mo(CO)6 with cyclopentadiene at 300 °C claimed that the dark red product was the pentacarbonyl Cp2Mo2(CO)5. After Cp2Mo2(CO)6 was synthesized via the anion CpMo(CO)3obtained from Mo(CO)6 and sodium cyclopentadienide, the formulation of the Mo(CO)6/cyclopentadiene pyrolysis product was corrected to Cp2Mo2(CO)6. However, in view of the later discovery44 of the MotMo triple-bonded product Cp2Mo2(CO)4 from the thermal reaction of Mo(CO)6 and cyclopentadiene under other conditions, it is likely that the originally claimed Cp2Mo2(CO)5 could have been a mixture of Cp2Mo2(CO)6 and Cp2Mo2(CO)4. At the time of the first synthesis of Cp2Mo2(CO)6 (and possibly Cp2Mo2(CO)4) from the thermal reaction of Mo(CO)6 with cyclopentadiene,3 the ability of metal-metal bonds to hold a molecule together had not yet been established. For this reason, the absence of bridging carbonyl groups in Cp2Mo2(CO)6, which was even clear in 1954 by the presence of only terminal infrared ν(CO) frequencies in the infrared spectrum,3 led to the postulation of unusual CnOn rings of carbonyl groups to hold the halves of the molecule together in order to avoid the postulation of a direct metal-metal bond.46 We now know that structures of this type are unfavorable, if not ridiculous. It appears that genuine Cp2Mo2(CO)5 has never been synthesized as a stable compound but only detected as a shortlived intermediate at room temperature18 or in low-temperature matrices.15-17 Our theoretical studies predict that Cp2Mo2(CO)5 is not likely to be a stable compound, since its disproportionation into Cp2Mo2(CO)6 + Cp2Mo2(CO)4 is highly exothermic. The related disproportionation of Cp2Cr2(CO)5 into Cp2Cr2(CO)6 + Cp2Cr2(CO)4 has not only been predicted theoretically but also observed experimentally.45 Photolysis of Cp2Mo2(CO)4 provides a possible approach to more highly unsaturated derivatives, as has been demonstrated (46) Cotton, F. A.; Liehr, A. D.; Wilkinson, G. J. Inorg. Nucl. Chem. 1955, 1, 175.


for the chromium analogue.16,47,48 The closed-shell singlet tricarbonyl Cp2Mo2(CO)3 requires a formal Mo -Mo quadruple bond for both molybdenum atoms to satisfy the 18-electron rule if all of the carbonyl groups are the usual 2-electron donors. None of the low-energy Cp2Mo2(CO)3 structures are predicted to have metal-metal distances sufficiently shorter than that in Cp2Mo2(CO)4 to suggest a Mo-Mo quadruple bond (Figure 4). The low-energy structures of Cp2Mo2(CO)3 are the triplet 3T-1 with only the usual 2-electron-donor carbonyl groups and the singlet 3S-2 with one 4-electron-donor bridging carbonyl group, as indicated by a relatively short Mo-O distance and a low predicted ν(CO) frequency (Figure 4). In both cases the MotMo distances are similar to that in Cp2Mo2(CO)4, suggesting formal molybdenum-molybdenum triple bonds in both cases leading to 17-electron metal configurations for the binuclear triplet 3T-1 and the favored 18-electron configuration with one 4-electrondonor bridging carbonyl group for the singlet 3S-2. An interesting feature of the structures of the more highly unsaturated Cp2Mo2(CO)2 is the absence of four-electron-donor carbonyl groups in the low-energy structures (Figure 5). The lack of four-electron-donor η2-µ-CO groups in any of the lowenergy Cp2Mo2(CO)2 structures contrasts with the four-electrondonor carbonyl groups found in most of the low-energy structures of the highly unsaturated Cp2Mo2(CO). The molybdenum-molybdenum distances in the low-energy Cp2Mo2(CO)2 structures are similar to those in the triply bonded derivative Cp2Mo2(CO)4 (i.e., around 2.5 Å) rather than the much shorter distancesthatwouldbeexpectedforthemolybdenum-molybdenum quintuple bond required to give both metal atoms the favored 18-electron configuration. In this connection, note that the formal Mo-Mo quadruple-bond distance of 2.09 Å in Mo2(O2CCH3)4 determined by X-ray crystallography49 is much shorter than the Mo-Mo bond distances of any of the structures discussed in this paper. The lowest energy Cp2Mo2(CO)2 structure, namely the triplet 2T-1 (Figure 5), is particularly interesting, since both of its carbonyl groups are semibridging carbonyl groups with the short Mo-C distances to the same molybdenum atom. The molybdenum atom in a CpMo(CO)2 unit with an MotMo triple bond to another molybdenum atom has the favored 18-electron configuration. The other portion of the 2T-1 structure, namely a CpMo unit triply bonded to another molybdenum atom and only loosely bonded to the carbonyl groups, as indicated by longer Mo-C distances of ∼2.5 Å, would formally have only a 14-electron configuration. The unpaired electrons in the triplet


2T-1 are associated with the molybdenum atom only loosely bonded to the carbonyl groups, as indicated by a spin density of ∼2 on this molybdenum atom. The fact that at least one molybdenum atom in any of the low-energy Cp2Mo2(CO)2 structures must have less than the favored 18-electron configuration for the singlet or a 17-electron configuration for the triplet suggests that Cp2Mo2(CO)2 should be particularly reactive, as indicated by the thermodynamically slightly favored (by ∼1 kcal/mol) disproportionation into Cp2Mo2(CO)3 and Cp2Mo2(CO) (Table 17). Structures of Cp2M2(CO)n derivatives with both Cp rings bonded to the same metal atom and only carbonyl groups bonded to the other metal atom were found in our previous study on Cp2Re2(CO)n derivatives.50 However, these are relatively high energy structures. Similar, likewise relatively high energy structures have now been found for the Cp2Mo2(CO)n derivatives studied in this work, namely 5S-3 and 5T-3 for Cp2Mo2(CO)5 (Figure 2), 3T-3 for Cp2Mo2(CO)3 (Figure 4), 2T-4 and 2S-2 for Cp2Mo2(CO)2 (Figure 5), and 1S-3 and 1T-3 for Cp2Mo2(CO) (Figure 6). However, Cp2MofMo(CO)5 (5S-3 in Figure 2) might be synthesized as a metastable derivative by the interaction of a reactive Cp2Mo fragment51 generated from Cp2MoH2 with a reactive Mo(CO)5 fragment.

Acknowledgment. We are indebted to the 111 Project (B07012), National Natural Science Foundation (20873045), Research Fund for the Doctoral Program of Higher Education (200800071019), and Excellent Young Scholars Research Fund of BIT (2008Y0713) in China and the U.S. National Science Foundation (Grants CHE-0749868 and CHE0716718) for partial support of this work. Supporting Information Available: Tables S1-S9, giving the theoretical harmonic vibrational frequencies for the 28 structures of Cp2Mo2(CO)n (n ) 1-6) using the BP86 method, Tables S10-S37, giving theoretical Cartesian coordinates for the 28 structures of Cp2Mo2(CO)n (n ) 1-6) using the MPW1PW91/SDD method, Tables S38-S40, giving dissociation Gibbs free energies (∆G in kcal/mol) for Cp2Mo2(CO)n (n ) 1- 6), Tables S41-S43, giving dissociation enthalpies (∆H in kcal/mol) for Cp2Mo2(CO)n (n ) 1-6), Tables S44-S48, giving infrared-active ν(CO) vibrational frequencies (cm-1) predicted by the MPW1PW91 and BP86 methods for the lowest energy isomers of Cp2Mo2(CO)n, and text giving the complete Gaussian 03 reference.38 This material is available free of charge via the Internet at http://pubs.acs.org. OM801170E

(47) Robbins, J. J.; Wrighton, M. S. Inorg. Chem. 1981, 20, 1133. (48) Virrels, I. G.; Nolan, T. F.; George, M. W.; Turner, J. J. Organometallics 1997, 16, 5879. (49) Cotton, F. A.; Mester, Z. C.; Webb, T. R. Acta Crystallogr. 1974, B30, 2768.

(50) Xu, B.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. Inorg. Chem. 2008, 47, 6779. (51) Geoffroy, G. L.; Bradley, M. G. J. Organomet. Chem. 1977, 134, C27.

Binuclear Cyclopentadienylmolybdenum Carbonyl ... - ACS Publications

Recommend Documents