The Nature of M−B Versus M B Bonds in Cationic Terminal Borylene

Oct 26, 2009 - The [M]−BR bonding in the cationic borylene complexes has a greater degree of ionic (60.6−66.8%) than covalent character. The orbit...
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Organometallics 2009, 28, 6442–6449 DOI: 10.1021/om900640p

The Nature of M-B Versus MdB Bonds in Cationic Terminal Borylene Complexes: Structure and Energy Analysis in the Borylene Complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]+, [(η5-C5H5)(CO)2M(BMes)]+, and [(η5-C5H5)(CO)2M(BNMe2)]+ (M = Fe, Ru, Os) Krishna K. Pandey,*,† Agustı´ Lled os,*,‡ and Feliu Maseras*,§ †

School of Chemical Sciences, Devi Ahilya University Indore, Indore 452017, India, ‡Departament de Quı´mica, Universitat Aut onoma de Barcelona, 08193 Bellaterra (Barcelona), Spain, and §Institute of Chemical Research of Catalonia (ICIQ), 43007 Tarragona, Spain Received July 21, 2009

Density functional theory calculations have been performed for the terminal cationic borylene complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ, [(η5-C5H5)(CO)2M(BMes)]þ, and [(η5-C5H5)(CO)2M(BNMe2)]þ (M=Fe, Ru, Os) using the exchange correlation functional BP86. The optimized bond lengths and angles of the complexes are in excellent agreement with experiment. The M-B bond distances in the complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ (I, M=Fe; II, M=Ru; III, M=Os) are similar to those expected for single bonds on the basis of covalent radii predictions. In contrast, the optimized M-B bond distances in the complexes [(η5-C5H5)(CO)2M(BMes)]þ and [(η5-C5H5)(CO)2M(BNMe2)]þ correspond to a Pauling bond order of 1.73-1.42. The contribution of the electrostatic interaction ΔEelstat is significantly larger in all borylene complexes than the covalent bonding ΔEorb, the [M]-BR bonding in the cationic borylene complexes having a greater degree of ionic (60.6-66.8%) than covalent character. The orbital interactions between metal and boron in [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ, [(η5-C5H5)(CO)2M(BMes)]þ, and [(η5-C5H5)(CO)2Fe(BNMe2)]þ arise mainly from MrBR σ donation. The π-bonding contribution is, in all complexes, much smaller (9.0-17.3% of total orbital contributions).

Introduction In the 11 years since the first report of structurally characterized terminal transition metal borylene complexes in 1998,1,2 the field has blossomed rapidly in terms of synthesis, structure, bonding, and reactivity. So far, 12 structurally characterized neutral terminal transition-metal borylene com*To whom correspondence should be addressed. E-mail: kkpandey. [email protected] (K.K.P.); [email protected] (A.L.); [email protected] (F.M.). (1) Braunschweig, H.; Kollann, C.; Englert, U. Angew. Chem., Int. Ed. 1998, 37, 3179–3180. (2) Cowley, A. H.; Lomeli, V.; Voigt, A. J. Am. Chem. Soc. 1998, 120, 6401–6402. (3) Braunschweig, H.; Colling, M.; Kollann, C.; Stammler, H. G.; Neumann, B. Angew. Chem., Int. Ed. 2001, 40, 2298–2300. (4) Braunschweig, H.; Colling, M.; Kollann, C.; Merz, K.; Radacki, K. Angew. Chem., Int. Ed. 2001, 40, 4198–4200. (5) Braunschweig, H.; Colling, M.; Hu, C.; Radacki, K. Angew. Chem., Int. Ed. 2003, 42, 205–208. (6) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K. Angew. Chem., Int. Ed. 2006, 45, 162–165. (7) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K. Organometallics 2006, 25, 5159–5164. (8) Blank, B.; Colling-Hendelkens, M.; Kollann, C.; Radacki, K.; Rais, D.; Uttinger, K.; Whittell, G. R.; Braunschweig, H. Chem. Eur. J. 2007, 13, 4770–4781. (9) Braunschweig, H.; Burzler, M.; Kupfer, T.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 7785–7787. (10) Braunschweig, H.; Forster, M.; Kupfer, T.; Seeler, F. Angew. Chem., Int. Ed. 2008, 47, 5981–5983. pubs.acs.org/Organometallics

Published on Web 10/26/2009

plexes (Chart 1)1-12 and 9 terminal cationic transition-metal borylene complexes (Chart 2)13-19 have been reported. Additionally, a number of base-stabilized adducts formed by the coordination of a Lewis base (main group or transition metal) (11) Braunschweig, H.; Kupfer, T.; Radacki, K.; Schneider, A.; Seeler, F.; Uttinger, K.; Wu, H. J. Am. Chem. Soc. 2008, 130, 7974–7983. (12) Alcaraz, G.; Helmstedt, U.; Clot, E.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2008, 130, 12878–12879. (13) Coombs, D. L.; Aldridge, S.; Jones, C.; Willock, D. J. J. Am. Chem. Soc. 2003, 125, 6356–6357. (14) Coombs, D. L.; Aldridge, S.; Rossin, A.; Jones, C.; Willock, D. J. Organometallics 2004, 23, 2911–2926. (15) (a) Aldridge, S.; Jones, C.; Gans-Eichler, T.; Stasch, A.; Kays, D. L.; Coombs, N. D.; Willock, D. J. Angew. Chem., Int. Ed. 2006, 45, 6118–6122. (b) Kays, D. L.; Day, J. K.; Aldridge, S.; Harrington, R. W.; Clegg, W. Angew. Chem., Int. Ed. 2006, 45, 3513–3516. (c) Pierce, G. A.; Aldridge, S.; Jones, C.; Gans-Eichler, T.; Stasch, A.; Coombs, N. D.; Willock, D. J. Angew. Chem., Int. Ed. 2007, 46, 2043–2046. (16) Vidovic, D.; Findlater, M.; Reeske, G.; Cowley, A. H. Chem. Commun. 2006, 3786–3787. (17) Braunschweig, H.; Radacki, K.; Uttinger, K. Angew. Chem., Int. Ed. 2007, 46, 3979. (18) Pierce, G. A.; Vidovic, D.; Kays, D. L.; Coombs, N. D.; Thompson, A. L.; Jemmis, E. D.; De, S.; Aldridge, S. Organometallics 2009, 28, 2947–2960. (19) De, S.; Pierce, G. A.; Vidovic, D.; Kays, D. L.; Coombs, N. D.; Jemmis, E. D.; Aldridge, S. Organometallics 2009, 28, 2961–2975. (20) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786–1801. (21) Braunschweig, H.; Colling, M. J. Organomet. Chem. 2000, 614, 18–26. (22) Braunschweig, H.; Colling, M. Coord. Chem. Rev. 2001, 223, 1–51. r 2009 American Chemical Society

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Chart 1. Selected Structurally Characterized Neutral Terminal Metal Borylene Complexes

to the two- or three-coordinate boron center have also been reported.20-32 A number of interesting review articles have been written during these periods by Braunschweig et al.20-29 and Aldridge et al.30-32 A qualitative picture of the metal-boron bond in LnM-BR borylene complexes can be attained with simple molecular orbital arguments based on donation and back-donation contributions (Figure 1). The M-B bond arises from donation (23) Braunschweig, H.; Colling, M. Eur. J. Inorg. Chem. 2003, 393–403. (24) Braunschweig, H. Adv. Organomet. Chem. 2004, 51, 163–192. (25) Braunschweig, H.; Rais, D. Heteroat. Chem. 2005, 16, 566–571. (26) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006, 45, 5254–5274. (27) Braunschweig, H.; Kollann, C.; Seeler, F. Struct. Bonding (Berlin) 2008, 130, 1–27. (28) Anderson, C. E; Braunschweig, H.; Dewhurst, R. D. Organometallics 2008, 27, 6381–6389. (29) Braunschweig, H.; Dewhurst, R. D. Angew. Chem., Int. Ed. 2009, 48, 1893–1895. (30) Aldridge, S.; Coombs, D. L. Coord. Chem. Rev. 2004, 248, 535–559. (31) Aldridge, S.; Kays, D. L. Main Group Chem. 2006, 5, 223–249. (32) Vidovic, D.; Pierce, G. A.; Aldridge, S. Chem. Commun. 2009, 1157–1171.

from the σ-symmetric electron lone pair on BR to an empty d(σ) orbital of the transition metal and back-donation from filled d(π) orbitals of the metal into the degenerate p(π) orbitals of B. The p(π) orbitals of B are vacant if R has no filled π-symmetric orbitals, but they become partly filled by BrR π donation if R has π-symmetric lone pair orbitals. This simple picture of the M-BR bond has been deepened with the help of DFT calculations. A number of theoretical studies of terminal neutral metal borylene complexes have been published as research papers33-40 as well as review articles.41,42 The calculated geometries of terminal cationic (33) Ehlers, A. W.; Baerends, E. J.; Bickelhaupt, F. M.; Radius, U. Chem. Eur. J. 1998, 4, 210–221. (34) Radius, U.; Bickelhaupt, F. M.; Ehlers, A. W.; Goldberg, N.; Hoffmann, R. Inorg. Chem. 1998, 37, 1080–1090. (35) Macdonald, C. L. B.; Cowley, A. H. J. Am. Chem. Soc. 1999, 121, 12113–12126. (36) Boehme, C.; Frenking, G. Chem. Eur. J. 1999, 5, 2184–2190. (37) Uddin, J.; Boehme, C.; Frenking, G. Organometallics 2000, 19, 571–582. (38) Chen, Y.; Frenking, G. Dalton Trans. 2001, 434–440.

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Figure 1. Schematic representation of the M-BR orbital interactions.

borylene complexes [(η5-C5H5)(CO)2Fe{B(η5-C5Me5)}]þ, [(η5-C5H5)(CO)2Fe(BMes)]þ, [(η5-C5H5)(CO)2Fe(BNMe2)]þ, and [(η5-C5H5)(CO)2Ru(BNMe2)]þ at different levels of theory (BLYP/LANL2DZ or B3LYP/LANL2DZ) have been reported previously.14,18,19,43 An energy decomposition analysis of the M-B bond in these iron cationic borylene complexes has been also published.43 To the best of our knowledge, the differences between the bonding situation of the formally M-BR and MdBR linkages have never been studied before. We decided to investigate the nature of M-B versus MdB bonds in the terminal cationic metal borylene complexes with an energy decomposition analysis (EDA). In this paper, the nine cationic terminal borylene complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ (I, M = Fe; II, M = Ru; III, M = Os), [(η5C5H5)(CO)2M(BMes)]þ (IV, M = Fe; V, M = Ru; VI, M = Os), and [(η5-C5H5)(CO)2M(BNMe2)]þ (VII, M = Fe; VIII, M = Ru; IX, M = Os) have been investigated at the DFT level using BP86. The main goals of the present study are (i) to investigate the geometries and to analyze the nature of M-B and MdB bonds of the terminal cationic borylene complexes and (ii) to provide a quantitative differentiation, based on an energy decomposition analysis (EDA), between the M-BR and the MdBR bonds in terminal cationic borylene complexes. There are two questions at the center of the discussion. One addresses the degree of ionic and covalent character of the M-B and MdB bonds. The second question addresses the extent of the MfB π-back-bonding contribution to the metal-ligand orbital interactions (Figure 1).

Computational Methods Calculations of the cationic terminal complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ (I, M = Fe; II, M = Ru; III, M = Os), [(η5-C5H5)(CO)2M(BMes)]þ (IV, M = Fe; V, M = Ru; VI, M = Os), and [(η5-C5H5)(CO)2M(BNMe2)]þ (VII, M = Fe; (39) Uddin, J.; Frenking, G. J. Am. Chem. Soc. 2001, 123, 1683–1693. (40) Bollwein, T.; Brothers, P. J.; Hermann, H. L.; Schwertfeger, P. Organometallics 2002, 21, 5236–5242. (41) Boehme, C.; Uddin, J.; Frenking, G. Coord. Chem. Rev. 2000, 197, 249–276. (42) Frenking, G.; Fr€ ohlich, N. Chem. Rev. 2000, 100, 717–774. (43) Aldridge, S.; Rossin, A.; Coombs, D. L.; Willock, D. J. Dalton Trans. 2004, 2649–2654. (44) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (45) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (46) (a) Chang, C.; Pelissier, M.; Durand, Ph. Phys. Scr. 1986, 34, 394–404. (b) Heully, J.-L.; Lindgren, I.; Lindroth, E.; Lundquist, S.; Martensson-Pendrill, A.-M. J. Phys. B 1986, 19, 2799–2815. (c) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597–4610. (d) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1996, 105, 6505–6516. (e) van Lenthe, E.; van Leeuwen, R.; Baerends, E. J.; Snijders, J. G. Int. J. Quantum Chem. 1996, 57, 281–293. (f) van Lenthe, E.; Ehlers, A. E.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943–8953.

Pandey et al. VIII, M = Ru; IX, M = Os) have been performed at the nonlocal DFT level of theory using the exchange functional of Becke44 and the correlation functional of Perdew45 (BP86). Scalar relativistic effects have been considered using the ZORA formalism.46 Uncontracted Slater-type orbitals (STOs) using triple-ζ basis sets augmented by two sets of polarization functions were employed for the SCF calculations.47 The (1s)2 core electrons of boron, carbon, nitrogen and oxygen, (1s2s2p)10 core electrons of iron, (1s2s2p3s3p3d)28 core electrons of ruthenium, and (1s2s2p3s3p3d4s4p4d)46 core electrons of osmium were treated by the frozen-core approximation.48 An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular densities and to present the coulomb and exchange potentials accurately in each SCF cycle.49 The calculations were performed utilizing the program package ADF-2008.01.50 The binding interactions in the complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ (I, M = Fe; II, M = Ru; III, M = Os), [(η5-C5H5)(CO)2M(BMes)]þ (IV, M = Fe; V, M = Ru; VI, M = Os) and [(η5-C5H5)(CO)2M(BNMe2)]þ (VII, M = Fe; VIII, M = Ru; IX, M = Os) between the metal fragments (singlet state) and borylene BR fragments (singlet state) have been analyzed with Cs symmetry using the energy decomposition scheme of ADF, which is based on the methods of Morokuma51 and Ziegler and Rauk.52 The bond energy ΔE between fragments can be decomposed as

ΔE ¼ ΔEint þ Eprep

ð1Þ

Here, ΔEprep is the energy required to promote the structures of the free fragments from their equilibrium structure in the electronic ground state to the geometry and electronic state which they take up in the molecule:

ΔEprep ¼ Etotal ðdistorted fragmentsÞ Etotal ðfragments in the equilibrium structureÞ

ð2Þ

ΔEint in eq 1 is the instantaneous interaction energy between the two fragments in the molecule. It can be decomposed into three main components:

ΔEint ¼ ΔEelstat þ ΔEPauli þ ΔEorb

ð3Þ

ΔEelstat describes the classical Coulomb interaction between the fragments, which is attractive in most cases. The term ΔEPauli, which is called exchange repulsion or Pauli repulsion, takes into account the destabilizing two-orbital three- or fourelectron interactions between occupied orbitals of both fragments. ΔEPauli is calculated by enforcing the Kohn-Sham (47) Snijders, J. G.; Baerends, E. J.; Vernooijs, P. At. Data Nucl. Data Tables 1982, 26, 483. (48) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41–51. (49) Krijn, J.; Baerends, E. J. Fit Functions in the HFS-Method; Internal Report (in Dutch); Vrije Universiteit Amsterdam: Amsterdam, The Netherlands, 1984. (50) Baerends, E. J.; Autschbach, J. A.; Berces, A.; Bo, C.; Boerrigter, P. M.; Cavallo, L.; Chong, D. P.; Deng, L.; Dickson, R. M.; Ellis, D. E.; Fan, L.; Fischer, T. H.; Fonseca Guerra, C.; van Gisbergen, S. J. A.; Groeneveld, J. A.; Gritsenko, O. V.; Gr€ uning, M.; Harris, F. E.; van den Hoek, P.; Jacobsen, H.; van Kessel, G.; Kootstra, F.; van Lenthe, E.; Osinga, V. P.; Patchkovskii, S. Philipsen, P. H. T.; Post, D. Pye, C. C.; Ravenek, W.; Ros, P.; Schipper, P. R. T.; Schreckenbach, G.; Snijders, J. G.; Sola, M.; Swart, M.; Swerhone, D.; te Velde, G.; Vernooijs, P.; Versluis, L.; Visser, O.; Wezenbeek, E.; Wiesenekker, G.; Wolff, S. K.; Woo, T. K.; Ziegler T. ADF 2008-01; Scientific Computing & Modelling NV, Amsterdam, The Netherlands. (51) (a) Morokuma, K. J. Chem. Phys. 1971, 55, 1236–1244. (b) Morokuma, K. Acc. Chem. Res. 1977, 10, 294–300. (52) (a) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1–10. (b) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1558–1565. (c) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1755–1759.

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Table 1. Selected Optimized Geometrical Parameters for the Cationic Terminal Borylene Complexes [(η5-C5H5)(CO)2M{B(η5C5Me5)}]þ, [(η5-C5H5)(CO)2M(BMes)]þ, and [(η5-C5H5)(CO)2M(BNMe2)]þ (M = Fe, Ru, Os)a [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ

[(η5-C5H5)(CO)2M(BMes)]þ

[(η5-C5H5)(CO)2M(BNMe2)]þ

M = Fe (I) M = Ru (II) M = Os (III) M = Fe (IV) M = Ru (V) M = Os (VI) M = Fe (VII) M = Ru (VIII) M = Os (IX) Bond Distances M-B B-C

B-N M-CO C-O

2.007 1.810 1.815 1.812 1.810 1.815

2.112 1.806 1.811 1.808 1.806 1.811

2.122 1.805 1.809 1.807 1.805 1.809

1.817 1.487

1.932 1.487

1.955 1.486

1.743 1.159

1.872 1.159

1.880 1.161

1.764 1.151

1.901 1.150

1.905 1.152

1.833

1.952

1.971

1.355 1.765 1.151

1.353 1.900 1.150

1.353 1.905 1.152

178.6 88.6 93.7

179.9 87.7 91.0

178.8 88.7 90.1

Bond Angles M-B-C

137.2 139.0 135.8 137.5 137.7 137.2 139.0

M-B-N B-M--CO 88.3 C(O)-M-C(O) 93.6 a

137.5 137.7 137.2 137.4 137.9

137.4 137.9 137.9

179.1

178.9

178.1

87.4 90.7

88.2 90.5

88.4 94.3

87.9 91.5

88.3 90.9

Distances are in A˚, and angles are in degrees.

Geometries. The important optimized bond lengths and angles of the terminal cationic borylene complexes [(η5C5H5)(CO)2M{B(η5-C5Me5)}]þ (I, M = Fe; II, M = Ru; III, M=Os), [(η5-C5H5)(CO)2M(BMes)]þ (IV, M = Fe; V, M = Ru; VI, M = Os) and [(η5-C5H5)(CO)2M(BNMe2)]þ (VII, M=Fe; VIII, M=Ru; IX, M=Os) at the BP86/TZ2P level are presented in Table 1. The optimized geometries of the ruthenium complexes (II, V, VIII) are shown in Figure 2. The calculated geometries of terminal cationic borylene complexes [(η5-C5H5)(CO)2Fe{B(η5-C5Me5)}]þ, [(η5-C5H5)(CO)2Fe(BMes)]þ, [(η5-C5H5)(CO)2Fe(BNMe2)]þ, and [(η5C5H5)(CO)2Ru(BNMe2)]þ at different levels of theory (BLYP/LANL2DZ or B3LYP/LANL2DZ) have been reported previously.14,18,19,43 Because the present calculations

were carried out with larger basis sets, we believe that they are more reliable. The structural data for the iron and ruthenium terminal cationic transition-metal borylene complexes were reported (see Chart 2). Although osmium complexes are not known so far, we have also calculated the structures of osmium borylene complexes (III, VI, and IX). The optimized bond distances for Fe-B and Ru-B are in close agreement with the experimental values. The M-B bond distances in the complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ (I, M = Fe; II, M=Ru; III, M=Os; 2.007, 2.112, and 2.122 A˚ for I-III, respectively) are similar to those expected for single bonds on the basis of covalent radii predictions (Fe-B = 1.99 A˚, Ru-B = 2.07 A˚, Os-B = 2.08 A˚).58 B-C(η5-C5Me5) bond distances (about 1.81 A˚) are significantly longer than normal B-C single bonds (1.59 A˚)58 but are similar to those reported for the B-η5-bonded Cp* in the X-ray structure of [B(η5-C5Me5)(η1-C5Me5)]þ.59 It can be inferred from the structural data that there is a single M-B bond in complexes I-III. The M-B bond distances in the complexes [(η5-C5H5)(CO)2M(BMes)]þ (IV, M = Fe; V, M = Ru; VI, M = Os; 1.817, 1.932, and 1.955 A˚ for IV-VI, respectively) and [(η5C5H5)(CO)2M(BNMe2)]þ (VII, M = Fe; VIII, M = Ru; IX, M = Os; 1.833, 1.952, and 1.977 A˚ for VIII-IX, respectively) are shorter than those expected for single bonds on the basis of covalent radii predictions (Fe-B = 1.99 A˚, Ru-B = 2.07 A˚, Os-B = 2.08 A˚).58 Using the relationship between bond order and bond distance suggested by Pauling,60 the optimized M-B

(53) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899–926. (54) Schaftenaar, G. MOLDEN3.4; CAOSCAMM Center, The Netherlands, 1998. (55) Diefenbach, A.; Bickelhaupt, M. B.; Frenking, G. J. Am. Chem. Soc. 2000, 122, 6449–6458. (56) Pandey, K. K. Coord. Chem. Rev. 2009, 253, 37–55. (57) Pandey, K. K.; Lled os, A. Inorg. Chem. 2009, 48, 2748–2759.

(58) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon: Oxford, U.K., 1984. (59) Voigt, A.; Filipponi, S.; Macdonald, C. L. B.; Gorden, J. D.; Cowley, A. H. Chem. Commun. 2009, 911–912. (60) Pauling L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: New York, 1960; p 239. The relationship of bond order to length is given by dn = d1 - 0.71 log n, where n is the bond order and d1 and dn are the lengths of bonds with bond orders 1 and n, respectively.

determinant of the molecule, which results from superimposing both fragments, to obey the Pauli principle through antisymmetrization and renormalization. The last term in eq 3, ΔEorb, gives the stabilizing orbital interactions between occupied and virtual orbitals of the two fragments. ΔEorb can be further partitioned into contributions by the orbitals that belong to different irreducible representations of the point group of the system. It has been suggested that the covalent and electrostatic character of a bond is given by the ratio ΔEelstat/ΔEorb.41,42,55-57 The electronic structures of the complexes were examined by NBO analysis.53 All MO pictures were made by using the MOLDEN program.54

Results and Discussion

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Figure 2. Optimized geometries of ruthenium borylene complexes [(η5-C5H5)(CO)2Ru{B(η5-C5Me5)}]þ (II), [(η5-C5H5)(CO)2Ru(BMes)]þ (V), and [(η5-C5H5)(CO)2Ru(BNMe2)]þ (VIII). Selected bond lengths and angles are given in Table 1.

bond distances correspond to Pauling bond orders of 1.73 (IV), 1.56 (V), 1.50 (VI), 1.66 (VII), 1.47 (VIII), and 1.42 (IX). The results reveal that the M-B bond in the complexes IV-IX is not truly a double bond, suggesting that M-B π-bonding may be weaker in these complexes. The B-C optimized bond distances in IV-VI (1.487 A˚) are also shorter than expected for single bonds on the basis of covalent radii predictions (B-C = 1.59 A˚)58 and correspond to a Pauling bond order of 1.40. The B-N bond distances in the complexes [(η5-C5H5)(CO)2M(BNMe2)]þ (nearly 1.354 A˚) correspond to a Pauling bond order of 1.71. The B-N π-bonding in the complexes VII-IX is stronger than the B-C π-bonding in the complexes IV-VI. The M-B-C/N bond angles in IV-IX deviate slightly from linearity. In the three series of compounds a steady increase of the M-B bond distance is found on going from iron to osmium: for I-III, 2.007 A˚ (I), 2.112 A˚ (II), 2.122 A˚ (III); for IV-VI, 1.817 A˚ (IV), 1.932 A˚ (V), 1.955 A˚ (VI); for VII-IX, 1.833 A˚ (VII), 1.952 A˚ (VIII), 1.971 A˚ (IX). Bonding Analysis of M-BR and MdBR Bonds. Structural analysis has shown significant differences between the M-B bonds in complexes I-III and complexes IV-IX. A deeper insight into these differences can be obtained with theoretical bonding analysis. We begin the analysis of the bonding situation of M-B and MdB bonds in the borylene complexes I-IX with a discussion of bond orders and atomic charges. Table 2 gives the Wiberg bond indices (WBI),61 the NPA charges, and the results of the natural bond orbital (NBO) analysis. Table 2 shows that the WBI values of the M-B bonds in the complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ (I, M = Fe; II, M = Ru; III, M = Os) are smaller than the WBI (61) Wiberg, K. B. Tetrahedron 1968, 24, 1083–1096.

Chart 2. Selected Structurally Characterized Cationic Terminal Metal Borylene Complexes

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Table 2. Wiberg Bond Indices (WBI), NPA Charges, and Results of the NBO Analysis in the Cationic Terminal Borylene Complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ, [(η5-C5H5)(CO)2M(BMes)]þ, and [(η5-C5H5)(CO)2M(BNMe2)]þ (M = Fe, Ru, Os) [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ

[(η5-C5H5)(CO)2M(BMes)]þ

[(η5-C5H5)(CO)2M(BNMe2)]þ

M = Fe (I) M = Ru (II) M = Os (III) M = Fe (IV) M = Ru (V) M = Os (VI) M = Fe (VII) M = Ru (VIII) M = Os (IX) WBI M-B B-C B-N M-C(CO) C-O

0.55 0.34

0.63 0.35

0.84 2.13

0.95 2.13

0.69 0.35 1.09 1.07 2.09

0.92 0.95 1.11 0.98 2.22

0.88 0.99 1.14 0.86 2.20

0.98 1.00

0.74

0.87

0.94

0.96 2.17

0.76 2.21

0.86 2.22

0.98 2.17

NPA Charges M B C5H5 C5Me5 Mes NMe2 CO

-0.41 0.61 0.19 0.15

-0.21 0.58 0.03 0.18 -0.23

0.23

0.21

0.03 0.52 -0.01 0.18 -0.20 -0.28 0.14

-1.34 1.11 0.58

-0.28 0.85 0.13

-0.07 0.77 0.06

-0.51 0.94 0.29

-0.27 0.87 0.13

-0.05 0.80 0.08

-0.20 -0.25 0.44

-0.23 0.25

0.22

0.28

0.26

0.20

1.819 54.28 37.12 0.11 62.76 0.02

1.752 55.72 36.38 0.06 63.56 0.00

1.815 53.31 36.03 0.09 63.87 0.02

1.733 47.22 33.58 11.36 54.28 0.78

45.72 48.58 51.32

44.28 58.52 41.32 0.10

46.69 57.58 42.37 0.16

52.78 56.56 43.38 0.05

NBO Bond Analysis M-B σ Bond occupation %M %s %p %d %f

1.765 49.43 35.26 0.14 64.60 0.00

1.806 48.92 35.96 0.14 63.89 0.01

1.838 46.99 35.65 0.13 64.21 0.01

1.740 64.88 26.69 16.10 58.21 0.00

1.777 56.51 37.12 0.11 62.75 0.02 B

%B %s %p %d

50.57 59.94 40.06 0.00

51.08 54.00 45.99 0.01

53.01 51.53 48.46 0.01

35.14 51.60 48.12 0.28

values of the MdB bonds in the complexes [(η5-C5H5)(CO)2M(BMes)]þ (IV, M = Fe; V, M = Ru; VI, M = Os) and [(η5-C5H5)(CO)2M(BNMe2)]þ (VII, M=Fe; VIII, M= Ru; IX, M=Os). The WBI values of the B-C bonds (0.35) in the complexes I-III are significantly lower than the WBI values of B-C bonds (1.0) in complexes IV-VI and of B-N bonds (1.0) in complexes VII-IX. One can infer from these observations that there is some multiple bonding in the M-B, B-C/B-N bonds of the complexes IV-IX. The calculated natural population analysis (NPA) charge distributions indicate that the C5Me5 group carries positive charge in complexes I-III, while the Mes and NMe2 groups are negatively charged in the complexes IV-IX. The boron atom carries significantly larger positive charge in IV-IX than in I-III. On going from iron to osmium in complexes I-IX, the negative charge on metal decreases sharply. A more definitive picture of M-B bonding is obtained through NBO analysis of the delocalized Kohn-Sham orbitals. The characteristics of the M-B σ-bonding orbitals are given in Table 2. The M-B π-bonding orbital is not observed in the NBO analysis of all the complexes under investigation. This indicates weaker M-B π bonding even in the borylene complexes IV-IX (as noticed by Pauling bond order). In the complexes I-III the M-B σ bonding orbital is slightly polarized toward the boron atom (almost similar contributions of metal and boron atoms), while in the complexes IV-VIII the M-B σ-bonding orbital is polarized toward the metal atom. The occupations for M-B σ-bonding orbitals are nearly

43.49 48.39 51.51 0.10

1.80. On going from iron to osmium, the percentage of s character at the boron of the M-B σ-bond atom decreases. Energy Analysis of M-B and MdB Bonding. In addition to the charge decomposition analysis using the NBO method, we also carried out an energy decomposition analysis of the M-B and MdB bonds in the terminal cationic borylene complexes I-IX. The results are given in Table 3 and Figure 3. Energy decomposition analysis of the neutral borylene complexes was reported by Frenking et al.41,42 Table 3 gives bond dissociation energies (BDE) for M-B bonds. The M-B bonds in cationic metal borylene complexes are quite strong. The values are remarkably greater than those calculated for neutral borylene complexes. For instance, the calculated M-B BDE in (CO)4Fe{B(η5-C5H5)} was 78 kcal mol-1,41 whereas it is 110.5 kcal mol-1 in [(η5C5H5)(CO)2Fe{B(η5-C5Me5)}]þ. BDE values of 151.0 and 125.9 kcal mol-1 have been previously reported for [(η5C5H5)(CO)2Fe(BMes)]þ and [(η5-C5H5)(CO)2Fe(BNMe2)]þ, respectively.43 Our values, obtained with larger basis sets, are notably lower (121.4 and 102.2 kcal mol-1, respectively). The tabulated results for osmium complexes reveal the expected periodic trend in bond strengths due to d-orbital extent: the Os-B bonds are slightly stronger than the corresponding bonds in iron and ruthenium complexes. On going from [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ to [(η5-C5H5)(CO)2M(BNMe2)]þ complexes, we note a steady increase in bond dissociation energy. It is surprising that M-B bond dissociation energies for the complexes [(η5-C5H5)(CO)2M(BNMe2)]þ are smaller than M-B bond dissociation energies for the complexes

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Table 3. Energy Decomposition Analysisa of Cationic Terminal Borylene Complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ, [(η5-C5H5)(CO)2M(BMes)]þ, and [(η5-C5H5)(CO)2M(BNMe2)]þ (M = Fe, Ru, Os) at the BP86/TZ2P Level [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ M = Fe (I) ΔEint ΔEPauli ΔEelstatb ΔEorb ΔEσ(a0 ) ΔEπ(a00 )c ΔEprep ΔE(-BDE)d

-118.1 184.0 -183.0 (60.6) -119.1 -107.5 -11.6 (9.7) 7.6 -110.5

[(η5-C5H5)(CO)2M(BMes)]þ

[(η5-C5H5)(CO)2M(BNMe2)]þ

M = Ru (II) M = Os (III) M = Fe (IV) M = Ru (V) M = Os (VI) M = Fe (VII) M = Ru (VIII) M = Os (IX) -120.7 206.8 -208.4 (63.6) -119.1 -108.1 -11.0 (9.2) 9.7 -111.0

-131.4 243.9 -244.6 (65.2) -130.7 -110.0 -11.7 (9.0) 9.5 -121.9

-125.3 261.8 -240.4 (62.1) -146.7 -122.8 -23.9 (16.3) 3.9 -121.4

-127.8 298.6 -277.8 (65.2) -148.6 -123.9 -24.6 (16.6) 6.7 -121.1

-141.3 343.5 -324.1 (66.8) -160.9 -134.6 -26.3 (15.7) 7.4 -133.9

-110.9 230.6 -210.9 (61.8) -130.6 -108.5 -22.0 (16.8) 8.5 -102.2

-112.1 258.1 -239.5 (64.7) -130.7 -108.1 -22.6 (17.3) 5.2 -106.9

-125.3 299.6 -280.8 (66.1) -144.1 -119.4 -24.7 (17.1) 5.9 -119.4

a Energy contributions in kcal mol-1. b The values in parentheses are the percentage contributions to the total attractive interactions, reflecting the ionic character of the bond. c The values in parentheses are the percentage contributions of π bonding to the total orbital interactions ΔEorb. d Bond dissociation energy with negative sign.

[(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ. We will discuss this feature from the energy decomposition analysis. Figure 3 shows the trends in bond dissociation energy and the different terms of the energy decomposition analysis for all the complexes studied. The breakdown of the interaction energy ΔEint into the repulsive term ΔEPauli and the attractive interactions ΔEorb and ΔEelstat shows that ΔEPauli has the largest absolute value for the complexes IV-IX, while its absolute value is almost similar to the electrostatic interactions for the complexes I-III (Table 3 and Figure 3). The calculated data given in Table 3 show that the relative interaction energies can be arranged in the following order: [(η5-C5H5)(CO)2M(BNMe2)]þ < [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ < [(η5-C5H5)(CO)2Ru(BMes)]þ. The contribution of the electrostatic interaction ΔEelstat is significantly larger in all borylene complexes I-IX than the covalent bonding term ΔEorb: that is, the [M]-BR bonding in the cationic borylene complexes I-IX has a greater degree of ionic character (60.6-66.8%). Table 3 also gives a breakdown of the orbital interactions ΔEorb, into contributions of MrBR σ donation and MfBR π back-donation. It is significant to note that the π-bonding contribution is, in all complexes, much smaller (9.0-17.3% of total orbital contributions) than the σ-bonding contribution. Similar results have been reported for neutral terminal borylene complexes by Frenking et al.41,42 A relatively larger π contribution is found in the complexes [(η5-C5H5)(CO)2M(BNMe2)]þ, and a smaller π contribution is observed in the complexes [(η5C5H5)(CO)2M{B(η5-C5Me5)}]þ. The largest π contribution is found in the ruthenium borylene complex [(η5-C[5H5)(CO)2Ru(BNMe2)]þ, where the RufB π back-donation has 17.3% of the ΔEorb (Table 3). The complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ are somewhat different from the complexes [(η5-C5H5)(CO)2M(BNMe2)]þ and [(η5-C5H5)(CO)2M(BMes)]þ, as they contain a nido-pentacarbahexaborane as a ligand. In this case, all orbitals of the apical boron, apart from the exohedral one, which carries the formal lone pair, are engaged in the cluster skeleton. Hence, a π back-bond is not possible without cluster breaking/ rearrangement. The results reveal that, in addition to the B(C5Me5) ligand, BMes and BNMe2 borylene ligands dominantly behave as σ donors. It is clear from these observations that the M-B bonding in the terminal borylene complexes [(η5-C5H5)(CO)2M(BMes)]þ and [(η5-C5H5)(CO)2M(BNMe2)]þ is not truly MdB double bonding. We want to clarify why the bond dissociation energy (BDE) and the interaction energy (ΔEint) are unexpectedly smaller for the complexes [(η5-C5H5)(CO)2M(BNMe2)]þ

Figure 3. Values of the energy contributions of the Pauli repulsive interactions, bond dissociation energy, π bonding, σ bonding, orbital interaction (covalent interaction), and electrostatic interaction (ionic contribution) to the M-B bonding in the terminal borylene complexes [(η5-C5H5)(CO)2M{B(η5C5Me5)}]þ (I, M = Fe; II, M = Ru; III, M = Os), [(η5C5H5)(CO)2M(BMes)]þ (IV, M = Fe; V, M = Ru; VI, M = Os) and [(η5-C5H5)(CO)2M(BNMe2)]þ (VII, M = Fe; VIII, M = Ru; IX, M = Os).

than for the complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ. In complexes I-III the repulsive term ΔEPauli is almost perfectly counterbalanced by the attractive electrostatic contribution ΔEelstat. In contrast, in complexes with the BNMe2 ligand (VII-IX), ΔEPauli is about 20 kcal mol-1 greater than ΔEelstat. A similar trend is found in complexes IV-VI, but in these compounds with BNMe2 the larger ΔEorb contribution (about 15 kcal mol-1 larger) leads to the highest BDE. We note that, on going from iron to osmium, the interaction energy increases in all three sets of complexes. Further, the osmium complexes possess both the highest orbital interactions (ΔEorb) and the highest electrostatic interactions (ΔEelstat). Aldridge and co-workers reported bond dissociation energy analysis of Fe-B bonds in [(η5-C5R5)(CO)2Fe(BMes)]þ (R = Me, H), [(η5-C5H5)(CO)2Fe(BNMe2)]þ, and related complexes with a BLYP exchange-correlation functional.43 The σ and π contributions to the bonding energies were determined using an adapted Mulliken analysis, which separates the bonding density into contributions from atomic orbitals of σ and π symmetry. Our calculated values of the π contribution to the metal-boron bond are roughly half what has been reported previously.43 The discrepancy between

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Figure 4. Plot of some relevant orbitals of the iron borylene complexes (A) [(η5-C5H5)(CO)2Fe{B(η5-C5Me5)}]þ (I), (B-D) [(η5-C5H5)(CO)2Fe(BMes)]þ (IV), and (E-G) [(η5-C5H5)(CO)2Fe(BNMe2)]þ (VII).

energy-partitioned schemes and charge decomposition analysis in the relative contributions from σ and π symmetry covalent interactions to the M-E bonds was previously noted.43 To visualize the differences in M-B and MdB bonding in terminal cationic borylene complexes, envelope plots of some relevant orbitals of the iron complexes [(η5-C5H5)(CO)2Fe{B(η5-C5Me5)}]þ (I), [(η5-C5H5)(CO)2Fe(BMes)]þ (IV), and [(η5-C5H5)(CO)2Fe(BNMe2)]þ (VII) are given in Figure 4. Figure 4A gives a pictorial description of the Fe-B σ bonding in complex I. It is important to note that a Fe-B π orbital has not been found in complex I. Figure 4B shows Fe-B σ bonding in complex IV, while parts C and D of Figure 4 give a pictorial description of the Fe-B π orbital and B-C(Mes) π orbital, respectively, in the complex [(η5C5H5)(CO)2Fe(BMes)]þ (IV). Figure 4E shows Fe-B σ bonding and parts F and G of Figure 4 represent Fe-B and B-N π orbitals in the complex [(η5-C5H5)(CO)2Fe(BNMe2)]þ (VII). Fe-B π orbitals are perpendicular to the B-C π orbital in IV and B-N π orbitals in VII.

(I, M=Fe; II, M=Ru; III, M=Os) correspond to a Pauling single-bond order. In contrast, the optimized M-B bond distances in the complexes [(η5-C5H5)(CO)2M(BMes)]þ and [(η5-C5H5)(CO)2M(BNMe2)]þ correspond to a Pauling bond order of 1.73-1.42. The contribution of the electrostatic interaction ΔEelstat is significantly larger in all borylene complexes I-IX than the covalent bonding ΔEorb: that is, the [M]-BR bonding in the cationic borylene complexes I-IX has a greater degree of ionic character (60.6-66.8%). The orbital interactions between metal and boron in [(η5C5H5)(CO)2M{B(η5-C5Me5)}]þ, [(η5-C5H5)(CO)2M(BMes)]þ, and [(η5-C5H5)(CO)2Fe(BNMe2)]þ arise mainly from MrBR σ donation. The π-bonding contribution is, in all complexes, much smaller (9.0-17.3% of total orbital contributions) than the σ-bonding contribution.

Conclusion

Acknowledgment. Financial support from the Spanish MICINN (Grants CTQ2008-06866-CO2-01/BQU and CTQ2008-06866-CO2-02/BQU), Consolider Ingenio 2010 (Projects CSD2006-0003 and CSD2007-00006), Generalitat de Catalunya (Distinci o per a la Promoci o de la Recerca Universitaria 2004), and the ICIQ foundation is gratefully acknowledged.

We have presented an energy analysis where the bonding situations in M-B and MdB bonds in terminal cationic borylene complexes are compared. The M-B bond distances in the complexes [(η5-C5H5)(CO)2M{B(η5-C5Me5)}]þ

Supporting Information Available: Tables giving Cartesian coordinates of the optimized geometries of the terminal cationic metal borylene complexes I-IX. This material is available free of charge via the Internet at http://pubs.acs.org.