Structure and Bonding Energy Analysis of Cobalt, Rhodium, and

Dec 4, 2009 - Blank , B.; Colling-Hendelkens , M.; Kollann , C.; Radacki , K.; Rais , D.; Uttinger , K.; Whittell , G. R.; Braunschweig , H. Chem.—E...
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Organometallics 2010, 29, 142–148 DOI: 10.1021/om900896f

Structure and Bonding Energy Analysis of Cobalt, Rhodium, and Iridium Borylene Complexes [(η5-C5H5)(CO)M(BNX2] (X = Me, SiH3, SiMe3) and [(η5-C5H5)(PMe3)M{BN(SiH3)2}] (M = Co, Rh, Ir) Krishna K. Pandey*,† and Djamaladdin G. Musaev*,‡ †

‡

School of Chemical Sciences, Devi Ahilya University Indore, Indore 452001, India and Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, Georgia 30322 Received October 13, 2009

Geometry, electronic structure, and bonding analysis of the terminal neutral borylene complexes of cobalt, rhodium, and iridium [(η5-C5H5)(CO)M(BNMe2)] (I, M = Co, II, M = Rh, III, M = Ir), [(η5-C5H5)(CO)M{BN(SiH3)2)}] (IV, M = Co, V, M = Rh, VI, M = Ir), [(η5-C5H5)(CO)M{BN(SiMe3)2)}] (VII, M = Co, VIII, M = Rh, IX, M = Ir), and [(η5-C5H5)(PMe3)M{BN(SiH3)2}] (X, M = Co, XI, M = Rh, XII, M = Ir) were investigated at the BP86 level of theory. The calculated geometry parameters of iridium borylene complex [(η5-C5H5)(CO)Ir{BN(SiMe3)2}] are in excellent agreement with their available experimental values. Pauling bond order of the optimized structures of I-XII shows that the M-B bonds in these complexes are nearly MdB double bonds, which is also supported by the performed energy decomposition analysis. The orbital interactions between the metal and boron arise mainly from MrBNX2 σ-donation. In all complexes, the π-bonding contribution is smaller (26.2-37.0% of total orbital contributions) and increases via M = Rh < Co < Ir. In all the complexes, the M-B π-bond orbitals are highly polarized toward the metal atom. Thus, in the BNX2 ligands, boron dominantly behaves as a σ-donor. The calculated MdBNX2 interaction energy increases in all four sets of complexes in the order Co e Rh < Ir. The contributions of the electrostatic interactions, ΔEelstat, are significantly larger in all studied borylene complexes than the covalent bonding ΔEorb: the MdBNX2 bonding in the neutral borylene complexes has a greater degree of ionic character (61.2-68.5%). The iridium complexes possess the highest orbital interactions, ΔEorb, and electrostatic interactions, ΔEelstat.

1. Introduction In the eleven years since the first report of structurally characterized terminal transition metal borylene complexes in 1998,1,2 the research of this field has blossomed rapidly in terms of synthesis, structure, bonding, and reactivity. So far

12 structurally characterized neutral terminal transition metal borylene complexes1-12 and nine terminal cationic transition metal borylene complexes13-19 (see Charts 1 and 2 of the Supporting Information, respectively) have been reported. Additionally, a number of base-stabilized adducts formed by the coordination of a Lewis base (main group or

*Corresponding authors. (D.G.M.) [email protected]; (K.K.P.) [email protected]. (1) Braunschweig, H.; Kollan, C.; Englert, U. Angew. Chem., Int. Ed. 1998, 37, 3179. (2) Cowley, A. H.; Lomeli, V.; Voigt, A. J. Am. Chem. Soc. 1998, 120, 6401. (3) Braunschweig, H.; Colling, M.; Kollann, C.; Stammler, H. G.; Neumann, B. Angew. Chem., Int. Ed. 2001, 40, 2298. (4) Braunschweig, H.; Colling, M.; Kollann, C.; Merz, K.; Radacki, K. Angew. Chem., Int. Ed. 2001, 40, 4198. (5) Braunschweig, H.; Colling, M.; Hu, C.; Radacki, K. Angew. Chem., Int. Ed. 2003, 43, 205. (6) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K. Angew. Chem., Int. Ed. 2006, 45, 162. (7) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K. Organometallics 2006, 25, 5159. (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. (9) Braunschweig, H.; Burzler, M.; Kupfer, T; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 7785. (10) Braunschweig, H.; Forster, M.; Kupfer, T; Seeler, F. Angew. Chem., Int. Ed. 2008, 47, 5981.

(11) Braunschweig, H.; Kupfer, T.; Radacki, K.; Schneider, A.; Seeler, F.; Uttinger, K.; Wu, H. J. Am. Chem. Soc. 2008, 130, 7974. (12) Alcaraz, G.; Helmstedt, U.; Clot, E.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2008, 130, 12878. (13) Coombs, D. L.; Aldridge, S.; Jones, C.; Willock, D. J. J. Am. Chem. Soc. 2003, 125, 6356. (14) Coombs, D. L.; Aldridge, S.; Rossin, A.; Jones, C.; Willock, D. J. Organometallics 2004, 23, 2911. (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. (b) Kays, D. L.; Day, J. K.; Aldridge, S.; Harrigton, R. W.; Clegg, W. Angew. Chem., Int. Ed. 2006, 45, 3513. (c) Pierce, G. A.; Aldridge, S.; Jones, C.; Gans-Eichler, T.; Stasch, A.; Cooms, M. D.; Willock, D. J. Angew. Chem., Int. Ed. 2007, 46, 2043. (16) Vidovic, D.; Findlater, M.; Reeske, G.; Cowley, H. Chem. Commun. 2006, 3786. (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. (19) De, S.; Pierce, G. A.; Vidovic, D.; Kays, D. L.; Coombs, N. D.; Jemmis, E. D.; Aldridge, S. Organometallics 2009, 28, 2961.

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transition metal) to the two- or three-coordinate boron center have also been prepared.20-32 Understanding the nature of M-borylene bonding in these species is of utmost importance and could lead to synthesis of new transition metal-borylene complexes with interesting reactivity. Theoretical approaches have been proven to be an indispensible part of the studies of terminal metal borylene complexes.33-42 Previously, the BLYP/LANL2DZ and B3LYP/LANL2DZ approaches have been applied to study the geometry and electronic structure 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 [(η5-C5H5)(CO)2Ru(BNMe2)]þ.14,18,19,43 To the best of our knowledge, the structure and MdBNX2 bonding analysis of the terminal neutral metal borylene complexes of cobalt, rhodium, and iridium have never been studied before. Here, for the first time, we report the geometry and electronic structure of, as well as nature of, MdBNX2 bonds in the terminal neutral metal borylene complexes of cobalt, rhodium, and iridium complexes [(η5-C5H5)(CO)M(BNMe2)] (I, M = Co, II, M = Rh, III, M = Ir), [(η5-C5H5)(CO)M{BN(SiH3)2)}] (IV, M = Co, V, M = Rh, VI, M = Ir), [(η5C5H5)(CO)M{BN(SiMe3)2)}] (VII, M = Co, VIII, M = Rh, IX, M = Ir), and [(η5-C5H5)(PMe3)M{BN(SiH3)2}] (X, M = Co, XI, M = Rh, XII, M = Ir). We intend to answer two questions: one of them addresses the degree of ionic and covalent character of the M-B, while the second question addresses the extent of the MfB π-back-bonding contribution to the MdBNX2 bond (see Figure 1 for schematic presentation of the MdBNX2 bond). We elucidate the role of the transition metal atoms and substituent X of BNX2 ligands in the stability of the M-BNX2 bond. One should note that cobalt borylene complexes studied in this paper are not known so far, and an X-ray structure of the rhodium (20) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786. (21) Braunschweig, H.; Colling, M. J. Organomet. Chem. 2000, 614-615, 18. (22) Braunschweig, H.; Colling, M. Coord. Chem. Rev. 2001, 223, 1. (23) Braunschweig, H.; Colling, M. Eur. J. Inorg. Chem. 2003, 393. (24) Braunschweig, H. Adv. Organomet. Chem. 2004, 51, 163. (25) Braunschweig, H.; Rais, D. Heteroat. Chem. 2005, 16, 566. (26) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006, 45, 5254. (27) Braunschweig, H.; Kollann, C.; Seeler, F. Struct. Bonding (Berlin) 2008, 130, 1. (28) Anderson, C. E; Braunschweig, H.; Dewhurst, R. D. Organometallics 2008, 27, 6381. (29) Braunschweig, H.; Dewhurst, R. D. Angew. Chem., Int. Ed. 2009, 48, 1893. (30) Aldridge, S.; Coombs, D. L. Coord. Chem. Rev. 2004, 248, 535. (31) Aldridge, S.; Kays, D. L. Main Group Chem. 2006, 5, 223. (32) Vidovic, D.; Pierce, G. A.; Aldridge, A. Chem. Commun. 2009, 1157. (33) Ehlers, A. W.; Baerends, E. J.; Bickelhaupt, F. M.; Radius, U. Chem.;Eur. J. 1998, 4, 210. (34) Radius, U.; Bickelhaupt, F. M.; Ehlers, A. W.; Goldberg, N.; Hoffmann, R. Inorg. Chem. 1998, 37, 1080. (35) Macdonald, C. L. B.; Cowley, A. H. J. Am. Chem. Soc. 1999, 121, 12113. (36) Boehme, C.; Frenking, G. Chem.;Eur. J. 1999, 5, 2184. (37) Uddin, J.; Boehme, C.; Frenking, G. Organometallics 2000, 19, 571. (38) Chen, Y.; Frenking, G. Dalton Trans. 2001, 434. (39) Uddin, J.; Frenking, G. J. Am. Chem. Soc. 2001, 123, 1683. (40) Bollwein, T.; Brothers, P. J.; Hermann, H. L.; Schwerdtfeger, P. Organometallics 2002, 21, 5236. (41) Boehme, C.; Uddin, J.; Frenking, G. Coord. Chem. Rev. 2000, 197, 249. (42) Frenking, G.; Frohlich, N. Chem. Rev. 2000, 100, 717. (43) Aldridge, S.; Rossin, A.; Coombs, D. L.; Willock, D. J. Dalton Trans. 2004, 2649.

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

borylene complex [(η5-C5H5)(CO)Rh(BN(SiMe3)2))] is not available yet.10

2. Computational Procedure Calculations of the neutral terminal borylene complexes [(η5C5H5)(CO)M(BNMe2)] (I, M = Co, II, M = Rh, III, M = Ir), [(η5-C5H5)(CO)M{BN(SiH3)2)}] (IV, M = Co, V, M = Rh, VI, M = Ir), [(η5-C5H5)(CO)M{BN(SiMe3)2)}] (VII, M = Co, VIII, M = Rh, IX, M = Ir), and [(η5-C5H5)(PMe3)M{BN(SiH3)2}] (X, M = Co, XI, M = Rh, XII, M = Ir) 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 the boron, carbon, nitrogen, and oxygen, (1s2s2p)10 core electrons of cobalt, (1s2s2p3s3p3d)28 core electrons of rhodium, and (1s2s2p3s3p3d4s4p4d)46 core electrons of iridium 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 between the metal {[(η5-C5H5)(CO)M and [(η5C5H5)(PMe3)M} and borylene BNX2 fragments in complexes I-XII has been analyzed at the Cs symmetry using the energy decomposition scheme of the ADF package, which is based on the Morokuma51 and Ziegler-Rauk52 methods. On the basis of these studies, the bond energy ΔE between the fragments can be decomposed as

ΔE ¼ ΔEint þ ΔE prep

ð1Þ

(44) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (45) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (46) (a) Chang, C.; Pelissier, M.; Durand, Ph. Phys. Scr. 1986, 34, 394. (b) Heully, J.-L.; Lindgren, I.; Lindroth, E.; Lundquist, S.; MartenssonPendrill, A.-M. J. Phys. B 1986, 19, 2799. (c) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597. (d) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1996, 105, 6505. (e) van Lenthe, E.; van Leeuwen, R.; Baerends, E. J.; Snijders, J. G. Int. J. Quantum Chem. 1996, 57, 281. (f) van Lenthe, E.; Ehlers, A. E.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943. (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. (49) Krijn, J.; Baerends, E. J. Fit Functions in the HFS-Method, Internal Report (in Dutch); Vrije Universiteit: Amsterdam, The Netherlands, 1984. (50) Baerends, E. J. ADF 2008-01; Scientific Computing & Modelling NV: The Netherlands. (51) (a) Morokuma, K. J. Chem. Phys. 1971, 55, 1236. (b) Morokuma, K. Acc. Chem. Res. 1977, 10, 294. (52) (a) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1. (b) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1558. (c) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1755.

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Table 1. Selected Optimized Geometrical Parameters for Neutral Terminal Borylene Complexes [(η5-C5H5)(CO)M(BNX2] (X = Me, SiH3, SiMe3) and [(η5-C5H5)(PMe3)M{BN(SiH3)2}] (M = Co, Rh, Ir),a I-XII [(η5-C5H5)(CO)M(BNMe2)] M= Co, I

M= Rh, II

M= Ir, III

[(η5-C5H5)(CO)M{BN(SiH3)2}] M= Co, IV

M= Rh, V

M= Ir, VI

[(η5-C5H5)(CO)M{BN(SiMe3)2] M= Co, VII

M= Rh, VIII

M= Ir, IX

[(η5-C5H5)(PMe3)M{BN(SiH3)2] M= Co, X

M= Rh, XI

M= Ir, XII

Bond Distances 1.747

1.849

1.861

1.407

1.407

1.406

2.114

2.233

2.235

1.768 2.053 2.101 2.101 2.131 2.131

1.771 2.257 2.334 2.334 2.384 2.384

1.768 2.256 2.328 2.328 2.398 2.398

179.7

179.5[175.9(3)]b 179.5

179.5

179.7

117.5

116.8

117.2

119.8

119.1

118.7

125.0 180.0

126.4 179.5

125.6 179.5

120.4

121.8

122.6

M-B

1.767

1.873

1.884

1.761

1.878

1.886

1.780

1.885

B-N

1.378

1.377

1.377

1.389

1.386

1.386

1.382

1.378

M-CO M-P C-O N-C N-Si M-C(Cp)

1.704

1.841

1.843

1.707

1.843

1.844

1.700

1.834

1.897 [1.892(3)]b 1.379 [1.365(4)]b 1.839

1.170 1.464

1.166 1.465

1.169 1.463

1.169

1.165

1.169

1.172

1.169

1.171

2.069 2.110 2.110 2.125 2.125

2.321 2.255 2.255 2.364 2.364

2.268 2.329 2.329 2.381 2.381

1.780 2.068 2.110 2.110 2.123 2.123

1.781 2.261 2.329 2.329 2.365 2.365

1.781 2.267 2.329 2.329 2.374 2.374

1.813 2.072 2.114 2.114 2.124 2.124

1.815 2.267 2.335 2.335 2.367 2.367

1.813 2.270 2.334 2.334 2.382 2.382

178.5 122.5

179.9 122.2

175.6 122.2

178.5

179.4

179.2

178.0

119.8

119.6

119.7

115.1

115.5

115.6

170.8

179.4

179.4

120.3 179.9

120.7 178.7

120.4 178.6

Bond Angles M-B-N B-N-C B-N-Si C-N-C Si-N-Si M-C-O a

Distances are in A˚ and angles are in deg. b X-ray structure data for [(η5-C5Me5)(CO)Ir(BNSiMe3)2].10

where ΔEprep is the energy required to promote of the free fragments from their equilibrium structure in the electronic ground state to that which they take up in the molecule. It can be divided into two components as

E prep ¼ E total ðdistorted fragmentsÞ - E total ðfragments in the equilibrium structureÞ ð2Þ In eq 1, ΔEint is the instantaneous interaction energy between the two fragments of the molecule. It can be decomposed into three main components:

ΔEint ¼ ΔEelstat þ ΔE Pauli þ ΔEorb

ð3Þ

where ΔEelstat describes the classical Coulomb interaction between the fragments; ΔEPauli, which is called exchange repulsion or Pauli repulsion, takes into account the destabilizing two-orbital three- or four-electron interactions between the occupied orbitals of both fragments, and ΔEorb is orbital interactions between the occupied and virtual orbitals of the two fragments. It has been suggested that the covalent and electrostatic character of the bond can be given by the ratio ΔEelstat/ΔEorb.41,42,53-55 The electronic structures of the studied complexes were examined by NBO analysis.56 All MO pictures were made by using the MOLDEN program.57 One should note that the above-presented computational techniques were previously successfully applied by Frenking and co-workers58 to analyze the bonding in the numerous transition metal complexes.

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

3. Results and Discussion 3.1. Geometries. The important bond distances and angles of the complexes I-XII calculated at the BP86/TZ2P level of theory are presented in Table 1. Their geometry structures are shown in Figure 2 (only for the case of M = Rh, i.e., for complexes II, V, VIII, and XI; the structures of the M = Co and Ir complexes are very similar to those presented in this figure and therefore are not included in this figure; the optimized Cartesian coordinates of all studied complexes are given in the Supporting Information). As mentioned above, the studied cobalt borylene complexes are not known so far, and an X-ray structure of the rhodium borylene complex [(η5-C5H5)(CO)Rh(BN(SiMe3)2))] is not available yet.10 Therefore, we cannot compare the calculated values for studied Co and Rh complexes with the experimental data. However, as seen in Table 1, the calculated geometry parameters of iridium borylene complex [(η5-C5H5)(CO)Ir{BN(SiMe3)2}] are in excellent agreement with their available experimental values.10 We expect the same accuracy for the other studied metal-borylene complexes of Co, Rh, and Ir. As seen in Table 1, the M-B bond distances in the studied complexes (I-XII) are shorter than those expected for a M-B single bond estimated on the basis of covalent radii predictions (Co-B = 1.95 A˚, Rh-B = 2.09 A˚, Os-B = 2.06 A˚).59 Using the relationship between the bond order and bond distance suggested by Pauling,60 the Pauling bond (58) Frenking, G.; Wichmann, K.; Frohlich, N.; Loschen, C.; Lein, M.; Frunzke, J.; Rayon, V. M. Coord. Chem. Rev. 2003, 238-239, 55. (59) (a) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon: Oxford, 1984. (b) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960. (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, d1 and dn are the lengths of bonds with bond order 1 and n, respectively.

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Figure 2. Optimized geometries of rhodium borylene complexes [(η5-C5H5)(CO)Rh(BNMe2)] (II), [(η5-C5H5)(CO)Rh{BN(SiH3)2}] (V), [(η5-C5H5)(CO)Rh{BN(SiMe3)2}] (VIII), and [(η5-C5H5)(PMe3)Rh{BN(SiH3)2)] (XI). The important bond lengths and angles are given in Table 1.

orders of the optimized M-B bond distances in these complexes are 1.8 (I), 2.0 (II), 1.8 (III), 1.8 (IV), 2.0 (V), 1.8 (VI), 1.7 (VII), 1.9 (VIII), 1.7 (IX), 1.9 (X), 2.2 (XI), and 1.9 (XII). Thus, the M-B bonds in the complexes I-XII are nearly MdB double bonds. Upon going from M = Co to M = Ir, the calculated MdB bond distance increases in the order 1.767 A˚ (I) < 1.873 A˚ (II) < 1.884 A˚ (III); 1.761 A˚ (IV) < 1.878 A˚ (V) < 1.886 A˚ (VI); 1.780 A˚ (VII) < 1.885 A˚ (VIII) < 1.897 A˚ (IX); and 1.747 A˚ (X) < 1.849 A˚ (XI) < 1.861 A˚ (XII). On substitution of the CO ligand in I-III by PMe3, X-XII, the calculated MdB bond distances are slightly decreased: 1.767 A˚ (I) > 1.747 A˚ (X); 1.873 A˚ (II) > 1.849 A˚ (XI), and 1.884 A˚ (III) > 1.861 A˚ (XII). Furthermore, the nature of the substituent X of ligand BNX2 has an insignificant effect on the nature of the MdBNX2 bonding: the MdB bond distance in complexes I-IX only slightly increases upon X going from Me to SiH3 and SiMe3. The B-N optimized bond distances, 1.377-1.407 A˚, in I-XII are also shorter than that expected for the single B-N bond based on covalent radii predictions (B-N = 1.55 A˚).59 As seen in Table 1, the calculated B-N bond distances are within 1.377-1.389 and 1.406-1.407 A˚ for complexes I-IX and X-XII, respectively. Again, it changes only very slightly upon changing the X group of BNX2 from Me to SiH3 and SiMe3. Unexpectedly, the nature of the auxiliary ligand affects strongly the B-N bonding. Indeed, on replacing the CO ligand by PMe3 (i.e., upon going from complexes I-III to X-XII), the B-N bond of the corresponding complexes is elongated by about 0.03 A˚. The above-presented trend in geometries is consistent with the calculated Pauling bond orders of the optimized B-N bond distances in these complexes, which are ∼1.7 for I-IX and ∼1.6 for X-XII. Thus, the B-N bonding in X-XII is relatively weaker than the B-N bonding in complexes I-IX. The M-B-N bond

angles in these complexes are deviated slightly from linearity, and the B-N-C/Si bond angles are nearly 120°. 3.2. Bonding Analysis of the MdBR Bonds of the Complexes I-XII. We begin the analysis of the MdB bonding in the borylene complexes I-XII with a discussion of bond orders and atomic charges. In Table 2 we presented the calculated Wiberg bond indices (WBI),61 NPA charges, and the results of the natural bond orbital (NBO) analysis. A seen from Table 2, the WBI values of the MdB bonds of I-XII are in the range 0.97-1.33. Like the MdB bond distances, upon going from M = Co to M = Ir, the WBI values of the MdB bonds increase as 1.03 (I) < 1.10 (II) < 1.23 (III); 1.02 (IV) < 1.08 (V) < 1.21 (VI); 0.97 (VII) < 1.04 (VIII) < 1.17 (IX), and 1.15 (X) < 1.19 (XI) < 1.33 (XII). The substitution of the CO ligand by the PMe3 ligand, i.e., upon going from complexes I-III to X-XII, the WBI values of the MdB bonds in these complexes increase as 1.03 (I) < 1.15 (X); 1.10 (II) < 1.19 (XI) and 1.23 (III) < 1.33 (XII). It is significant to note that the WBI values for the MdB bonds are slightly larger than those for M-C(CO) bonds; that is, the MdBNX2 bonds in these borylene complexes are stronger than M-CO bonds. The WBI values for the B-N bonds are within 0.94-1.09 in complexes I-XII: they are slightly larger for complexes I-III, with a CO ligand, than complexes X-XII, with a PMe3 ligand. These trends are consistent with that reported above for the calculated geometries. As seen in Table 2, the calculated natural (NPA) charge distributions indicate that the C5H5 ligands carry 0.29-0.40e negative charge in these complexes, while the B atom and BNX2 groups are positively charged. The boron atom carries

(61) Wiberg, K. A. Tetrahedron 1968, 24, 1083.

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Table 2. Wiberg Bond Indices (WBI), NPA Charges, and Results of the NBO Analysis in Neutral Terminal Borylene Complexes [(η5C5H5)(CO)M(BNX2] (X = Me, SiH3, SiMe3) and [(η5-C5H5)(PMe3)M(BN(SiH3)2] (M = Co, Rh, Ir), I-XII [(η5-C5H5)(CO)M(BNMe2)]

[(η5-C5H5)(CO)M{BN(SiH3)2}]

[(η5-C5H5)(CO)M{BN(SiMe3)2]

M= Co, I

M= Co, IV

M= Co, VII

M= Rh, II

M= Ir, III

M= Rh, V

M= Ir, VI

M= Rh, VIII

M= Ir, IX

[(η5-C5H5)(PMe3)M{BN(SiH3)2] M= Co, X

M= Rh, XI

M= Ir, XII

WBI M-B B-N M-C(CO) C-O M-P

1.03 1.00 0.91 2.02

1.10 1.03 0.98 2.05

1.23 1.05 1.13 2.01

1.02 1.00 0.90 2.03

1.08 1.03 1.08 2.07

1.21 1.04 1.12 2.02

0.97 1.05 0.92 2.00

1.04 1.08 0.99 2.05

1.17 1.09 1.13 2.01

1.15 0.94

1.19 0.97

1.33 0.97

0.57

0.59

0.70

-0.03 0.49 -0.38 0.06

-0.16 0.58 -0.40 0.18

-0.10 0.51 -0.40 0.12

0.35

0.38

0.38

NPA Charges M B C5H5 BNX2 CO PMe3

0.04 0.61 -0.31 0.27 -0.00

-0.02 0.65 -0.34 0.33 0.03

0.06 0.60 -0.34 0.28 0.00

0.04 0.61 -0.29 0.24 0.01

-0.03 0.66 -0.32 0.30 0.05

0.07 0.60 -0.33 0.25 0.01

0.04 0.61 -0.32 0.29 -0.01

-0.02 0.66 -0.36 0.35 0.03

0.05 0.61 -0.36 0.31 0.00

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

1.771 44.32 50.63 0.02 49.35 0.00 55.68 59.94 40.06 0.00

1.821 46.35 50.48 0.02 49.48 0.02 53.65 67.61 32.35 0.04

1.870 48.55 51.04 0.01 48.93 0.02 51.45 58.72 41.24 0.04

1.775 43.69 50.53 0.07 49.40 0.00 56.31 73.82 26.10 0.08

1.816 45.33 50.95 0.03 49.00 0.02 54.67 59.53 40.44 0.03

1.862 47.34 51.95 0.02 48.00 0.02 52.66 69.75 30.19 0.06

1.918 58.94 8.72 0.03 91.25 0.00 41.06 71.78 28.13 0.08

1.921 59.88 10.79 0.02 89.18 0.01 40.12 69.31 30.55 0.06

1.858 47.59 51.92 0.02 48.03 0.03 52.41 68.16 37.78 0.06

1.792 42.31 51.51 0.02 48.47 0.00 57.67 62.19 37.76 0.05

1.798 44.46 51.20 0.01 48.75 0.04 55.54 60.42 39.55 0.03

1.858 46.87 51.67 0.04 48.25 0.04 53.13 59.52 40.44 0.05

M-B π-Bond occupation %M %s %p %d %f B %B %s %p %d

1.826 85.80 0.00 0.00 99.99 0.01

1.835 86.83 0.00 0.04 99.94 0.02

1.810 85.71 0.00 0.06 99.93 0.01

1.796 86.92 0.00 0.02 99.98 0.00

1.838 89.20 0.00 0.01 99.98 0.01

1.797 86.73 0.00 0.04 99.95 0.01

1.807 86.92 0.00 0.04 99.96 0.00

1.811 90.62 0.00 0.02 99.98 0.00

1.801 90.13 0.00 0.03 99.96 0.01

1.924 81.82 0.00 0.00 100.00 0.00

1.944 84.34 0.00 0.01 99.99 0.00

1.929 81.15 0.00 0.02 99.97 0.01

14.20 0.00 99.90 0.10

13.17 0.00 99.73 0.27

14.29 0.00 99.77 0.23

13.08 0.00 99.88 0.12

10.80 0.00 99.75 0.25

13.27 0.00 99.77 0.23

13.08 0.00 99.92 0.08

9.38 0.00 99.72 0.28

9.87 0.00 99.76 0.24

18.38 0.00 99.93 0.07

15.66 0.00 99.79 0.21

18.85 0.00 99.82 0.18

a significantly larger (ca. 0.49-0.66e) positive charge. The metal atoms and carbonyl group are almost neutral. A more definitive picture of MdB bonding is obtained through NBO analysis of the delocalized Kohn-Sham orbitals. The characteristics of the MdB σ- and π-bonding orbitals are listed in Table 2. In most of the complexes I-XII, the M-B σ-bonding orbital is slightly polarized toward the boron atom (i.e., the B-center contributes more to the bonding orbital) except the complexes [(η5-C5H5)(CO)M{BN(SiMe3)2)}] (VII, M = Co, VIII, M = Rh), where the M-B σ-bonding orbitals are polarized toward the metal atom. The occupations for M-B σ-bonding orbitals are in the range 1.771-1.921. In all four sets of borylene complexes I-XII, the M-B π-bond orbitals are highly polarized toward the metal atom (i.e., the M-center contributes more to the bonding orbital). The contributions of boron to the πbonding are small (see Table 2). This conclusion is consistent with the nature of the bonding (HOMO) orbitals given in Figure 3 for the rhodium complex [(η5-C5H5)(CO)Rh{BN(SiMe3)2}] (VIII). In this figure, the HOMO gives a pictorial

description of the RhdB π-bonding, but HOMO-1 shows RhdB σ-bonding, while HOMO-4 is a B-N π-orbital. 3.3. Energy Decomposition Analysis of the MdB Bonding of Complexes I-XII. Besides the charge decomposition analysis at the NBO level we also carried out an energy decomposition analysis of the MdB bonds in the calculated metal-borylene complexes [(η5-C5H5)(CO)M(BNX2] (X = Me, SiH3, SiMe3) and [(η5-C5H5)(PMe3)M{BN(SiH3)2}] (M = Co, Rh, Ir). The results are given in Table 3 and Figure 4. Energy decomposition analyses of the neutral borylene complexes [(CO)5Cr(BR)], [(CO)4Fe(BR)], [(CO)3Ni(BR)] (R = BF, BNH2, BO-), [(CO)4Fe(BCp)], [(CO)4Fe(BN(SiH3)2)], and [(CO)5W(BN(SiH3)2)] were reported by Frenking et al.41,42 The tabulated bond dissociation energy in Table 3 reveals that the IrdB bonds are slightly stronger than the corresponding cobalt and rhodium complexes. Figure 4 shows a diagram of the π-bonding, σ-bonding, interaction energies 4Eint, orbital interaction energies 4Eorb, and electrostatic interactions 4Eelstat components of the above-presented

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147

Figure 3. Plot of some relevant orbitals of the rhodium borylene complex [(η5-C5H5)(CO)Rh{BN(Si(Me3)2)}]. Table 3. Energy Decomposition Analysisa of Neutral Terminal Borylene Complexes [(η5-C5H5)(CO)M(BNX2] (X = Me, SiH3, SiMe3) and [(η5-C5H5)(PMe3)M(BN(SiH3)2] (M = Co, Rh, Ir) at BP86/TZ2P, I-XII [(η5-C5H5)(CO)M(BNMe2)] M= Co, I -101.3 231.4 -207.5 (62.4%) -125.2 ΔEorb -86.0 ΔEσ(a0 ) -39.2 ΔEπ(a00 )c (31.3%) 2.5 ΔEprep ΔE(-BDE)d -98.8

ΔEint ΔEPauli ΔEelstatb

[(η5-C5H5)(CO)M{BN(SiH3)2}]

M= Rh, II

M= Ir, III

M= Co, IV

M= Rh, V

M= Ir, VI

-102.3 283.3 -259.9 (67.4%) -125.7 -88.1 -37.6 (29.9%) 6.0 -96.3

-125.0 334.7 -312.3 (67.9%) -147.4 -104.2 -43.2 (29.3%) 10.4 -114.6

-98.2 223.6 -197.5 (61.4%) -124.2 -86.3 -37.9 (30.5%) 1.4 -96.8

-99.0 264.3 -241.2 (66.4%) -122.2 -86.2 -36.0 (29.5%) 5.5 -93.5

-122.1 319.2 -294.7 (66.8%) -146.6 -104.5 -42.1 (28.7%) 10.2 -111.9

[(η5-C5H5)(CO)M{BN(SiMe3)2] M= Co, VII -97.9 229.6 -206.5 (63.1%) -121.0 -87.7 -33.3 (27.5%) 1.8 -96.1

M= Rh, VIII -99.0 279.9 -258.0 (68.1%) -120.9 -88.9 -32.0 (26.5%) 5.7 -93.3

[(η5-C5H5)(PMe3)M{BN(SiH3)2]

M= Ir, IX

M= Co, X

-122.0 328.8 -309.3 (68.5%) -142.5 -105.1 -37.4 (26.2%) 10.7 -111.3

-99.4 223.0 -197.2 (61.2%) -125.2 -78.9 -46.3 (37.0%) 0.8 -98.6

M= Rh, XI -102.5 281.2 -254.0 (66.2%) -129.7 -86.0 -43.7 (33.7%) 8.7 -93.8

M= Ir, XII -126.4 336.7 -307.6 (66.4%) -155.4 -105.5 -49.9 (32.1%) 13.8 -112.6

a Energy contributions in kcal/mol. b Values in parentheses are the percentage contribution to the total electrostatic interactions reflecting the ionic character of the bond. c Values in parentheses are the percentage contribution of π-bonding to the total orbital interactions ΔEorb. d Bond dissociation energy with negative sign.

Figure 4. Values of the energy contributions of the π-bonding, σ-bonding, interaction energies, orbital interactions (covalent interaction), and electrostatic interactions (ionic contribution) to the M-B bonding in terminal borylene complexes I-XII.

bonding energies. The breakdown of the interaction energy 4Eint into the repulsive term 4EPauli and the attractive terms 4Eorb and 4Eelstat shows that the 4EPauli repulsive interactions have the larger absolute value for the studied complexes I-XII (Table 3). The contributions of the electrostatic interaction terms ΔEelstat to the MdBNX2 bonding are significantly larger in all borylene complexes I-XII than the covalent bonding ΔEorb term. Thus, the MdBNX2 bond in the studied borylene complexes of M = Co, Rh, and Ir has a greater degree of ionic character (61.2-68.5%). Table 3 also gives a breakdown of the orbital interactions ΔEorb into the MrBNX2

σ-donation and MfBNX2 π-back-donation components. It is significant to note that the π-bonding contribution is, in all studied complexes, smaller (26.2-37.0% of total orbital contributions) than the σ-bonding contribution. Similar results have been reported for neutral terminal borylene complexes [(CO)5Cr(BR)], [(CO)4Fe(BR)], [(CO)3Ni(BR)] (R = BF, BNH2, BO-), [(CO)4Fe(BCp)], [(CO)4Fe(BN(SiH3)2)], and [(CO)5W(BN(SiH3)2)] by Frenking et al.41 The relatively larger π-contribution is found for the complexes X-XII, [(η5-C5H5)(PMe3)M{BN(SiH3)2}], and a smaller π-contribution is observed for the complexes VIII-X, [(η5-C5H5)(CO)2M{BN(SiMe3)2}]. From the data presented in Table 3 it could be concluded that (1) in the BNX2 ligands, boron dominantly behaves as a σ-donor, (2) the MdB bonding in these terminal borylene complexes I-XII has nearly MdB double-bond character, (3) the interaction energy increases in all four sets of complexes via the order Co e Rh < Ir, (4) the π-orbital contribution to the MdBNX2 interaction energy increases via M = Rh < Co < Ir in all four sets of complexes, and (5) the σ-orbital contribution to the MdBNX2 interaction energy reduces but its π-orbital contribution increases upon replacing the CO auxiliary ligand (in complexes I-III) by a PMe3 ligand (in complexes X-XII) for all three transition metals. Thus, the PMe3 ligand acts as a stronger π-ligand but a weaker σ-ligand than CO.

4. Conclusions From the above-presented theoretical studies of the structure and bonding in 12 neutral borylene complexes of cobalt,

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Organometallics, Vol. 29, No. 1, 2010

rhodium, and iridium one can draw the following conclusions: 1. Here, for the first time (except the Ir complex IX), we reported the geometry and electronic structure of, as well as analyzed the nature of, MdBNX2 bonds in the terminal neutral metal borylene complexes of cobalt, rhodium, and iridium complexes [(η5-C5H5)(CO)M(BNX2)] (where M = Co, Rh, and Ir, and X = Me, SiH3, and SiMe3). The calculated geometry parameters of iridium borylene complex [(η5-C5H5)(CO)Ir{BN(SiMe3)2}], IX, are in excellent agreement with their available experimental values.10 2. The M-B bonds in these complexes are nearly MdB double bonds, where the σ-bonding orbital is slightly polarized toward the B-center, while the π-bonding orbital is highly polarized toward the metal atom. In all studied complexes, the π-bonding contribution to the total Md BNX2 bond is significantly smaller than that of the σbonding and increases upon going from M = Co to Ir. Thus, in the BNX2 ligands boron dominantly behaves as a σ-donor. 3. The σ-orbital contribution to the MdBNX2 bonding reduces but its π-orbital contribution increases upon replacing the auxiliary ligand CO (in complexes I-III) by PMe3 (in complexes X-XII) for all three transition metals. Thus, the PMe3 ligand acts as a stronger π-ligand but a weaker σligand than CO. Furthermore, the substitution of the CO ligand in I-III by PMe3, X-XII, slightly decreases the calculated MdB bond distances. 4. The nature of the X substituent of BNX2 has an insignificant effect on the character of the MdBNX2 bonding: the

Pandey and Musaev

MdB bond distance in complexes I-IX only slightly increases upon X going from Me to SiH3 and SiMe3. 5. The calculated WBI values for MdB bonds are slightly larger than that for M-C(CO) bonds; that is, the MdBNX2 bonds in these borylene complexes are stronger than M-CO bonds. 6. The contributions of the electrostatic interactions ΔEelstat to the MdBNX2 bonds are significantly larger in all borylene complexes I-XII than the covalent bonding ΔEorb. Thus, the MdBNX2 bonding in the borylene complexes has a greater degree of ionic character. Furthermore, the iridium complexes possess the highest orbital interactions, ΔEorb, and electrostatic interactions, ΔEelstat. We believe that a more detailed understanding of the bonding in metal-borylene complexes is a requisite, particularly for the synthesis of terminal transition metal borylene complexes, as well as the designing of efficient borylenetransfer processes. In this aspect, the above-presented findings are important contributions to the fast developing metal-borylene chemistry.9-11

Acknowledgment. K.K.P. acknowledges the Visiting Fellowship at the Cherry L. Emerson Center of Emory University. Supporting Information Available: Cartesian coordinates of the optimized geometries of metal borylene complexes I-XII (Table S1), Charts 1 and 2, and complete ref 50. This material is available free of charge via the Internet at http://pubs.acs.org.