Analysis of Bonding in Cyclopentadienyl Transition-Metal Boryl Complexes Anthony A. Dickinson, David J. Willock,* Richard J. Calder, and Simon Aldridge* Department of Chemistry, Cardiff University, P.O. Box 912, Park Place, Cardiff, U.K. CF10 3TB Received June 13, 2001 Analysis of the bonding in transition-metal boryl complexes of the type [(C 5 R 5 )M(CO) 2 - BX 2 ] has been carried out by density functional methods, to quantify the relative contributions to the metal boryl linkage from ionic and covalent interactions. Covalent (orbital) terms account for 60-70% of the overall attractive interaction between metal and boryl fragments, with σ donation from the boryl ligand overwhelmingly predominating over π back-donation even in the most favorable cases (e.g. 84.1:15.8 and 81.9:18.0 for CpFe(CO) 2 BH 2 and CpFe- (CO) 2 B(C 6 F 5 ) 2 , respectively). Introduction Transition-metal boryl complexes (L n M-BX 2 ) have been the subject of considerable recent research effort, 1 not least because of their implication in synthetically useful organic transformations such as the hydrobora- tion and diboration of carbon-carbon multiple bonds. 2 More recently the involvement of cyclopentadienyl transition-metal boryl complexes in both stoichiomet- ric 3,4 and catalytic 4,5 functionalization of alkanes and arenes has been demonstrated, notably by Hartwig. 3,5 It has been suggested that the unusual regiochemistry and activity of such systems may be due to the Lewis acidic properties of the boryl ligand, which provide favorable kinetics for the formation of boron-carbon bonds. 3e Such studies of reactivity have been complimented by numerous structural investigations in which the nature of the metal-boron bond has been probed by crystal- lographic and spectroscopic methods. 1 One of the sig- nificant questions investigated by such studies is the potential for the strongly σ donor boryl ligand also to act as a π acid by utilizing the vacant boron-based orbital of π symmetry (Chart 1). Metal boron bond lengths, together with the relative orientation of metal and boryl fragments and the IR stretching frequencies of ancillary carbonyl ligands have typically been used to probe the extent of back-bonding. 1c,3,6 In the majority of compounds studied to date it has been concluded that π interactions represent at most a relatively minor contribution to the overall metal boron bond. Such a conclusion is not unexpected, given the strongly π donating boryl substituents (e.g. X 2 ) cat, o-O 2 C 6 H 4 ) and π acceptor spectator ligands (e.g. CO) commonly em- ployed in precedented synthetic routes. 1 In such cases π donation from the metal to the ancillary carbonyl ligands and from the π donor substituents X to the boron (1) (a) Wadepohl, H. Angew. Chem., Int. Ed. 1997, 36, 2441. (b) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 110, 1882. (c) Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R,; Wright, L. J. 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Principal Orbital Interactions for Transition Metal Boryl Complexes: σ Donor and π Acceptor Properties of the Boryl Ligand 1146 Organometallics 2002, 21, 1146-1157 10.1021/om0105122 CCC: $22.00 © 2002 American Chemical Society Publication on Web 02/19/2002