Analysis of the bonding in XH 3 Cu + (XB, Al, Ga) complexes

High‐level density functional theory (DFT) calculations on XH 3 Cu + (XB, Al, Ga) complexes show that the attachment of the metal cation to the base takes place through agostic‐type interactions. These interactions that can be viewed as dative bonds from the σ XH bonding orbitals of the base toward low‐lying empty 4 s orbitals of the metal cation, and back‐donations from the lone pairs of the metal into the σ   * XHantibonding orbitals of the neutral, are particularly favored when the XH bonds have a high X +δ H −δ polarity. Accordingly, the AlH 3 and GaH 3 Cu + binding energies are very similar, but much larger than that of BH 3 . Depopulation of the σ XH bonding orbital and the concomitant population of the σ   * XHantibonding orbital involved in the agostic interaction result in a significant weakening of the corresponding XH linkages, whose bond length increases and whose stretching frequency appears red‐shifted. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006