Nature of the Chemical Bond between a Transition Metal and a Group-13 Element:  Structure and Bonding of Transition Metal Complexes with Terminal Group-13 Diyl Ligands ER (E = B to Tl; R = Cp, N(SiH₃)₂, Ph, Me)

The equilibrium geometries and first TM−ER bond dissociation energies of 35 transition metal complexes with group-13 ligand atoms [(CO) 4 Fe−ECp], [(CO) 4 Fe−EN(SiH 3 ) 2 ], [(CO) 5 W−EN(SiH 3 ) 2 ], [(CO) 4 Fe−EPh], and [TM(ECH 3 ) 4 ] (TM = Ni, Pd, Pt; E = B, Al, Ga, In, Tl) have been calculated at the DFT level of theory using gradient-corrected exchange and correlation functionals (BP86). The bonding situation in the complexes was examined with the help of the NBO and CDA partitioning schemes. The calculations show that the bond energies of the TM−ER bonds are rather high and follow in all cases the trend B > Al > Ga ≈ In > Tl. The TM−ER bonds where R is a poor π-donor (Ph, CH 3 ) are shorter and stronger than those where R is a π-donating group (Cp, N(SiH 3 ) 2 ). The NBO analysis of the TM−E interactions indicates that the nature of the TM−ER bond is mainly ionic and that the covalent contributions are less important. The covalent bond order of the TM−ER bonds is always < 1. The TM←ER σ-donation is clearly larger than the TM→ER π-back-donation when R is a strong π-donor. TM→ER π-back-donation becomes larger and may even become bigger than the TM←ER σ-donation when R is a weak π-donor.