Bonding nature and reaction behavior of inter-element linkages with transition metal complexes. A theoretical study

The B–C bond in (HO)2B–CH3 is stronger than the C–C bond in ethane, because a hyperconjugation interaction is formed between boryl pπ orbital and C–H bonding orbital of CH3. The M–B(OH)2 bond (M=Pd or Pt) is also stronger than the M–CH3 bond, because of the presence of back-donating interaction between M dπ and boryl pπ orbitals and the considerably large orbital overlaps between B(OH)2 sp2 and M valence orbitals. Also, the M–XH3 bond (X=C, Si, Ge, or Sn) becomes weaker in the order M–SiH3>M–GeH3>M–SnH3>M–CH3. This result is easily interpreted in terms of the energy level and the expansion of the XH3 sp3 orbital. In the activation reaction of the (HO)2B–XH3 σ-bond, the empty pπ orbital of the boryl group forms the charge-transfer interaction with the M dπ orbital in the transition state (TS), to stabilize the TS and as a result to facilitate σ-bond activation. The allyl–methyl reductive elimination of Pd(CH3)(η3-C3H5)(PH3) requires a very large activation energy, in spite of the very large exothermicity. On the other hand, allyl–silyl, allyl–germyl, and allyl–stannyl reductive eliminations occur with a moderate activation barrier, while they are moderately exothermic (X=Si or Ge) or moderately endothermic (X=Sn). This difference between methyl and the others is clearly interpreted in terms of the presence of hypervalency of silyl, germyl, and stannyl elements.