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

The B-C bond in (HO)(2)B-CH3 is stronger than the C-C bond in ethane, becau se a hyperconjugation interaction is formed between boryl p(pi) orbital and C-H bonding orbital of CH3. The M-B(OH)(2) bond (M = Pd or Pt) is also str onger than the M-CH, bond, because of the presence of back-donating interac tion between M d(pi) and boryl p(pi) orbitals and the considerably large or bital overlaps between B(OH)(2) sp(2) and M valence orbitals. Also, the M-X H3 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 sp(3) orbital. In the activation reactio n of the (HO)(2)B-XH3 sigma-bond, the empty p(pi) orbital of the boryl grou p forms the charge-transfer interaction with the M d(pi) orbital in the tra nsition state (TS), to stabilize the TS and as a result to facilitate sigma -bond activation. The allyl-methyl reductive elimination of Pd(CH3)(eta(3)- C3H5)(PH3) requires a very large activation energy, in spite of the very la rge exothermicity. On the other hand, allyl-silyl, allyl-germyl, and allyl- stannyl reductive eliminations occur with a moderate activation barrier, wh ile they are moderately exothermic (X = Si or Ge) or moderately endothermic (X = Sn). This difference between methyl and the others is clearly interpr eted in terms of the presence of hypervalency of silyl, germyl, and stannyl elements. (C) 2000 Elsevier Science S.A. All rights reserved.