Analysis of the E – E Bond in Group - 13 Complexes [ ( PMe 3 ) 2 ( E 2 H n ) ] ( E = B – In , n = 4 , 2 , 0

Quantum chemical calculations at the BP86/def2-TZVPP level have been carried out for the donor-acceptor complexes [(PMe3)2(E2Hn)] for n = 4, 2, 0. The focus of this works lies on the E–E bonding situation. The electronic structure of the molecules was analyzed with the EDA-NOCV method and with NBO calculations. The EDA-NOCV analysis of the E–E interactions in [(PMe3)2(E2Hn)] (n = 4, 2, 0) provide deep insights into the nature and the strength of the bonds. The calculated intrinsic interactions ΔEint suggest that the trend for the bond strength of the E–E single bond [(PMe3)(H)2E– E(H)2(PMe3)] has the order B > Ga > Al > In. The orbital interactions ΔEorb which exhibit the same trend as ΔEint have one dominant contribution which comes from the coupling of the singly occupied orbitals in the (PMe3)(H)2E fragments. A slightly different trend B > Ga ~ In > Al is found for the interaction energy ΔEint of the E–E bonds in [(PMe3)(H)E–E(H)(PMe3)]. The orbital term ΔEorb which has the order B > Ga > In > Al has one major and one minor component which in case of the boron compound may be identified with a σ and a π bond. The heavier homologues [(PMe3)(H)E–E(H)(PMe3)] (E = Al – In) have pyramidally coordinated atoms E. The dominant orbital interactions in the latter species come from the formation of a “slipped” π bond while the minor component comes from the formation of the σ bond. This can be explained with the change in the hybridization of the orbitals at atom E along the formation of the E–E bond. The compounds [(PMe3)E–E(PMe3)] exhibit three different types of bonding situations depending on atoms E. The boron system [(PMe3)B≡B(PMe3)] has a classical triple bond which consist of a σ bond that provides 56 % to the orbital interactions and two degenerate π bonds which contribute 40 % to the covalent bonding. The aluminium and gallium complexes [(PMe3)E–E(PMe3)] (E = Al, Ga) are also triply bonded species where the covalent bonding has one strong and two weaker components. The strong component comes from the “slipped” π bond while the minor components come from the formation of the σ and π bonds. The indium complex [(PMe3)In–In(PMe3)] has only an In–In single bond and two electron lone pairs at the indium atoms. The charge donation Me3P → E2(Hn) ← PMe3 has for all atoms E the trend for n 4 > 2 > 0.