Natural bond orbital analysis of molecular interactions: Theoretical study of W(CO)₅ complexes with E(PH₃)₂ and NHE_Me ligands (E=C, Si, Ge, Sn, Pb)

The complexes with ligands carbodiphosphorane-analogues (called tetrylones) [(CO)₅ W-{E(PH₃)₂}] (W5-EP₂) and N-heterocyclic carbene-analogues (called tetrylenes) [(CO)₅ W-{NHE_Me}] (W5-NHE_Me) when E=C-Pb have been studied using natural bond orbital (NBO) method. The NBO analysis provides a consistent picture of the chemical bonding is two entire families of transition metal complexes of tetrylone and tetrylene ligands in term of donor-acceptor interactions, showing the correlation of these interactions with Wiberg bond indies (WBI), natural partial charges, and the energetically highest lying occupied molecular orbitals for σ and π orbitals of free ligands E(PH₃)₂ and NHE_Me. Analysis of the bonding situation reveals that in E(PH₃)₂ and NHE_Me ligands, the energy level of the π orbital rises, whereas that of the σ orbital decreases as atom E becomes heavier. The complexes with head-on-bonded ligands have (CO)₅ W←E donation which comes from the σ-lone-pair orbital of E(PH₃)₂ and NHE_Me where E=C for tetrylones and E=C, Si, Ge for tetrylenes, whereas the (CO)₅ W←E donation in the side-on bonded complexes when E becomes heavier arises from the π-lone-pair orbital of E(PH₃)₂ and NHE_Me ligands which is the HOMO of the free ligands. This makes the heavier adducts of tetrylones and tetrylenes become stronger donors than the lighter systems. The NBO analysis suggests that the E(PH₃)₂ ligands are strong σ-donors and strong π-acceptors while the NHE_Me ligands are strong σ-donors and weak π-acceptors. This is possible for tetrylones that have two lone-pair orbitals available for donation, whereas the tetrylenes have only one lone-pair orbital available for donation. (author)