Comparison of side-on and end-on coordination of E₂ ligands in complexes [W(CO)₅E₂] (E = N, P, As, Sb, Bi, Si ^-, Ge^-, Sn^-, Pb^-)

Complexes of W(CO)₅ with neutral diatomic pnictogen ligands N₂, P₂, As₂, Sb₂, and Bi₂ and anionic Group 14 ligands Si₂^2-, Ge ₂^2-, Sn₂^2-, and Pb₂ ^2- coordinated in both side-on and end-on fashion have been optimized by using density functional theory at the BP86 level with valence sets of TZP quality. The calculated bond energies have been used to compare the preferential binding modes of each respective ligand. The results were interpreted by analyzing the nature of the interaction between the ligands and the metal fragment using an energy partitioning method. This yields quantitative information regarding the strength of covalent and electrostatic interactions between the metal and ligand, as well as the contributions by orbitals of different symmetry to the covalent bonding. Results show that all the ligands studied bind preferentially in a side-on coordination mode, with the exception of N₂, which prefers to coordinate in an end-on mode. The preference of the heavier homologues P₂-Bi₂ for binding in a side-on mode over the end-on mode in the neutral complexes [(CO)₅WE₂] comes mainly from the much stronger electrostatic attraction in the former species. The energy difference between the side-on and end-on isomers of the negatively charged complexes with the ligands Si₂^2-, Ge₂^2-, Sn ₂^2-, and Pb₂^2- is much less and it cannot be ascribed to a particular bonding component.