Transition-metal complexes [(PMe₃)₂Cl ₂M(E)] and [(PMe₃)₂(CO)₂M(E)] with Naked group 14 atoms (E = C-Sn) as ligands; part 1: parent compounds

The equilibrium geometries and bond dissociation energies of 16valence-electron(VE) complexes [(PMe₃)₂Cl ₂M(E)] and 18-VE complexes [(PMe₃)₂(CO) ₂M(E)] with M = Fe, Ru, Os and E = C, Si, Ge, Sn were calculated by using density functional theory at the BP86/TZ2P level. The nature of the M-E bond was analyzed with the NBO charge decomposition analysis and the EDA energy-decomposition analysis. The theoretical results predict that the heavier Group 14 complexes [(PMe₃)₂Cl₂M(E)] and [(PMe₃)(CO)₂M(E)] with E = Si, Ge, Sn have C _2v., equilibrium geometries in which the PMe₃ ligands are in the axial positions. The complexes have strong M-E bonds which are slightly stronger in the 16-VE species 1ME than in the 18-VE complexes 2ME. The calculated bond dissociation energies show that the M-E bonds become weaker in both series in the order C > Si > Ge > Sn; the bond strength increases in the order Fe < Ru < Os for 1ME, whereas a U-shaped trend Ru < Os < Fe is found for 2ME. The M-E bonding analysis suggests that the 16-VE complexes 1IME have two electron-sharing bonds with σ and π symmetry and one donoracceptor π bond like the carbon complex. Thus, the bonding situation is intermediate between a typical Fischer complex and a Schrock complex. In contrast, the 18-VE complexes 2ME have donor-acceptor bonds, as suggested by the Dewar-Chatt-Duncanson model, with one M ← E σ donor bond and two M→E π-acceptor bonds, which are not degenerate. The shape of the frontier orbitais reveals that the HOMO-2 σ MO and the LUMO and LUMO+1 π MOs of 1ME are very similar to the frontier orbitais of CO.