Transition-Metal Complexes [(PMe3)2Cl2M(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 16-valence-electron(VE) complexes [(PMe3)(2)Cl2M(E)] and 18-VE complexes [(PMe3)(2)(CO)(2)M(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)(2)Cl2M(E)] and [(PMe3)(2)(CO)(2)M(E)] with E=Si, Ge, Sn have C-2 nu 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 IME, whereas a U-shaped trend Ru < Os < Fe is found for 2ME. The M-E bonding analysis suggests that the 16-VE complexes ME have two electron-sharing bonds with o and pi symmetry and one donor-acceptor pi 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 o donor bond and two M-E pi-acceptor bonds, which are not degenerate. The shape of the frontier orbitals reveals that the HOMO-2 sigma MO and the LUMO and LUMO+1 pi* MOs of 1ME are very similar to the frontier orbitals of CO.