Quadruple bonding of bare group-13 atoms in transition metal complexes

Abstract

Density functional theory calculations at the M06-D3/def2-TZVPPD level of the group-13 anion complexes EFe(CO)₃⁻ (E = B–Tl) and the isoelectronic neutral and charged boron adducts BTM(CO)₃^q (TM^q = Fe⁻, Ru⁻, Os⁻, Co, Rh, Ir, Ni⁺, Pd⁺, Pt⁺) give tetrahedral (C_3v) geometries in the ¹A₁ electronic ground state as equilibrium structures. The analysis of the bonding situation with the energy decomposition analysis in combination with natural orbital for chemical valence method suggests that the E–TM(CO)₃^q bonds possess four bonding components: (a) one strong electron-sharing σ bond E–TM(CO)₃^q; (b) two π backdonations E⇇TM(CO)₃^q and (c) one weak σ donation E→TM(CO)₃^q. The relative strength of the four bonding components depends on the charge of the system, the transition metal TM and the group-13 atom E. The σ donation E→TM(CO)₃^q is in all systems rather weak while the associated charge migration is not negligible. A similar situation of the bonding of terminal group-13 atoms Ga and In is found in Ga–TM(GaCp)₄⁺ and E–Pt(PMe₃)₃⁺ (TM = Ni, Pd, Pt; E = Ga, In), which are model compounds for the stable complexes GaTM(GaCp*)₄⁺ (TM = Ni, Pt) and InPt(PPh₃)₃⁺. The quadruple bonds E→TML₂ are hybrids of electron-sharing and dative bonds.