Regium Bonds to Activated Methyl Groups: Transition to Covalent Bond and a Pentacoordinate Carbon Center

Quantum chemical calculations probe the nature of the regium bonds formed by MR, where M refers to Group 11 Cu, Ag, and Au, and R is Me, Cl, or CN. The Lewis base in each dyad is one of the methyl groups from BeMe₂, MgMe₂, or MeMgCl. A rather strong M···C regium bond is formed in each case, with interaction energies ranging from -13 all the way up to -72 kcal/mol. The strongest interactions involve Au and induce an inversion of the methyl group, which occupies a position roughly halfway between the Au and the Mg center. Both the Au-C and C-Mg bonds contain high degrees of covalency, leaving the central C in an approximate trigonal bipyramidal pentavalent coordination. The regium bond in these complexes contains a large contribution from polarization energy, comparable to the electrostatic contribution.