Multiple bonding in homonuclear Group 13 ethene analogues

The nature and strength of the bonding in the neutral (MR2)2 and reduced (MR2)2 n− (n=0, 1, 2) complexes of the Group 13 elements (M=B, Al, Ga or In) have been investigated using non-local density functional theory. The reduced systems are found to be genuinely multiply bonded and estimates of the π-bond strengths suggest that these weaken considerably down the group and are compared with those between Group 14 elements. Formation of the doubly reduced complexes is disfavored for the heavier elements by the repulsion between the negatively charged centers and the weakness of the MM π-interaction. Calculations on Li2[M2R4] derivatives, however, suggest that strong solvation is the key to the successful syntheses of these systems. trans-Bending of the [M2R4]2− anions is investigated and is predicted to increase down the group. A careful treatment of the electronic causes of this bending is reported, and differs from previous analyses of the non-classical shapes of isoelectronic Group 14 molecules.