Regium Bonding to Carbones: Transition to Covalency and σ/π Lone Pair Selectivity

Carbones possess two carbon-centered lone pairs of approximately σ and π character, but the factors governing their selective engagement with a Lewis acid remain poorly understood. Here, AuCN is employed as a compact regium bond Lewis acid to probe the electronic and geometric preferences of a series of carbones, unsaturated carbon bases, and conventional carbene C(NH2)2. DFT calculations reveal exceptionally strong and highly directional regium bonds. The four amine-supported carbones exhibit BSSE-corrected interaction energies with AuCN of approximately -100 kcal mol-1 and undergo pronounced pyramidalization around the central carbon, identifying the out-of-plane π-type lone pair as the preferred donor. In contrast, carbones containing unsaturated N substituents, phosphines, or carbene C(NH2)2 engage AuCN through an in-plane σ-type lone pair. Electrostatic and orbital terms contribute almost equally to stabilization, accompanied by substantial charge transfer, partial shared-electron character, and weakening of the Au-C(CN) bond within the AuCN fragment. The results reveal ligand-dependent variations in Au-C(carbone) interaction strength and covalent character within a continuous bonding framework.