K(CO)−8 and Rb(CO)−8 Cube-Like Alkali Octacarbonyls Satisfying the 18-Electron Rule

Inspired by recent discoveries of alkaline-earth octacarbonyls and based on extensive first-principles theory investigations, we predict herein the possibility of perfect cube-like alkali octacarbonyls M(CO)−8 (Oh, 3A1g) (1) (M = K, Rb) which satisfy the 18-electron rule in an octacoordinate cubic field with (n − 1)d-orbital participation of alkali metal center playing an important role in M–CO coordination interaction. The slightly elongated neutral M(CO)8 (D4h, 2B1g) (3) turn out to be similar with the experimentally observed Ba(CO)+8 monocation in an octacoordinate tetragonal field. Detailed orbital analyses indicate that, similar to neutral alkaline-earth octacarbonyls, M(CO)−8 monoanions possess the superatomic electronic configuration of S2P6D8F2, with the alkali–CO coordination interaction mainly originating from M(dπ) → (CO)8(π*) backdonations which result in strong red shifts of the C–O stretching frequencies relative to free CO.