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⁻ (O_h, ³A_1g) (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 (D_4h, ²B_1g) (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(d_π) → (CO)₈(π*) backdonations which result in strong red shifts of the C–O stretching frequencies relative to free CO. [ABSTRACT FROM AUTHOR]