Impact of metal exchange on the electronic structure and optical properties of isostructural octa-coordinated MoIV/CdII and WIV/CdII polynuclear cyanide polymers

Abstract

The electronic structure and related electronic properties of two novel isostructural octacyanometallates Cd₂(H₂O)₄[Mo^IV(CN)₈]·2H₂O and Cd₂(H₂O)₄[W^IV(CN)₈]·2H₂O are described from a combined experimental and computational approach. The impact that the octacoordinated heavy metals W and Mo have on the electronic structure and optical response of isostructural materials whose electronic properties strongly depend on the crystal structure is discussed. It is found that the effect of the polarization power of the metal centers, combined with the ligand field of cyanos, produces considerable changes in the electronic structure and, consequently, in the band gap energy. The ab initio calculations, which were performed with generalized gradient PBE and hybrid HSE06 density functionals, accurately reproduce the electronic structure of [Mo(CN)₈]⁴⁻ and [W(CN)₈]⁴⁻ building units, revealing that the electronic transitions associated with the band gap energy have an origin in the charge transfer phenomena of metal to ligand nature. Moreover, the optical band gap transitions have an allowed indirect behavior in the Γ → M → D direction which is associated with the (x² − y²) → π* transition. The allowed direct and indirect (experimental and theoretical) band gap energies together with the exciton effective masses are reported.