Mössbauer study of octacyanotungstate anions

Abstract

¹⁸² W Mössbauer spectra are reported for various W^IV and W^V octacyanides, including both crystalline materials and frozen solutions. Theoretical interpretation indicates that the quadrupole coupling constants, e²qQ, in the W^IV species arise from partial cancellation of the dominant contribution from the d² configuration by effective population of the nominally empty metal d orbitals through ligand→metal σ-bonding. Predictions that e²qQ should be negative in dodecahedral [W(CN)₈]^4–, and positive with |e²qQ| smaller in square-antiprismatic [W(CN)₈]^4–, have been confirmed experimentally. On this basis [W(CN)₈]^4– is assigned a dodecahedral structure in Li₄[W(CN)₈],nH₂O and square-antiprismatic in Cd₂[W(CN)₈],8H₂O, and it is inferred that the solidstate structures are retained in frozen solutions of K₄[W(CN)₈](dodecahedral) and H₄[W(CN)₈](square antiprismatic). The quadrupole coupling in [(EtNC)₄W(CN)₄] is discussed. In the W^V species e²qQ≈ 0, indicating that the above mentioned cancellation is virtually exact in the d¹ configuration, and leading to a crude empirical value of 16 mm s^–1 for |e²qQ| due to one d electron. The effect on the W^IV coupling constants of distortions from ideal geometry is elucidated. The bonding in [W(CN)₈]^n–(n= 3 or 4) appears to be essentially covalent with the W atom carrying at most a small formal charge.