Mössbauer study of octacyanotungstate anions
Abstract
182 W Mössbauer spectra are reported for various WIV and WV octacyanides, including both crystalline materials and frozen solutions. Theoretical interpretation indicates that the quadrupole coupling constants, e2qQ, in the WIV 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 WV 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 WIV 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.