EPR spectra of eight-coordinated complexes of the early transition metals with sulfur and selenium donor ligands. 3. Vanadium(IV) and niobium(IV) dithio- and diselenocarbamates

Single-crystal and powder EPR studies of the eight-coordinated complexes tetrakis(N,N-diethyldithiocarbamato)vanadium(IV), tetrakis(N,N-diethyldithiocarbamato)niobium(IV), and tetrakis(N,N-diethyldiselenocarbamato)vanadium(IV), diluted in the diamagnetic Ti(IV) analogues, are reported. Two, chemically inequivalent, crystallographic sites are present in the unit cell of the two isomorphous host compounds. They are differently populated by the paramagnetic ions, the most asymmetric site accounting for about 75% of the total V(IV) or Nb(IV) concentration. All the different EPR signals arise from discrete eight-coordinated species, with a molecular geometry close to the ideal triangular dodecahedron (D2d). The mixing coefficients in a ground-state orbital of the type a|x2 - y2? - b|z2? have been obtained from the experimental results and related to the deviation from pure D2d symmetry. However, no simple relationship could be found between this distortion and the type of ligands or metal ion involved. Low-intensity satellite lines have been observed in the spectrum of V/Ti(Et2dsec)4 and attributed to the hyperfine coupling of the unpaired electron with four different selenium nuclei. No splitting from the other four selenium ligands could be observed. This result has been related to the inequivalence of the A and B ligand sites of the dodecahedron and, specifically, to the different overlap between the metal ion ground-state orbital and the ligand ?? orbitals.