The ground-state electronic structures of K3V(ox)3Â·3H2O, Na3V(ox)3Â·5H2O, and NaMgAl1â€“xVx(ox)3Â·9H2O (0 < x <= 1, ox = C2O42â€“) have been studied by Fourierâ€“transform electronic absorption and inelastic neutron scattering spectroscopies. High-resolution absorption spectra of the 3Î“(t2g2) â†’ 1Î“(t2g2) spin-forbidden electronic origins and inelastic neutron scattering measurements of the pseudo-octahedral [V(ox)3]3â€“ complex anion below 30 K exhibit both axial and rhombic components to the zero-field-splittings (ZFSs). Analysis of the ground-state ZFS using the conventional S = 1 spin Hamiltonian reveals that the axial ZFS component changes sign from positive values for K3V(ox)3Â·3H2O (D â‰ˆ +5.3 cmâ€“1) and Na3V(ox)3Â·5H2O (D â‰ˆ +7.2 cmâ€“1) to negative values for NaMgAl1â€“xVx(ox)3Â·9H2O (D â‰ˆ â€“9.8 cmâ€“1 for x = 0.013, and D â‰ˆ â€“12.7 cmâ€“1 for x = 1) with an additional rhombic component, |E|, that varies between 0.8 and 2 cmâ€“1. On the basis of existing crystallographic data, this phenomenon can be identified as due to variations in the axial and rhombic ligand fields resulting from outer-sphere H-bonding between crystalline water molecules and the oxalate ligands. Spectroscopic evidence of a crystallographic phase change is also observed for K3V(ox)3Â·3Y2O (Y = H or D) with three distinct lattice sites below 30 K, each with a unique ground-state electronic structure.
Whereas there are hundreds of known iron(II) spin-crossover compounds, only a handful of cobalt(II) spin-crossover compounds have been discovered to date, and hardly an in depth study on any of them exists. This review begins with an introduction into the theoretical aspects to be considered when discussing spin-crossover compounds in general and cobalt(II) systems in particular. It is followed by case studies on [Co(bpy)3]2+ and [Co(terpy)2]2+ (bpyÂ =Â 2,2â€²-bipyridine, terpyÂ =Â 2,2â€²:6â€²,2â€³-terpyridine) presenting and discussing results from magnetic susceptibility measurements, X-ray crystallography, optical spectroscopy, and EPR spectroscopy.