Low-spin six-co-ordinate cobalt( II ) complexes. A solution study of tris(violurato)cobaltate( II ) ions

The formation of complexes of CoII with violuric acid (1H,3H-pyrimidine-2,4,5,6-tetrone 5-oxime), H3vi, and its monomethyl (H2mvi) and dimethyl (Hdmvi) derivatives in dimethyl sulphoxide–water (80 : 20) has been investigated. Potentiometric, calorimetric, magnetic, and e.s.r. measurements clearly evidence that [Co(H2vi)3]–, [Co(Hmvi)3]–, and [Co(dmvi)3]– are low-spin octahedral complexes. The high spin–low spin change occurs upon binding of the third ligand.