The colours of simple salts of the violurate anion

Salts of the violurate anion with the alkali and alkaline earth metals, the d¹⁰ ions Zn²⁺ and Cd²⁺, Mn²⁺, Pb²⁺, Ag⁺ and the lanthanides show a variety of spectacular colours in the solid state. The metal ions have no intrinsic absorption in the visible region (apart from the weak spin-forbidden bands of Mn²⁺) and do not normally show charge-transfer absorption. The colours are ascribed to the n→π* transition of the violurate anion. As confirmation of this assignment the visible absorption of the K⁺ salt is shown to be polarized perpendicular to the plane of the violurate anion. Low temperature (∼20 K) absorption spectra of the Na⁺, K⁺ and Rb⁺ salts are reported.