An EPR Study of the Photoexcited Triplet State of Tetrahedral d° Transition Metal Anions

Abstract

The excited states of tetrahedral anions which in their groundstate have a d° transition metal at the centre — such as VO₄ ^3-, CrO ₄ ^2-, MnO₄ ^-, and similar compounds with 4d and 5d metals — have since long been the object of study. In an MO description the first excitation corresponds to the “charge-transfer” t₁ → 2e, in which the t₁ MO’s are linear combinations of 2p AO’s on the oxygens, and the 2e MO’s involve the d_x2_-y2 and d_z2 AO’s on the metal (1). Four electronic states result, the energies of which have been predicted to lie in the order ¹T₂ > ¹T₁ > ³T₂ ≅ ³T₁ (2). In this picture the long-wavelength absorption band of such an ion is the crystal-field-induced ¹T₁ ← ¹A₁ transition while the second, much stronger band is the dipole-allowed ¹T₂ ← ¹A₁ transition.