A theoretical model has been developed to describe the valence tautomeric transformation in cobalt compounds with sulfur-containing ligands. Taking into account the molecular structure of the crystals demonstrating this transformation, the cooperative interaction between the molecular modes forming the spectrum of a single valence tautomeric complex and crystalline modes assuring the cooperativity in the system is assumed to be responsible for the charge transfer-induced spin transition between the configurations low-spin CoIII(sq)(cat) → high-spin CoII(sq)2. The problem of interacting complexes is solved by the mean-field approximation. The mean values of the products of the electronic diagonal matrices and the coordinates of the local modes for the two configurations participating in the transformation play the role of the order parameters. On the basis of the developed model, a comprehensive explanation of the magnetic characteristics of the Co(tBu2sq)2(3-tnp)2 and Co(tBu2sq)2(2-tnp)2 compounds is given. The effects of dipole–dipole interaction on the valence tautomeric transformation, accompanied by the redistribution of the electronic density inside each valence tautomeric complex, have also been examined. When the energy of the dipole–dipole interaction is comparable to that of other interactions involved in the model, the former can suppress or facilitate the spin transition. Calculations demonstrate that in the compound Co(tBu2sq)2(btnp)2, this interaction suppresses the valence–tautomeric transformation, which explains why this complex does not demonstrate a spin transition in the whole temperature range.
Klokishner et al. (Fri,) studied this question.