A microscopic model of low-spin/high-spin transitions in solids containing transition metal ions with d(6) configuration is described. The underlying physics of the transition is related to the large totally symmetric relaxation of ligands accompanying the two electron promotion from the t(2) to e orbitals. The electron distribution function, the electronic entropy, and the heat capacity are determined from the analysis of the free energy. The qualitative estimation of the relaxation is in agreement with x-ray determination. The electronic and lattice entropies are comparable because of the large lattice relaxation. The stepwise low-spin/high-spin transition of Fe2+ with temperature is demonstrated and, hence, the temperature induced abrupt changes in the properties of solids. The weak long-range intermolecular interaction is not excluded but it is not necessary for the explanation of a variety of experimental observations.