Spin crossover (SCO) phenomenon is referred to the transitions between high-spin (hs) to low spin (ls) or ls to hs states in metal complexes with d⁴-d⁷ electronic configurations. The phenomenon is routinely observed only in Fe^II and Fe^III complexes. Recently it has been shown that modulation of the orientation of the N₄(O^-)₂ donors in a series of hexadentate ligands has a dramatic effect on the spin state of MnIII complexes and that gradual thermal SCO may be attained when the phenolate oxygen donors are trans to each other. This geometry is achieved by varying the length of the alkyl chains in the starting tetraamine. The lengthening of the tetraamine link by two methylene groups (L2) orients the oxygen donors in trans geometry and produces the [MnL2]NO3 compound demonstrating SCO [1]. The magnetic moment of this compound manifests a significant temperature dependence. In the present communication a microscopic approach to the problem of SCO in crystals containing manganese(III) ions as structural units is developed and the main mechanisms governing this effect are revealed. In order to properly reproduce the energies of the low-lying levels of an isolated manganese (III) complex the model takes into account the following basis set including 73 states:formula . The interaction of the manganese ions with the crystal field of C2v symmetry splitting the cubic ³T₁ and 5E terms is included in the model. The range of the crystal field parameters for which the low-lying part of the energy spectrum of the MnIII ion consists of two non-degenerate levels arising from the states ³T₁ and 5E with the spin S=1 state being the ground one is determined. It is shown that in this case the ground level mainly originates from the ³T₁(T₂⁴) term i.e. the situation facilitating SCO takes place. The ls-hs transition is considered as a cooperative phenomenon driven by the interaction of the electronic shells of the MnIII ions with the all-round full symmetric deformation that is extended over the crystal lattice via theacoustic phonon field. The parameter of cooperative interaction is evaluated with the aid of experimental data on the elongation of all four equatorial Mn-N bond lengths. The SCO phenomenon is treated within the molecular field approximation. Quite good agreement has been obtained between the calculated and experimental values of the magnetic susceptibility and effective magnetic moment.FigureTemperature dependence of the magnetic susceptibility for the [ MnL2]NO3 compound: squares –experimental data [1], solid line – calculations