A microscopic approach to the problem of cooperative sin crossover in the [MnL2]NO3 crystal, which contains Mn-III ions as structural units, is elaborated on, and the main mechanisms governing this effect are revealed. The proposed model also takes into account the splitting of the low-spin T-3(1) (t(2)(4)) and high-spin E-5 (t(2)(3)e) terms by the low-symmetry crystal field. The low-spin -> high-spin transition has been considered as a cooperative phenomenon driven by interaction of the electronic shells of the Mn-III ions with the all-around full-symmetric deformation that is extended over the crystal lattice via the the observed thermal dependencies of the magnetic susceptibility and the effective acoustic phonon field. The model well explains magnetic moment.