Here we present a microscopic approach to the problem of cooperative spin transitions in crystals containing clusters of transition metal ions as structural units. The theory is applied to compounds based on the cyano-bridged pentanuclear clusters {[M(tmphen)2]3[M’(CN)6]2} (M3M’ 2) with a trigonal bipyramidal structure (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) (Fig.1). The compounds are shown to cardinally change their physical characteristics within the same ligand composition and geometrical structure of the constituent clusters by varying only the metal ions. In the FeII 3FeIII 2 and FeII 3CoIII 2 compounds the FeII ions demonstrate the low-spin(ls) –high-spin (hs) transformation (spin crossover) accompanied by the spin change S(FeII( 6 t2 ))=0→S(FeII( 4 2 ) t2e )=2 . An unprecedent charge transfer induced spin transition (CTIST) (ls-FeII)3(ls-OsIII)2 →(hs-FeIII)2(ls-FeII) (ls-OsII)2 has been observed for the Fe3Os2 compound. The spin conversion in the Fe3M’ 2 (M’=Fe, Co, Os) compounds has been proved by the Mössbauer spectroscopy, magnetic measurements and single crystal X-ray studies. Both spin-crossover and CTIST are considered as cooperative phenomena driven by the long-range interaction with the all-round full symmetric lattice deformation. Due to the proximity of the metal ions in the clusters the short-range interaction between these ions inside the cluster via the optic phonon field is taken into account as well. The model also involves the spin-orbital coupling operating within the cubic ( 4 2 ) 2 2 5T t e and ( 5 ) 2 2 2T t terms of the of the hs-FeII ions and ls-FeIII , ls-OsIII ions, respectively. For the FeII 3FeIII 2 cluster the Heisenberg exchange between the hs FeII and ls FeIII ions is included into consideration. In addition the model for CTIST in the Fe3Os2 compound accounts for the metal-metal electron transfer. The competition between short- and long-range interactions is shown to determine the type and the temperature of the spin transitions in the M3M’ 2 compounds. The developed model reproduces well the observed temperature dependence of the magnetic moments related to the spin transitions (Fig.2) and the Mössbauer spectra. The calculated temperature dependence of the electronic density redistribution is in a good agreement with the experimental data obtained from the Mössbauer spectra.Fig.1. Molecular structure of pentanuclear {[M(tmphen)2]3[M’(CN)6]2} clusterFig.2. Temperature dependence of cT product for FeII 3CoIII 2 compound (circles – experiment, solid line – theory).