A computational method targeted to Werner-type complexes is developed on the basis of quantum mechanical effective Hamiltonian crystal field (EHCF) methodology (previously proposed for describing electronic structure of transition metal complexes) combined with the Gillespie-Kepert version of molecular mechanics (MM). It is a special version of the hybrid quantum/MM approach. The MM part is responsible for representing the whole molecule, including ligand atoms and metal ion coordination sphere, but leaving out the effects of the d-shell. The quantum mechanical EHCF part is limited to the metal ion d-shell. The method reproduces with reasonable accuracy geometry and spin states of the Fe(II) complexes with monodentate and polydentate aromatic ligands with nitrogen donor atoms. In this setting a single set of MM parameters set is shown to be sufficient for handling all spin states of the complexes under consideration.

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rnA series of the d 6 iron(I1) complexes with bulky organic ligands (like [ Fe(bzpy),(NCS),]) can exist in two spin forms: in the low-spin ( S = 0) form at low temperature and in the high-spin ( S = 2) form at high temperature. In the crystal phase, the transition between these two forms may be either smooth or abrupt. Recently, the abrupt spin transitions were identified with the first-order transitions between different ordered phases occurring in the binary mixtures of the two spin forms of the complex. Here, we apply the method widely used in the field of binary metal alloys to the analysis of the spin transitions. The molecules undergoing the spin transition are modeled by octahedra of variable size which interact when they are immediate neighbors in the crystal lattice. We show that some simple assumptions concerning the intermolecular interaction and crystal geometry relaxation allows one to get the desired first-order phase transitions together with a satisfactory description for the crystal compressibility as a function of temperature.

The quantum mechanical effective Hamiltonian of crystal field (EHCF) methodology (previously developed for describing electronic structure of transition metal complexes) is combined with the Gillespie-Kepert version of molecular mechanics (MM) in order to describe multiple potential energy surfaces (PES) of the Werner-type complexes corresponding to different spin states of the latter. The procedure thus obtained is a special version of the hybrid quantum mechanics/molecular mechanics approach. The MM part is responsible for representing the whole molecule, including ligand atoms and metal ion coordination sphere, but leaving aside the effects of the d shell. The quantum mechanics part (EHCF) is restricted to the metal ion d shell. The method reproduces with considerable accuracy geometry and spin states of a wide range of Fe(II) and Co(II) complexes of various total spin and coordination polyhedra and containing both monodentate and polydentate ligands with aliphatic and aromatic nitrogen donor atoms. In this setting, a single MM parameters set is shown to be sufficient for dealing with all spin states and coordination numbers of the complexes.