Compounds having the general formula trans-M(II)(PY)4X2 (M(II) = V, Mn, Fe, Co, Ni, Ru; X = NCS, Cl, Br, I) have been synthesized, and the vanadium(II) and manganese(II) compounds have been characterized by ESR spectroscopy using the iron, cobalt, nickel, and ruthenium compounds as solid solvents. Furthermore, all compounds prepared have been characterized by means of X-ray powder diffraction and thereby space groups and unit cell parameters have been obtained in most cases. The spin-Hamiltonian parameters have been obtained for all the V(II) and Mn(II) compounds using a simulation procedure involving exact numerical diagonalization of the spin-Hamiltonian matrices. It was found that the paramagnetic trans-Fe(PY)4X2 and trans-CO(PY)4X2 could be satisfactorily used at room temperature as solid solvents for the V(II) and Mn(II) compounds with respect to the ESR measurements, while this was not the case for the trans-Ni(PY)4X2 compounds. This has been attributed to the orbitally degenerate ground states of iron(II) and cobalt(II). The trans-Ru(py)4X2 compounds, being diamagnetic, were also satisfactory solid solvents. The spin-Hamiltonian parameters obtained were found to be almost independent of the solid solvent, and typical values of the zero-field splitting parameters for the vanadium(II) compounds were for trans-V(PY)4(NCS)2 and trans-V(PY)4Cl2, absolute value of D almost-equal-to 0.2 cm-1, and for trans-V(PY)4Br2 and trans-V(PY)4I2, absolute value of D almost-equal-to 0.37 cm-1, and for the manganese(II) compounds, trans-Mn(PY)4(NCS)2, absolute value of D almost-equal-to 0 cm-1, trans-Mn(PY)4Cl2, absolute value of D almost-equal-to 0.2 cm-1, trans-Mn(PY)4-Br2, absolute value of D almost-equal-to 0.65 cm-1, and trans-Mn(PY)4I2, absolute value of D almost-equal-to 0.95 cm-1. In all cases except trans-Mn(PY)4(NCS)2 the rhombic zero-field splitting parameter E was found to below (<1/10D). The g values of the vanadium (II) and manganese(II) compounds were 1.98 and 2, respectively, and the hyperfine coupling constants were 66.0 X 10(-4) cm-1 less-than-or-equal-to absolute value of less-than-or-equal-to 70.0 X 10(-4) cm-1 and 77.0 X 10(-4) less-than-or-equal-to absolute value of D 83.0 x 10(-4) cm-1. These parameters are discussed in relation to the structures and compared to the parameters previously reported for the trans-Cr(pY)4XY+ compounds which are isoelectronic with the corresponding vanadium(II) compounds. This study is the first systematic characterization of a series of vanadium(II) compounds by means of ESR spectroscopy, and to our knowledge it contains the hitherto most extensive nonapproximative simulations of ESR spectra.
