SPIN EQUILIBRIUM IN 5-CO-ORDINATE LEWIS-BASE ADDUCTS OF COBALT(II) SCHIFF-BASE COMPLEXES WITH A S2N3 LIGAND ENVIRONMENT - MOLECULAR AND CRYSTAL-STRUCTURE OF [COL(4)(MIM)] AT 293 AND 103 K ,N'-BIS(5-MERCAPTO-3-METHYL-1-PHENYLPYRAZOL-4-YLME THYLENE)-O-PHENYLENEDIAMINE, MIM=N-METHYLIMIDAZOLE]

Four novel five-co-ordinate Lewis-base adducts of cobalt(II) Schiff-ba se complexes with a S2N3 ligand environment have been prepared and the ir variable-temperature magnetic susceptibilities determined (T = 77-3 00 K). The chi(Co)(-1) vs. T plots revealed a minimax behaviour charac teristic of an S = 1/2 reversible arrow 3/2 spin equilibrium. The corr esponding magnetic moments decrease in a gradual manner on lowering th e temperature. The thermodynamic parameters for the equilibrium were e stimated from the magnetic. moment data and Delta H degrees values wer e found to lie in the range of 273-757 cm(-1). Decrease of the in-plan e ligand field causes a decrease of the transition temperature and sta bilization of the high-spin relative to the low-spin ground state. The X-ray single-crystal structural determinations for [CoL(4)(mim)][H(2) L(4) = 1-phenylpyrazol-4-ylmethytene)-o-phenylenediamine, = N-methyl- imidazole] were carried out at 293 (mu(eff) = 3.6 mu(B)) and 103 K (mu (eff) = 23 mu(B)). The co-ordination geometry around the cobalt centre revealed a distorted trigonal bipyramid with one N and one S atom of the metallocycle as axial ligands, and the other N.S atoms and the N-i midazole nitrogen situated in an equatorial plane. Comparison of the m olecular structures at different temperatures indicates an occurrence of some configurational and conformational changes along with variatio ns in metal-ligand bond lengths associated with a spin transition. Low ering the temperature results in increase of one of the N-Co-S valence angles by 7.2 degrees, an increase in the Co-N(imidazole) bond length by 0.023 Angstrom but decreases in the other metal-ligand bonds by 0. 02-0.05 Angstrom.