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Studies on pyridine : thiourea and substituted thiourea complexes of cobalt (II) arylcarboxylates Wong, Chun Tong


A number of new thiourea and substituted thiourea complexes of cobalt(II) carboxylates, Co(RCO₂)₂•L₂ (where RCO₂ is acetate or arylcarboxylate and L is thiourea, ethylenethiourea or N,N'-dimethylthiourea), have been prepared and characterized, mainly by spectral studies in the infrared, near-infrared and visible regions and by magnetic susceptibility studies over the temperature range 80 - 320 ⁰K. These studies have shown that, the molecular structures involve a pseudo-tetrahedral ligand environment about cobalt with two oxygen and two sulphur atoms forming the immediate environment about the metal (CoO₂S₂ chromophore). Differences in the solid state electronic spectra of different carboxylate complexes are not simply related to the nature of the carboxylate group or sulphur-ligand involved and are ascribed to solid state crystal packing effects which result in variations in the detailed geometry about the cobalt ions. The magnetic properties of these complexes obey Curie-Weiss law with magnetic moments in the range 4.33 - 4.46 B.M. and Weiss constants in the range -2 to -7 ⁰K. The data have been analyzed, assuming a zero-magnetic-field splitting of the ground state spin levels caused by ligand field asymmetry. Pyridine complexes of the general formula Co(RCO₂)₂ • py₂ (where RCO₂ is acetate, trifluoroacetate or arylcarboxylate) have been prepared and spectral and cryomagnetic studies show the complexes to be pseudo- octahedral with four oxygen atoms and two nitrogen atoms forming the immediate environment about cobalt (CoO₄N₂ chromophore). The complexes have been classified into two groups. One group, which includes the acetate, trifluoroacetate and two ortho-substituted benzoate complexes, exhibits relatively high magnetic moments (4.95 - 5.02 B.M.) and cryomagnetic and electronic spectral properties consistent with tetragonal structures. These complexes are considered to have structures involving bridging carboxylate groups and trans-coordinated pyridine molecules. The other group of complexes which includes the benzoate and two para-substituted benzoate complexes have much lower room-temperature magnetic moments (4.6 - 4.7 B.M.) and in general exhibits spectral and cryomagnetic properties suggestive of lower symmetry about cobalt. These complexes are considered to have structures involving cis-coordination of pyridine molecules. No obvious correlation between structural type and the nature of the carboxylate group was found. The stabilities of these complexes towards pyridine dissociation were, however, found to be related to the nature of the anion involved, decreasing with increasing base strength of the carboxylate ion.

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