| Abstract
| - The coordination mode of thiosemicarbazones (Htsc, 1) has been observed to be directed by the steric bulk of the alkyl/aryl group (R) trans to the hydrazinic nitrogen. When R is large, a four-membered chelate ring (2) is formed, and a five-membered ring (3) is formed only when R is small in size. This is observed in the reaction of [M(bpy)2X2] (M = Ru, X = Cl or M = Os, X = Br) with benzaldehyde thiosemicarbazones and acetone thiosemicarbazone, respectively, affording complexes of type [M(bpy)2(tsc)]+.
- Twelve mixed-ligand thiosemicarbazone complexes of ruthenium and osmium, ten of general formula [M(bpy)2(bztsc−R)]ClO4, (M = Ru, Os; bpy = 2,2‘-bipyridine, Hbztsc−R = benzaldehyde thiosemicacbazone) and twoof type [M(bpy)2(actsc)]ClO4 (Hactsc = acetonethiosemicarbazone), have been synthesized and characterized.All the complexes are diamagnetic (low-spin d6, S = 0) and in acetonitrile solution show several intense metal-to-ligand charge-transfer (MLCT) transitions in the visible region. Structures of Hbztsc−OMe, [Ru(bpy)2(bztsc−NO2)]ClO4 and [Ru(bpy)2(actsc)]ClO4 have been determined by X-ray crystallography. Benzaldehyde thiosemicarbazone exists in the thione form with the phenyl group trans to the hydrazinic nitrogen. The benzaldehydethiosemicarbazone ligand coordinates to the metals through the hydrazinic nitrogen and sulfur with a bite angleof ∼67°, forming a four-membered chelate ring. However, the actsc ligand coordinates through the imine nitrogenand sulfur, forming a five-membered chelate ring with a bite angle of ∼81°. The difference in coordinationmodes of two types of thiosemicarbazone ligands, viz., bztsc−R and actsc, appears to result from the differencein steric bulk of the aryl and methyl group trans to the hydrazinic nitrogen. In acetronitrile solution they all showa reversible metal(II)−metal(III) oxidation in the range 0.18−0.58 V vs SCE followed by an irreversible oxidationin the range 1.11−1.60 V vs SCE. Two successive one-electron reductions of the coordinated bipyridine are alsoobserved in the range −1.53 to −1.96 V vs SCE.
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