Effect of size of metal ion on MS4N2 chromophore: Synthesis,spectral and single crystal X-ray structural studies on (2,2′-bipyridine)bis(N-cyclohexyl-N-methyldithiocarbamato)M(II) (M = zinc,cadmium)

Isomorphous complexes Zn(S₂CN(Me)Cy)₂(bipy)] (1) and Cd(S₂CN(Me)Cy)₂(bipy)] (2) (where Cy(Me)NCS₂⁻ = N-cyclohexyl-N-methyldithiocarbamate anion and bipy = 2,2′-bipyridine) have been synthesised. Their structures and spectroscopic properties have been studied by IR, ¹H and ¹³C NMR spectroscopy and single crystal X-ray analysis. IR spectrum of the complexes show the contribution of thioureide form to the structures. ¹H NMR spectra of the complexes showed the desheilding of methyl protons and H-1 of cyclohexyl group on complexation. The downfield shift of N¹³CS₂, methyl carbon and C-1 of cyclohexyl group carbon signals for 2 (205.5, 36.7 and 64.4 ppm) from the chemical shift value of 1 (204.2, 35.6 and 63.1 ppm) is attributed to the movement of more electron density from dithiocarbamate towards cadmium. Single crystal X-ray structures of 1 and 2 indicate that the central metal atom is in a distorted octahedral environment for both complexes. The presence of added 2,2′-bipyridine ligand in the coordination sphere of M(S₂CN(Me)Cy)₂ increases Zn–S distances and decreases S–Zn–S angles in 1 and slightly increases Cd–S distances in 2. S–Cd–S angles are not affected. This is due to the relatively larger size of the cadmium ion compared to zinc ion which alleviates the strain involved in transformation from tetrahedral to octahedral.