Structural studies in metal–purpurate complexes. Part 2. Crystal structures of diaquabis(purpurato)-calcium and -cadmium dihydrate. Stereochemistry of the [M(tridentate ligand)2(unidentate ligand)2] complexes

Abstract

The crystal structures of the title compounds, [ML₂(H₂O)₂]·2H₂O {[L]^–= purpurate, M = calcium (1) or cadmium (2)}, have been determined at 295 K by X-ray diffraction and refined by least squares to R 0.055 (3 253 ‘observed’ reflections) and 0.052 (3 351 ‘observed’ reflectlons) respectively. Crystals of both are triclinic, space group P. For (1), a= 13.727(6), b= 10.300(3), c= 9.223(4)Å, α= 96.45(3), β= 99.39(3), γ= 104.29(3)°, Z= 2; for (2), a= 13.623(5), b= 10.190(5), c= 9.314(2)Å, α= 97.74(3), β= 98.96(3), γ= 104.78(4)°Z= 2. In both compounds, the metal atom of the complex molecule is eight-co-ordinated by two tridentate purpurate ligands [(1) : Ca–N, 2.630(3), 2.560(3), Ca–O,2.400(3)–2.600(5), (2); Cd–N, 2.467(5), 2.383(5), Cd–O, 2.317(5)–2.630(4)Å], each twisted appreciably with respect to the median plane, and two pseudo-axial water molecules [Ca–O, 2.306(4), 2.428(3); Cd–O, 2.271(6), 2.451(5)Å; O–Ca–O, 169.1 (1); O–Cd–O, 167.0(2)°]. The stereochemistry is intermediate between that of a square antiprism and a hexagonal bipyramid as predicted by repulsion calculations.