The charge density in dichlorotetrakis(thiourea)nickel(II), [Ni{SC(NH2)2}4Cl2]: a contrast between spectroscopy and diffraction

Abstract

The charge density observed by X-ray diffraction at 140 K in [Ni(tu)4Cl2](tu = thiourea) is reported. The values for the chloride, thiourea, and NiII fragments closely resemble those of the free ion or molecule, except (a) each chloride σ-donates 0.16(14) e and π-donates 0.35(15) e; (b) each thiourea σ-donates 0.17(20) e to the nickel; (c) NiII, in its 3B1 ground state, has a 3d configuration dxy1.91(14), dxz,yz3.08(18), dx2–y21.12(14), dz21.39(14) with a substantial [1.6(7) e] 4p population. These figures are consistent with a conventional bonding model, in which 3dσ orbitals on NiIIσ-bond almost equally to chlorine and thiourea, and in which 3dπ and 4pπ orbitals π-bond only to chlorine. The residual density maps suggest a differential nephelauxetic effect is occuring, with metal-centred π orbitals larger than σ. A polarisation of the entire NiCl2fragment is also observed, perhaps caused by intermolecular effects. Previous results show that thiourea has a much larger effect on the spectra and magnetism than the chloride. This contrast with the charge-density results is explained by the greater dominance of overlap in spectroscopy, and of ligand energy levels (or crudely, ionisation potentials) in charge densities. This illustrates experimentally the theoretical observation that covalence (i.e. molecular orbital) parameters are best ‘directly’ observed by diffraction, since spectroscopy is often dominated in more ionic complexes by the less interesting terms due to orthogonalisation of metal and ligand orbitals.