X-Ray structural evidence for the influence of geometrical distortion on the spin-state of five-co-ordinate cobalt(II) and nickel(II) complexes

Abstract

The structures of the two isomorphous complexes [CoLI]I and [NiLI]I [L = the tripod-like quadridentate ligand NN-bis-(2-diphenylphosphinoethyl)-N-(2-diethylaminoethyl)amine] have been determined from X-ray diffractometer data, that of the Ni complex by Patterson and Fourier methods, and of the Co complex by symbolic addition methods. The nickel complex is low-, the cobalt complex high-spin.

Both complexes are monoclinic, space group P2₁/c, Z= 4. Cell dimensions are: Ni, a= 19·881(2), b= 9·468(1), c= 19·783(1)Å, β= 113·75(1)°; Co, a= 20·309(2), b= 9·432(1), c= 20·100(3)Å, β= 115·52(1)°. The structures were refined by full-matrix least-squares methods to R 6·9 (Co, 1875 independent reflections) and 5·2%(Ni, 3093 independent reflections).

The geometries of both complexes are trigonal-bipyramidal, but the nickel complex is distorted toward an elongated square-pyramid, whereas the cobalt complex is distorted toward a tetrahedron. This distortion can account for the different spin states of the two complexes.