To bend or not to bend:: Dilemma of the edge-sharing binuclear square planar complexes of d8 transition metal ions

The binuclear complexes of d(8) transition metal ions of type [L2M(mu-XRn)(2)ML2] (n = 0 to 2) show a wide variety uf bending angles between the coordination planes of the two metal atoms. A theoretical and structural analysis of those compounds with unsubstituted bridges, [L2M(mu-X)(2)ML2], tries to unravel the factors that determine the degree of bending of such compounds. A rationale is given for the structures of 139 crystallographically independent molecules. A driving force for bending of the molecules is the attractive metal...metal interaction that results from donor-acceptor interactions between the d(z)(2) and p(z) orbitals of the two metal atoms and is modulated by the nature of (a) the metal atom, (b) the terminal ligands, and (c) the bridging atoms, In all cases the energy difference between the planar and the bent form is within the 10 kcal/mol range, and steric effects seem to be important in preventing bending only for the bulkiest terminal ligands. For the studied bridging atoms, the stability of the bent form increases in the order O < S > Se > Te and F < Cl < Br < I, and for different metal atoms it increases in the order Ni(II) < Pd(II) < Pt(II) < Rh(I) < Ir(I).