STRUCTURE AND STABILITY OF CARBOXYLATE COMPLEXES .20. DIAQUA BIS(METHOXYACETATO) COMPLEXES OF NICKEL(II), COPPER(II), AND ZINC(II) - A STRUCTURAL STUDY OF THE DYNAMIC PSEUDO-JAHN-TELLER EFFECT

The crystal structure of trans-diaquabis(methoxyacetato)copper(II) C6H 14O8Cu, has been determined by neutron diffraction at 4.2 K (monoclini c, P2(1)/n, a = 6.88(1), b = 7.19(1), c = 9.77(2) Angstrom, gamma = 95 .7(1)degrees, (Z = 2)) and by X-ray diffraction at 125, 165, 205, 240, 265, 245, and 325 K. These measurements show that there is no phase c hange in the temperature range 4.2-325 K. The copper(II) coordination at 4.2 K is a tetragonally distorted elongated rhombic octahedron (Cu- OOC 1.955(1), Cu-OMe 2.209(1), and Cu-OH2 2.031(2) Angstrom). As the t emperature increases to 325 K, the Cu-OOC bonds shorten slightly to 1. 934(5) Angstrom, the Cu-OMe bonds shorten more markedly to 2.137(4) An gstrom, and Cu-OH2 lengthens to 2.155(6) Angstrom to give a tetragonal ly distorted compressed rhombic octahedron. For comparison the structu re of the isomorphous nickel(II) complex (monoclinic, P2(1)/n, a = 6.6 33(1), b = 7.192(1), c = 10.016(2) Angstrom, gamma = 98.30(2)degrees, (Z = 2)) has been redetermined at 295 K and the structure of the analo gous zinc(II) complex (orthorhombic, F2dd, a = 7.530(1), b = 13.212(1) , c = 21.876(2) Angstrom (Z = 8)) has also been determined. The nickel (II) complex has an almost regular trans (centrosymmetric) octahedral coordination (Ni-OOC 2.022(1), Ni-OMe 2.043(1), and Ni-OH2 2.077(2) An gstrom). However, zinc(II) has a very distorted octahedral coordinatio n with tile zinc atom on a 2-fold axis with the water molecules and th e methoxy ligators cis and the carboxylate ligators trans(Zn-OOC 1.985 (1), Zn-OMe 2.304(2), and Zn-OH2 2.038(2) Angstrom). The variation in the dimensions of the copper(II) coordination sphere is discussed in t erms of static (low temperature) and planar dynamic (high temperature) pseudo-Jahn-Teller effects.