SOLID-SOLUTIONS OF A JAHN-TELLER COMPOUND IN AN UNDISTORTED HOST .2. HIGH-SPIN, 6-COORDINATE CR-2+ WITHOUT JAHN-TELLER DISTORTION

A series of solid solutions of the type [Cr(x)Zn1-x(nic)2(H2O)4], in which nic represents C6H4NO2-, the conjugate base of nicotinic acid, have been prepared as single crystals and characterized by X-ray diffraction. The pure chromium end member is triclinic, space group P1BAR, with a cell volume V of 360.5(2) angstrom-3. The compound has a Jahn-Teller distortion. The pure zinc end member, which has no Jahn-Teller distortion, crystallizes in the monoclinic system, space group C2/m, with V = 728.0(1) angstrom-3. Eight solid solutions with chromium mole fractions in the range of 0.145(12) to 0.777(12) all adopt the structure of the undistorted zinc nicotinate host, with space group C2/m and V ranging only from 731.1(4) angstrom-3 to 740.6(3) angstrom-3 (at 20-degrees-C) for the lowest and highest Cr mole fractions, respectively. The population at the metal-atom site was established for each sample by X-ray diffraction. The questions of homogeneity in the crystalline samples and of the significance of the population refinement are discussed. A non-rigorous geometrical concordance exists between the triclinic and monoclinic end members. An extensive hydrogen bonding network in the zinc-type lattice confers stability on the mixed systems, in which high-spin, d4 chromium centers exist without Jahn-Teller distortion.