On the Charge Density Distribution in Cobalt(III) Ammine Hexacyanochromate(III) Complexes

Abstract

A charge density refinement using effective anharmonic isotropic thermal parameters was performed on the X-ray diffraction data for pentaammineaquacobalt(III) hexacyanochromate(III) and for hexaamminecobalt(III) hexacyanochromate(III), constraining the unit cells of the crystals to be electically neutral. Our previous refinements, which did not admit of anharmonicity, could not simultaneously give a good fit and provide electroneutrality of the unit cell. The effective anharmonic parameters correct a systematic error observed in these, and, less extremely, in other transition-metal complex crystals. Values of the derived charge density parameters change mostly at less than the 2σ level of significance in the new refinements except for overall ionic charges. It seems the limitations in our earlier refinements did not lead to serious errors in the chemical conclusions reached, apart from the ionic charges. In the [Co(NH₃)₅(OH₂)][Cr(CN)₆] case we observe five significant charge flows; from CN to Cr, σ bonding 0.5(3), and π back-bonding 0.6(2) e; from NH₃ and H₂O to Co, σ bonding 0.7(3), and π back-bonding 0.5(2) e; and a transfer of 1.5(4) from Cr(CN)₆ to Co(NH₃)₅(H₂O) through 17 CN…H hydrogen bonds, with N…H(3) noticeably more important. Bond overlap densities are also significant. The relative sizes of these charge flows are similar for the isomorphous hexaamminecobalt(III) hexacyanochromate(III) salt.