Chromium octahedral bonding geometries

Figure 4. A comparison of the octahedral bonding geometries of trans-tris(ben-zohydroxamato)chromium(III) and cis-tris(benzohydroxamato)iron(IH)...

Let us start with the Ni(II) complexes of the already mentioned scorpiand diammac (6,13-diammino-6,13-dimethyl-1,4,8,11-tetraazacy-clotetradecane) in its two cis and trans conformations. In contrast to the previously mentioned chromium-, iron-, and cobalt-diammac complexes, in which the geometry of [M(fra s-diammac)]" + was substantially octahedral and that of the [M(cw-diammac)]" + was substantially trigonal prismatic, in the case of both [Nif/raws-diammac)]2+ and [Ni(m-diammac)]2 + the structural differences are attenuated and both can be viewed as more or less distorted octahedral geometries, with two sets of averaged Ni-N bond lengths of 2.07 A and 2.13 A, respectively.161 162 This is reflected by the fact that both the two complexes exhibit in aqueous solution a chemically reversible Ni(II)/Ni(III) oxidation ([Nif/raws-diammac)]2 + E° = + 0.67 V vs. SHE [Ni(m-diammac)]2 + ... 

The crystal structure of L-histidinato-D-pencillaminatochromium(III) monohydrate has been determined The complex was synthesized from chromium(III) chloride, l-histidine and D-penicillamine in aqueous solution. The chromium(III) was coordinated to the N, S, and O of penicillamine and the N, O and imidazole (N) of histidine in an approximately octahedral geometry. Thus, both cysteine and penicillamine form complexes with Cr(III) which contain Cr-S bonds. 

The structural studies have suggested an octahedral geometry for chromium hexacarbonyl. The Ci C distance is found to be 1.92 A, while the C—O bond length is 1.16 A. The molecule is also found to be diamagnetic. 

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