Five-coordinate geometry

Krebs and co-workers synthesized a series of dinuclear copper(II) complexes as models for catechol oxidase 91 (365) (distorted SP Cu-Cu 2.902 A), (366) (distorted five-coordinate geometry Cu-Cu 3.002A), (367) (distorted SP Cu-Cu 2.995 A), (368) (distorted five-coordinate geometry Cu-Cu 2.938 A), and (369) (distorted SP Cu-Cu 2.874 A). These complexes were characterized by spectroscopic and electrochemical methods. From kinetic analysis, a catalytic order for catecholase activity (aerial oxidation of 3,5 -di - ter t-buty lcatec h o 1) was obtained.326... 

X-ray structure analyses of Rh(COCH3)(I)2(dppp) (14) and [Rh(I I)(I)(//-I)(dppp)]2 (15), where dppp l,3-bis(diphenylphosphino) propane, were reported. Unsaturated complex (14) possesses a distorted five-coordinate geometry that is intermediate between sbp and tbp structures.69 Under CO pressure, the rhodium/ionic-iodide system catalyzes either the reductive carbonylation of methyl formate into acetaldehyde or its homologation into methyl acetate. By using labeled methyl formate (H13C02CH3) it was shown that the carbonyl group of acetaldehyde or methyl acetate does not result from that of methyl formate.70... 

For mono-organo tin, five- and six-coordination are often found234. Five-coordinate geometry verging on the square pyramid is sometimes observed as in various RSnL2 compounds235 for L a bidentate S ligand. Seven-coordination is also represented, usually... 

The intriguing question is how the seven-coordinate geometry around the metal center favors its remarkable catalytic activity, knowing that in the native MnSOD and FeSOD enzymes the active metal center has a five-coordinate geometry (3a,14f30). All SOD... 

Thus while the Jahn-Teller theorem is generally invoked to account for the distorted octahedral geometries of the copper(II) ion, in terms of the first-order vibronic coupling the extension of the coupling to some second-order effects also allows some rationalization of the tetrahedral and five-coordinate geometries of the copper(II) ion.1063-1067... 

The d-d, CD and MCD spectra of cobalt carbonic anhydrase are pH dependent, as noted earlier. In general the d-d and MCD spectra of the enzyme in acidic pH resemble those of Co11 complexes of tetrahedral geometry. The spectra at alkaline pH are different from the spectra at acid pH, and suggest the presence of a distorted five-coordinate cobalt(II) centre. Thus it appears that in the active form of the enzyme the metal is present in a five-coordinate geometry. 

We have considered several other possibilities for vanadium bromo-peroxidase. A cis-(dioxo) arrangement, even with only two ligands at 1.72 A, gives an oxidation state by BVS of 6.30, which is also inconsistent with the known valence (5+) of the metal. Other six-coordinate arrangements with the sixth ligand as H20 at a long distance (2.2-2.3 A) produce similar results. However, a five-coordinate geometry, with the 2.11 A bonds as either V-O or V-N, has a calculated valence of 4.83 and 5.08, respectively, consistent with the known oxidation state of 5+. 

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