Vanadium trigonal bipyramidal coordination

Complexes of vanadium(III) typically have octahedral coordination, though other coordinations are certainly not unusual, particularly with bulky ligands where trigonal bipyramidal coordination is adopted. Nitrogen- and oxygen-containing mul-... 

Enzymes requiring vanadium for catalytic activity. Perhaps the best studied of these are the vanadium-dependent nitrogenases [EC 1.18.6.1]. Other vanadium-dependent enzymes include vanadium haloperoxidase, vanadium chloroperoxidase, and vanadium bromoper-oxidase. In the vanadium chloroperoxidase and bromo-peroxidase reactions, the vanadium(V) is coordinated in a trigonal bipyramidal site to a histidyl residue, three nonprotein oxygens, and, presumably, to a hydroxide. 

The complexes typically have square pyramidal (11) or bipyramidal structure (12) with vanadyl oxygen apical, the vanadium atom lying 0.035-0.055 A above the plane defined by the equatorial ligands.354 There have been reports of five-coordinate trigonal bipyramidal complexes (13) with structures determined by X-ray diffraction techniques. 

The basic relationship between the coordination mode and the number of electrons around the metal still applies in rather complicated systems such as those with three BEU- ligands. Complexes of general formula have been characterised with scandium, titanium and vanadium. All the complexes adopt distorted trigonal-bipyramidal geometry with the BH4- ligands at the equatorial positions (Scheme 16). 

Structure and Functions. The VHPOs contain vanadate(V) H2VO4 = V0(0)(0H)2 in their active site. Vanadium is covalently bonded to the Ns of the imidazole moiety of a histidine constituent of the protein, with the vanadium center in a trigonal-bipyramidal environment (Figs. 1 and 2). At pH 8.5, the dissociation constant is ca 45 nM. Oxidant is hydrogen peroxide, which is reduced to water. Hydrogenperoxide is activated by coordination to vanadium, forming a hydroperoxo species V0(0H)(H02)+ (eq. la) in a distorted tetragonal pyramidal... 

As observed in the (arylimido)vanadium(V) triisopropoxides, the crystal structure of the (phenylimido)vanadium(V) ethoxydichloride [(C6H5N)V(OEt)Cl2] (31) reveals the p-ethoxido-bridged dimeric structure with the V(l)-N(l) distance of 1.6554(14) A and the nearly linear C(l)-N(l)-V(l) angle of 173.46(13)°, in which each vanadium atom is coordinated in a trigonal-bipyramidal geometry by bridging... 

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