Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the electronic structures of open-shell bent vanadocene compounds with chelating dithiolate ligands, which are minimum molecular models of the active sites of pyranopterin Mo/W enzymes. The compounds Cp2V(dithiolate) [Where dithiolate is 1,2-ethenedithiolate (S2C2H2) or 1,2-benzenedithiolate (bdt), and Cp is cyclopentadienyl] provide access to a 17-electron, d(1) electron configuration at the metal center. Comparison with previously studied Cp2M(dithiolate) complexes, where M is Ti and Mo (respectively d(0) and d(2) electron configurations), allows evaluation of d(0), d(1), and d(2) electronic configurations of the metal center that are analogues for the metal oxidation states present throughout the catalytic cycle of these enzymes. A "dithiolate-folding effect" that involves an interaction between the vanadium d orbitals and sulfur p orbitals is shown to stabilize the d(1) metal center, allowing the d(1) electron configuration and geometry to act as a low-energy electron pathway intermediate between the d(0) and d(2) electron configurations of the enzyme.

Reactions of [MoO2 (acetylacetonate)(2)] with the proligands (N-hydroxyimino)diacetic acid (H(3)hidpa), R, R-2,2'-(N-hydroxyimino)dipropionic acid (R,R-H(3)hidpa) or R,S-2,2'-(N-hydroxyimino) dibutyric acid (R,S-H(3)hidba) yielded the compounds [PPh4][Delta,Lambda -Mo(hida)(2)]. CH2Cl2 1, [H5O2][Delta -Mo(R,R-hidpa)(2)] 2, [PPh4][Mo(R,S-hidba)(2)]. 2H(2)O 3a and Na[Delta,Lambda -Mo(R,S-hidba)(2)]. 1/4 Pr-i(2) O 3b, respectively. Reactions of H(3)hida with a methanolic solution of [PPh4][MoOCl4 (H2O)] in the presence of NaOH (ca. pH 8) provided an alternative synthesis for 1. The complex of 1 when transferred into CH2Cl2 using [PPh4]Br yielded brown block-like crystals from a CH2Cl2-EtOH solution, however, 2 and 3b were crystallised from H2O and MeCN solutions with [H5O2](+) and [Na](+) counter cations, respectively. X-Ray crystallography confirmed the same distinctive eight-co-ordinate geometry of the complex anions of 1, 2 and 3b as identified for Amavadin, the form in which vanadium(IV) is bound in Amanita muscaria mushrooms. EPR and UV/vis spectra recorded for 1, 2 and 3a are consistent with the presence of molybdenum(V). Cyclic voltammetric studies using a glassy carbon working electrode in CH2Cl2 for 1 exhibited a reversible Mo-VI/Mo-V and a quasi-reversible Mo-V/Mo-IV redox couple at E-1/2 = +0.96 and -0.99 V (vs. a saturated calomel electrode), respectively. Complex 3a also displayed a reversible Mo-VI/Mo-V redox couple at E-1/2 = +0.77 V, whereas the Mo-V/Mo-IV couple was irreversible (E-pc = -1.28 V). Additional electrochemical studies with 2 recorded a reversible Mo-VI/Mo-V redox couple in Me2SO (E-1/2 = +0.77 V), however in H2O this one-electron oxidation process is irreversible.

The development of the understanding of the co-ordination chemistry and the properties of Amavadin, the chemical form in which vanadium is accumulated by the Amanita genus of mushrooms, is reviewed.