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Sulfite-oxidizing enzymes from eukaryotes and prokaryotes have five-coordinate distorted square-pyramidal coordination about the molybdenum atom. The paramagnetic Mo(v) state is easily generated, and over the years four distinct CW EPR spectra have been identified, depending upon enzyme source and the reaction conditions, namely high and low pH (hpH and lpH), phosphate inhibited (Pi) and sulfite (or blocked). Extensive studies of these paramagnetic forms of sulfite-oxidizing enzymes using variable frequency pulsed electron spin echo (ESE) spectroscopy, isotopic labeling and density functional theory (DFT) calculations have led to the consensus structures that are described here. Errors in some of the previously proposed structures are corrected.

The electron spin echo envelope modulation (ESEEM) investigation of the high-pH (hpH) form of sulfite oxidase (SO) and sulfite dehydrogenase (SDH) prepared in buffer enriched with H(2)(17)O reveals the presence of three types of exchangeable oxygen atoms at the molybdenum center. Two of these oxygen atoms belong to the equatorial OH ligand and the axial oxo ligand, and are characterized by (17)O hyperfine interaction (hfi) constants of about 37 MHz and 6 MHz, respectively. The third oxygen has an isotropic hfi constant of 3-4 MHz and likely belongs to a hydroxyl moiety hydrogen-bonded to the equatorial OH ligand. This exchangeable oxygen atom is not observed in the ESEEM spectra of the Y236F mutant of SDH, where the active site tyrosine has been replaced by phenylalanine.

The crystal structure of [SnLCl3] has been determined and shown to consist of discrete distorted fac-octahedral molecules [L = tris(3,5-dimethylpyrazolyl)hydroborate]. The single-crystal EPR spectra, at room temperature and Q-band frequencies, of [MoE(L)Cl2] and [MoO(L)(NCS)2], E = O or S, diluted in the structurally similar [SnLCl3] have been recorded. The data from the different possible diluent sites in [SnLCl3] are consistent with exact, or nearly exact, monoclinic EPR symmetry for each of the molybdenum compounds which involves a rotation of the g and A tensors about an axis perpendicular to the mirror plane in each molecule. The angles of rotation are in the range 30–38° and are in good agreement with those obtained via simulation of the frozen-solution X-band EPR spectra of the compounds. The relationships between the g and A tensors, the molecular geometries, and the electronic structures are discussed.

The synthesis, crystal structure, spectroscopic and electrochemical properties of tris(quinoxaline-2,3-dithiolato)molybdate(IV), [Mo(qdt)3]2–, are reported. [PPh4]2[Mo(qdt)3]·H2O crystallizes in the space group P21/c, with a= 20.150(20), b= 14.014(5), c= 23.530(20)A, β= 104.34(8)°, and Z= 4. The anion has an essentially trigonal prismatic MoS6 core with Mo–S (average) 2.393(10)A. [Mo(qdt)3]2– undergoes reversible protonation on the ligand nitrogen atoms and the triply protonated species, formulated as [Mo(Hqdt)3]+, has been isolated and characterized as its chloride salt. Cyclic voltammetry indicates that [Mo(qdt)3]2– undergoes one-electron reduction and oxidation processes. The oxidation product, [Mo(qdt)3]–(e.s.r. parameters g= 2.002, Ā= 32.9), can be generated in solution by chemical oxidation.

The mononitrosyl complexes [Mo{HB(Me2pz)3}(NO)XY][HB(Me2pz)3–= hydrotris (3,5-dimethyl-1-pyrazolyl)borate; X = Y = F, Cl, I, OEt, OPh, SPh, or NHPh; X = Cl, Y = SPh, OPh, OEt, NHC6H4Br-p, NHC6H4Me-p, or NHEt; X = I, Y = OPh, NHC6H4Me-p, NHEt, or NHNMe2] have been studied in solution by 95Mo and 14N n.m.r. spectroscopy. The iodo complex, [Mo{HB(Me2pz)3}(NO)I2], exhibits a 95Mo resonance at δ 2 272, the most deshielded monomeric MoII resonance reported to date. Replacement of the iodo ligands of [Mo{HB(Me2pz)3}(NO)I2] by other ligands increases the shielding of the 95Mo nucleus according to the general trend, I– < Cl– < F– < thiolato < alkoxy < amido. A chemical shift range of δ 161–2 272 was observed for the complexes studied. The complexes exhibit an inverse halogen dependence of the chemical shift. The nitrosyl ligand 14N chemical shifts follow the same shielding trend as the 95Mo chemical shifts.

The structure of the title compound has been redetermined by single-crystal X-ray difraction from three-dimensional data collected by counter methods. The compound crystallizes in the monoclinic space-group P21/c with a= 6·408(4), b= 13·514(8), c= 16·821(9)A, β= 116·66(3)°, and Z= 4. The structure was refined by full-matrix least-squares methods to R 0·035 for 1209 independent reflections. The co-ordination geometry about the cobalt is a tetragonal pyramid with the NO group at the apex. The Co atom is 0·52 A above the basal plane formed by the four S atoms, and the nitrosyl group is disordered such that the O atom alternately lies above one or the other of the Co–S bonds of the same dithiocarbamate ligand. Co–N is 1·746(7)A and Co–S (mean) 2·263(4)A. An improved synthesis of this compound and its solution n.m.r. spectrum are reported.