The dioxo-Mo(VI) complexes LM0O2X [L = hydrotris(3,5-dimethylpyrazol-1 -yl)borate (La), hydrotris(3isopropylpyrazol-1 -yl)borate (Lb), hydrotris(3,5-dimethyl-1,2,4-triazol-1 -yl)borate (Lc); X = Cl, Br, NCS, OMe, OEt, OPh, SPr', SPh, SCH2Ph] have been synthesized and characterized by spectroscopic and structural methods.The infrared spectra of the complexes exhibit ( 2) bands at 940-920 and 910-890 cm-1, and the NMR spectra are indicative of molecular C, symmetry in solution. The X-ray crystal structures of three complexes are reported. LaMo02(SPh): monoclinic space group P2\lc, a = 18.265( 6) Á, b = 8.110(3) Á, c = 18.299(3) Á, ß = 117.06(2)°, V = 2414(1) Á* 123 with Z = 4. LbMo02(OMe): monoclinic space group Cllc, a = 30.365(4) Á, b = 8.373(1) Á, c = 19.646( 2) Á, ß = 113.28(1)°, V = 4588(1) Á3 5with Z = 8. LcMo02(SPh): orthorhombic space group P2,2,2,, a = 7.9302(13) k,b = 16.627(2) k,c= 17.543(2) k,V= 2313.1(9) Á3 with Z = 4. The structures were refined by full-matrix least-squares procedures to R values of 0.043,0.027, and 0.039, respectively. The mononuclear complexes feature facially tridentate N-donor ligands, mutually cis oxo and X ligands, and distorted octahedral geometries. The alkoxy and thiolate complexes undergo a reversible, one-electron reduction to form the corresponding dioxo-Mo(V) anions [LMovC>2X]-. The requirements for reversible, one-electron electrochemical reduction of dioxo-Mo(VI) complexes appear to be (i) minimal conformational change, restricting substitution trans to the oxo groups, upon reduction and (ii) a steric or electrostatic barrier to the close approach and dinucleation of the reduced species. A number of oxo-hydroxo-Mo(V) complexes of the type LMovO-(OH)X were generated by protonation of the anions [LMov02X]~. Chemical reduction by Bun4NSH results in the sequential generation of [LMov02X]-and [LMovOSX]-anions (except for X = OPh, SPh, and SPr1, when only [LMov02X]is formed). The Mo(V) complexes have been characterized by EPR spectroscopy.