Sky-blue Tp*WOCl₂ has been synthesized from the high-yielding reaction of Tp*WO₂Cl with boron trichloride in refluxing toluene. Dark-red Tp*WOI₂ was prepared via thermal decarbonylation followed by aerial oxidation of Tp*WI(CO)₃ in acetonitrile. From these precursors, an extensive series of mononuclear tungstenyl complexes, Tp*WOXY [X = Cl-, Y = OPh-, SPh-; X = Y = OPh-, 2-(n-propyl)phenolate (PP-), SPh-, SePh-; XY = toluene-3,4-dithiolate (tdt²−), quinoxaline-2,3-dithiolate (qdt²-), benzene-1,2-diselenolate (bds²-); Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate], was prepared by metathesis with the respective alkali-metal salt of X-/XY²- or (NHEt₃)₂(qdt). The complexes were characterized by microanalysis, mass spectrometry, electrochemistry, IR, electron paramagnetic resonance (EPR), and electronic absorption spectroscopies, and X-ray crystallography (for X = Y = OPh-, PP-, SPh-; XY = bds²-). The six-coordinate, distorted-octahedral tungsten centers are coordinated by terminal oxo [W≡O = 1.689(6)-1.704(3) Å], tridentate Tp*, and monodentate or bidentate O/S/Se-donor ligands. Spin Hamiltonian parameters derived from the simulation of fluid-solution X-band EPR spectra revealed that the soft-donor S/Se ligand complexes had larger g values and smaller 183W hyperfine coupling constants than the less covalent hard-donor O/Cl species. The former showed low-energy ligand-to-metal charge-transfer bands in the near-IR region of their electronic absorption spectra. These oxotungsten(V) complexes display lower reduction potentials than their molybdenum counterparts, underscoring the preference of tungsten for higher oxidation states. Furthermore, the protonation of the pyrazine nitrogen atoms of the qdt²- ligand has been examined by spectroelectrochemistry; the product of the one-electron reduction of [Tp*WO(qdtH)]+ revealed usually intense low-energy bands.

The reactions of TpiPrMoVIO2Cl with salicylanilides and NEt3 produce cis-TpiPrMoVIO2(2-OC6H4CONHR) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate, R = Ph, 4-C6H4Cl, 4-C6H4OMe). The N-methyl complex, TpiPrMoO2{2-OC6H4CON(Me)Ph}, is similarly prepared. Reduction of the amido complexes by cobaltocene produces green, EPR-active compounds, [CoCp2][TpiPrMoVO2(2-OC6H4CONHR)], that exhibit strong, low energy, ν(MoO2) IR bands at 895 and 790 cm−1 (cf. 935 and 900 cm−1 for the Mo(VI) analogues). The X-ray structures of all seven complexes have been determined. In each case, the Mo center exhibits a distorted octahedral coordination geometry defined by mutually cis oxo and phenolate ligands and a tridentate fac-TpiPr ligand. The Mo(V) anions exhibit greater Mo═O distances (av. 1.738 Å vs 1.695 Å) and O═Mo═O angles (av. 112.4° vs 102.9°) than their Mo(VI) counterparts, indicative of the presence of a three-center (MoO2), π* semioccupied molecular orbital in these d1 complexes. The amido Mo(VI) and Mo(V) complexes exhibit an intramolecular hydrogen-bond between the NH and Ophenolate atoms. Protonation of [CoCp2][TpiPrMoVO2(2-OC6H4CONHR)] by lutidinium tetrafluoroborate is quantitative and produces EPR-active, cis-(hydroxo)oxo-Mo(V) complexes, TpiPrMoVO(OH)(2-OC6H4CONHR), related to the low pH Mo(V) forms of sulfite oxidase.

The oxo-MoIV complexes formed in the reactions of cis-TpiPrMoVIO2(OAr-R) [TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate, –OAr-R = phenolate derivative] complexes with PEt3 or PEt2Ph in acetonitrile depend on the nature of the potential hydrogen-bonding group (R) incorporated into the phenolate ligand. Green, diamagnetic, oxo(phosphoryl)-MoIV complexes, TpiPrMoIVO(OAr-R)(OPR'3), are produced when R is absent or is a non-coordinating group such as 2-OMe and 3-NEt2; six-coordinate TpiPrMoO(OC6H4OMe-2)(OPEt3) was structurally characterized and exhibits a distorted octahedral geometry typical of such species. When R is a carbonyl functionality, complete oxygen atom transfer leads to green or purple, diamagnetic, chelate complexes of the type, TpiPrMoIVO(OAr-R-κ2O,O'). The R = 2-COEt, 2-CO2Me and 2-CO2Ph derivatives exhibit six-coordinate, distorted-octahedral structures possessing fac TpiPr, terminal oxo and bidentate O,O'-donor –OAr-R ligands. Where R is an amido functionality, CONHPh, the complexes, TpiPrMoIVO(OC6H4CONHPh-2-κ2O,O')·OPR'3 (R'3 = Et3, Et2Ph), are isolated. Here, the six-coordinate, distorted-octahedral complex forms an intermolecular NH···OPR'3 hydrogen bond to the lattice OPR'3 molecule. Thus, facile chelation of potential hydrogen-bonding phenolate ligands suppresses hydrogen-bond-stabilized aquation or hydroxylation cis to the oxo group in these MoIV complexes.

Reactivity studies of oxo-Mo(IV) complexes, Tp iPr MoO{2-OC₆H4₄C(O)R-ᴋ²O,O'} (R = Me, Et, OMe, OEt, OPh, NHPh), containing chelated hydrogen-bond donor/acceptor phenolate ligands are reported.