Molybdenum site structure of Escherichia coli YedY, a novel bacterial oxidoreductase

We report a structural characterization using X-ray absorption spectroscopy of the molybdenum site of Escherichia coli YedY, a novel oxidoreductase related to be the sulfite oxidase family of molybdenum enzymes. We find that the enzyme can exist in Mo(V) and Mo(IV) oxidation states but cannot be readily oxidized to the Mo(VI) form. Mo(V) YedY has molybdenum coordination similar to that of sulfite oxidase, with one Mo═O at 1.71 ?, three Mo-S at 2.39 ?, and one Mo-OH at 2.09 ?, which elongates to 2.20 ? upon reduction to Mo(IV), indicating Mo-OH(2) coordination. The Mo(V) enzyme also possesses a long Mo-O coordination at 2.64 ?, which may be due to oxygen coordination by Asn-45 O(δ), with Mo-O(δ) approximately trans to the Mo═O group. A comparison with sulfite oxidase indicates that YedY possesses a much more uniform Mo-S coordination, with a maximum permitted deviation of less than 0.05 ?. Our results indicate that the YedY active site shows considerable similarity to but also important differences from that of reduced forms of sulfite oxidase.     

Identity of the exchangeable sulfur-containing ligand at the Mo(V) center of R160Q human sulfite oxidase

In our previous study of the fatal R160Q mutant of human sulfite oxidase (hSO) at low pH (Astashkin et al. J. Am. Chem. Soc.2008, 130, 8471-8480), a new Mo(V) species, denoted "species 1", was observed at low pH values. Species 1 was ascribed to a six-coordinate Mo(V) center with an exchangeable terminal oxo ligand and an equatorial sulfate group on the basis of pulsed EPR spectroscopy and (33)S and (17)O labeling. Here we report new results for species 1 of R160Q, based on substitution of the sulfur-containing ligand by a phosphate group, pulsed EPR spectroscopy in K(a)- and W-bands, and extensive density functional theory (DFT) calculations applied to large, more realistic molecular models of the enzyme active site. The combined results unambiguously show that species 1 has an equatorial sulfite as the only exchangeable ligand. The two types of (17)O signals that are observed arise from the coordinated and remote oxygen atoms of the sulfite ligand. A typical five-coordinate Mo(V) site is compatible with the observed and calculated EPR parameters.     

Structural Insights into the Incorporation of the Mo Cofactor into Sulfite Oxidase from Site‐Directed Spin Labeling

Mononuclear molybdoenzymes catalyze a broad range of redox reactions and are highly conserved in all kingdoms of life. This study addresses the question of how the Mo cofactor (Moco) is incorporated into the apo form of human sulfite oxidase (hSO) by using site‐directed spin labeling to determine intramolecular distances in the nanometer range. Comparative measurements of the holo and apo forms of hSO enabled the localization of the corresponding structural changes, which are localized to a short loop (residues 263–273) of the Moco‐containing domain. A flap‐like movement of the loop provides access to the Moco binding‐pocket in the apo form of the protein and explains the earlier studies on the in vitro reconstitution of apo‐hSO with Moco. Remarkably, the loop motif can be found in a variety of structurally similar molybdoenzymes among various organisms, thus suggesting a common mechanism of Moco incorporation.     

Structural studies of the molybdenum center of the pathogenic R160Q mutant of human sulfite oxidase by pulsed EPR spectroscopy and 17O and 33S labeling

Electron paramagnetic resonance (EPR) investigation of the Mo(V) center of the pathogenic R160Q mutant of human sulfite oxidase (hSO) confirms the presence of three distinct species whose relative abundances depend upon pH. Species 1 is exclusively present at pH < or = 6, and remains in significant amounts even at pH 8. Variable-frequency electron spin echo envelope modulation (ESEEM) studies of this species prepared with (33)S-labeled sulfite clearly show the presence of coordinated sulfate, as has previously been found for the "blocked" form of Arabidopsis thaliana at low pH (Astashkin, A. V.; Johnson-Winters, K.; Klein, E. L.; Byrne, R. S.; Hille, R.; Raitsimring, A. M.; Enemark, J. H. J. Am. Chem. Soc. 2007, 129, 14800). The ESEEM spectra of Species 1 prepared in (17)O-enriched water show both strongly and weakly magnetically coupled (17)O atoms that can be assigned to an equatorial sulfate ligand and the axial oxo ligand, respectively. The nuclear quadrupole interaction (nqi) of the axial oxo ligand is substantially stronger than those found for other oxo-Mo(V) centers studied previously. Additionally, pulsed electron-nuclear double resonance (ENDOR) measurements reveal a nearby weakly coupled exchangeable proton. The structure for Species 1 proposed from the pulsed EPR results using isotopic labeling is a six-coordinate Mo(V) center with an equatorial sulfate ligand that is hydrogen bonded to an exchangeable proton. Six-coordination is supported by the (17)O nqi parameters for the axial oxo group of the model compound, (dttd)Mo(17)O((17)Otms), where H2dttd = 2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane; tms = trimethylsilyl. Reduction of R160Q to Mo(V) with Ti(III) gives primarily Species 2, another low pH form, whereas reduction with sulfite at higher pH values gives a mixture of Species 1 and 2, as well as the "primary" high pH form of wild-type SO. The occurrence of significant amounts of the "sulfate-blocked" form of R160Q (Species 1) at physiological pH suggests that this species may be a contributing factor to the lethality of this mutation.     

Active-site stereochemical control of oxygen atom transfer reactivity in sulfite oxidase

A number of both experimental and computational studies have recently been reported for symmetric, six-coordinate dioxomolybdenum(VI) complexes as models of the fully oxidized form of the molybdopterin enzyme sulfite oxidase (SO). Such studies have suggested that the two terminal oxo donors in SO are electronically equivalent. However, the consensus structure of the catalytically competent Mo(VI) active site in SO is five-coordinate square pyramidal, possessing two terminal oxo donors, an ene-1,2-dithiolate chelate and a cysteine sulfur donor ligand. Computational studies at the density functional level of theory have been performed on a minimal model of the SO active site, [Mo(VI)O2(S2C2Me2)(SCH3)]-, in C1 symmetry to evaluate the composition of the LUMO, which is the putative electron acceptor orbital in the oxygen atom transfer (OAT) reaction with the sulfite substrate. The LUMO in this model is principally composed of a Mo dxy - ppi* interaction between the Mo and the equatorial oxygen (Oeq), while the axial oxygen (Oax) possesses no contribution to this orbital. In fact, the LUMO+1 orbital which possesses a substantial amount of Oax character lies nearly 1 eV higher in energy than the LUMO. It has also been suggested that changes in the Oax-Mo-Sthiolate-C torsion angle during the course of enzyme catalysis may aid in selection of Oeq for OAT. Calculations were performed in which this torsion angle was varied by 20 degrees through 360 degrees . These calculations demonstrate that the Mo dxy -Oeq ppi* interaction, and therefore the Oeq atom character, always dominates the LUMO. The results presented here suggest that oxygen atom selection and activation are a direct function of the low-symmetry structure of the oxidized SO active site and provide a role for the ene-1,2-dithiolate in promoting OAT reactivity through its kinetic trans effect on the equatorial oxo donor.     

Nature of the oxomolybdenum-thiolate pi-bond: implications for Mo-S bonding in sulfite oxidase and xanthine oxidase

The electronic structure of cis,trans-(L-N(2)S(2))MoO(X) (where L-N(2)S(2) = N,N'-dimethyl-N,N'-bis(2-mercaptophenyl)ethylenediamine and X = Cl, SCH(2)C(6)H(5), SC(6)H(4)-OCH(3), or SC(6)H(4)CF(3)) has been probed by electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies to determine the nature of oxomolybdenum-thiolate bonding in complexes possessing three equatorial sulfur ligands. One of the phenyl mercaptide sulfur donors of the tetradentate L-N(2)S(2) chelating ligand, denoted S(180), coordinates to molybdenum in the equatorial plane such that the OMo-S(180)-C(phenyl) dihedral angle is approximately 180 degrees, resulting in a highly covalent pi-bonding interaction between an S(180) p orbital and the molybdenum d(xy) orbital. This highly covalent bonding scheme is the origin of an intense low-energy S --> Mo d(xy) bonding-to-antibonding LMCT transition (E(max) approximately 16000 cm(-)(1), epsilon approximately 4000 M(-)(1) cm(-)(1)). Spectroscopically calibrated bonding calculations performed at the DFT level of theory reveal that S(180) contributes approximately 22% to the HOMO, which is predominantly a pi antibonding molecular orbital between Mo d(xy) and the S(180) p orbital oriented in the same plane. The second sulfur donor of the L-N(2)S(2) ligand is essentially nonbonding with Mo d(xy) due to an OMo-S-C(phenyl) dihedral angle of approximately 90 degrees. Because the formal Mo d(xy) orbital is the electroactive or redox orbital, these Mo d(xy)-S 3p interactions are important with respect to defining key covalency contributions to the reduction potential in monooxomolybdenum thiolates, including the one- and two-electron reduced forms of sulfite oxidase. Interestingly, the highly covalent Mo-S(180) pi bonding interaction observed in these complexes is analogous to the well-known Cu-S(Cys) pi bond in type 1 blue copper proteins, which display electronic absorption and resonance Raman spectra that are remarkably similar to these monooxomolybdenum thiolate complexes. Finally, the presence of a covalent Mo-S pi interaction oriented orthogonal to the MOO bond is discussed with respect to electron-transfer regeneration in sulfite oxidase and Mo=S(sulfido) bonding in xanthine oxidase.     

High-resolution EXAFS of the active site of human sulfite oxidase: comparison with density functional theory and X-ray crystallographic results

Much of our knowledge about molybdenum enzymes has originated from EXAFS spectroscopy. This technique provides excellent bond-length accuracy but has only limited bond-length resolution. We have used EXAFS spectroscopy with an extended data range in an attempt to improve bond-length resolution for the molybdenum enzyme sulfite oxidase. The Mo site of sulfite oxidase has two oxygen and three Mo-S ligands (two from cofactor dithiolene plus a cysteine). For the oxidized (Mo(VI)) enzyme, we find that the three Mo-S bond lengths are very similar (within 0.05 A) at 2.41 A, as are the Mo=O ligands at 1.72 A. Density functional theory shows that this is consistent with the proposed active-site structure. The reduced (Mo(IV)) enzyme shows two Mo-S bond lengths at 2.35 A and one at 2.41 A (assigned to cofactor dithiolene and cysteine, respectively, from DFT), together with one Mo=O at 1.72 A and one Mo-OH(2) at 2.30 A.     

Oxo,sulfido, and tellurido Mo-enedithiolate models for xanthine oxidase: understanding the basis of enzyme reactivity

The active site of the mononuclear molybdenum enzyme xanthine oxidase has an LMoOS(OH) center that catalyzes the hydroxylation of substrate (L representing an enedithiolate ligand contributed by a pterin cofactor in the enzyme). Reaction of the enzyme with cyanide results in the replacement of the Mo=S group with a second Mo=O group, which results in loss of enzyme activity. To understand the basis for this loss of activity, we have computationally examined the interaction of a model for the LMoO2(OH) as well the LMoOTe(OH) congener of the enzyme with formamide (a substrate for the enzyme). Our electronic structure calculations for the oxo congener indicate a reduced electron density on the hydrogen being transferred from substrate in the course of the reaction, a shorter O-H bond in the transition state, and a longer nascent O-C bond of product, factors which combine to account for the loss of reactivity in the LMoO2(OH) species. Interestingly, our calculations indicate that the Te congener is characterized by an increased electron density on the hydrogen species being transferred, a longer Te-H bond in the transition state, and a shorter O-C nascent bond in the product and suggest that a Te congener of xanthine oxidase, were it to be prepared experimentally, should exhibit catalytic activity.     

Monodithiolene molybdenum(V, VI) complexes: a structural analogue of the oxidized active site of the sulfite oxidase enzyme family

The active sites of the xanthine oxidase and sulfite oxidase enzyme families contain one pterin-dithiolene cofactor ligand bound to a molybdenum atom. Consequently, monodithiolene molybdenum complexes have been sought by exploratory synthesis for structural and reactivity studies. Reaction of [MoO(S(2)C(2)Me(2))(2)](1-) or [MoO(bdt)(2)](1-) with PhSeCl results in removal of one dithiolate ligand and formation of [MoOCl(2)(S(2)C(2)Me(2))](1-) (1) or [MoOCl(2)(bdt)](1-) (2), which undergoes ligand substitution reactions to form other monodithiolene complexes [MoO(2-AdS)(2)(S(2)C(2)Me(2))](1-) (3), [MoO(SR)(2)(bdt)](1-) (R = 2-Ad (4), 2,4,6-Pr(i)(3)C(6)H(2) (5)), and [MoOCl(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (6) (Ad = 2-adamantyl, bdt = benzene-1,2-dithiolate). These complexes have square pyramidal structures with apical oxo ligands, exhibit rhombic EPR spectra, and 3-5 are electrochemically reducible to Mo(IV)O species. Complexes 1-6 constitute the first examples of five-coordinate monodithiolene Mo(V)O complexes; 6 approaches the proposed structure of the high-pH form of sulfite oxidase. Treatment of [MoO(2)(OSiPh(3))(2)] with Li(2)(bdt) in THF affords [MoO(2)(OSiPh(3))(bdt)](1-) (8). Reaction of 8 with 2,4,6-Pr(i)(3)C(6)H(2)SH in acetonitrile gives [MoO(2)(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (9, 55%). Complexes 8 and 9 are square pyramidal with apical and basal oxo ligands. With one dithiolene and one thiolate ligand of a square pyramidal Mo(VI)O(2)S(3) coordination unit, 9 closely resembles the oxidized sites in sulfite oxidase and assimilatory nitrate reductase as deduced from crystallography (sulfite oxidase) and Mo EXAFS. The complex is the first structural analogue of the active sites in fully oxidized members of the sulfite oxidase family. This work provides a starting point for the development of both structural and reactivity analogues of members of this family.     

The active site of arsenite oxidase from Alcaligenes faecalis

Arsenite oxidase, a member of the DMSO reductase family of molybdenum enzymes, has two molecules of guanosine dinucleotide molybdenum cofactor coordinating the molybdenum at the active site. X-ray absorption spectroscopy indicates that the Mo-S bonds shorten from 2.47 to 2.37 A upon reduction with the physiological substrate. It also indicates the presence of an oxo ligand at 1.70 A in both oxidized and reduced forms of the enzyme, together with a short, 1.83 A, Mo-O bond in the oxidized form that is lost upon reduction. Resonance Raman spectroscopy indicates that the two pterin dithiolene moieties have different aromaticities, with one, the Q-pterin, having a more discrete dithiolate structure while the other, the P-pterin, has considerable pi-delocalization. Our results indicate that the structure of arsenite oxidase is intermediate between that seen in other molybdenum enzymes, in which one ligand to the metal is provided by the polypeptide (serine, cysteine, or selenocysteine), and tungsten enzymes that lack a peptide ligand.     

Geometrical control of the active site electronic structure of pyranopterin enzymes by metal-dithiolate folding: aldehyde oxidase

Density functional calculations on geometry-optimized oxidized (Mo(VI)) and reduced (Mo(IV)) analogues of the isolated active site of aldehyde oxidase (MOP), a member of the xanthine oxidase family of pyranopterindithiolate enzymes, show that fold angle changes of the dithiolate ligand modulate the relative metal and dithiolate contributions to the frontier redox orbitals. Proton abstraction from the equatorial aqua ligand of the oxidized Mo(VI) site also flattens the metal dithiolate fold angle. It is proposed that static and/or dynamic changes in the structure of the protein surrounding the active site can induce changes in the dithiolate fold angle and thereby provide a mechanism for electronic buffering of the redox orbital, for fine-tuning the nucleophilicity of the equatorial aqua/hydroxide ligand, and for modulating the electron-transfer regeneration of the active sites of molybdenum and tungsten enzymes via a "dithiolate folding effect".     

Nature of the catalytically labile oxygen at the active site of xanthine oxidase

In this paper we report the results of molybdenum K-edge X-ray absorption studies performed on the oxidized active site of xanthine oxidase at pH 6 and 10. These results indicate that the active site possesses one terminal oxygen ligand (Mo=O), two thiolate ligands (Mo-S), one terminal sulfido ligand (Mo=S), and one Mo-OH moiety. EXAFS analysis demonstrates that the Mo-OH bond shortens from 1.97 A at pH 6 to 1.75 A at pH 10, which is consistent with the generation of a Mo-O- moiety. This study provides convincing structural evidence that the catalytic oxygen donor at the oxidized active site of xanthine oxidase is Mo-OH rather than the Mo-OH2 ligation previously suggested by X-ray crystallography. These results support a mechanism initiated by base-assisted nucleophilic attack of the substrate by Mo-OH.     

Pulsed EPR investigations of systems modeling molybdenum enzymes: hyperfine and quadrupole parameters of oxo-17O in [Mo 17O(SPh)4]-

Ka band ESEEM spectroscopy was used to determine the hyperfine (hfi) and nuclear quadrupole (nqi) interaction parameters for the oxo-17O ligand in [Mo 17O(SPh)4]-, a spectroscopic model of the oxo-Mo(V) centers of enzymes. The isotropic hfi constant of 6.5 MHz found for the oxo-17O is much smaller than the values of approximately 20-40 MHz typical for the 17O nucleus of an equatorial OH(2) ligand in molybdenum enzymes. The 17O nqi parameter (e2qQ/h = 1.45 MHz, eta approximately = 0) is the first to be obtained for an oxo group in a metal complex. The parameters of the oxo-17O ligand, as well as other magnetic resonance parameters of [Mo 17O(SPh)4]- predicted by quasi-relativistic DFT calculations, were in good agreement with those obtained in experiment. From the electronic structure of the complex revealed by DFT, it follows that the SOMO is almost entirely molybdenum d(xy) and sulfur p, while the spin density on the oxo-17O is negative, determined by spin polarization mechanisms. The results of this work will enable direct experimental identification of the oxo ligand in a variety of chemical and biological systems.     

17O ESEEM evidence for exchange of the axial oxo ligand in the molybdenum center of the high pH form of sulfite oxidase

A 17O ESEEM investigation of the high pH form of chicken sulfite oxidase using hyperfine sublevel correlation (HYSCORE) spectroscopy at 29.25 GHz has revealed a new type of exchangeable 17O ligand that is characterized by a significantly smaller hyperfine interaction ( approximately 5 MHz) than that previously detected by CW EPR. This new type of exchangeable oxygen ligand is assigned to the axial oxo group of the Mo(V) center.     

Analogues for the molybdenum center of sulfite oxidase: oxomolybdenum(V) complexes with three thiolate sulfur donor atoms

cis,trans-(L-N2S2)Mo(V)O(SR) [L-N2S2H2 = N,N'-dimethyl-N,N'-bis(mercaptophenyl)ethylenediamine; R = CH2Ph, CH2CH3, and p-C6H4-Y (Y = CF3, Cl, Br, F, H, CH3, CH2CH3, and OCH3)] are the first structurally characterized mononuclear Mo compounds with three thiolate donors, as occurs at the Mo active site in sulfite oxidase. X-ray crystal structures of the cis,trans-(L-N2S2)Mo(V)O(SR) compounds, where R = CH2Ph, CH2CH3, p-C6H4-OCH3, and p-C6H4-CF3, show a similar coordination geometry about the Mo atom with all three sulfur thiolate donors in the equatorial plane. This coordination geometry places two adjacent S ppi orbitals parallel to the Mo=O bond, analogous to the orientation in the ene-dithiolate ligand in sulfite oxidase; the third S ppi orbital lies in the equatorial plane. Charge-transfer transitions from the S p to the Mo d orbitals occur at approximately 28,000 cm(-1) (epsilon: 4,400-6,900 L mol(-1)] cm(-1)) and 15,500 cm(-1) (epsilon: 3,200-4,900 L mol(-1) cm(-1)). The EPR parameters are nearly identical for all the cis,trans-(L-N2S2)Mo(V)O(SR) compounds (g1 approximately 2.022, g2 approximately 1.963, g3 approximately 1.956, Al approximately 58.4 x 10(-4) cm(-1), A2 approximately 23.7 x 10(-4) cm(-1), A3 approximately 22.3 x 10(-4) cm(-1)) and are typical of an oxo-Mo(V) center coordinated by multiple thiolate donors. The g and A tensors are related by a 24 degrees rotation about the coincident g2 and A2 tensor elements, reflecting the approximate Cs coordination symmetry. These EPR parameters more closely mimic those of the low pH form of sulfite oxidase and the "very rapid" species of xanthine oxidase than previous model compounds with two or four thiolate donors. The cis,trans-(L-N2S2)Mo(V)O(SR) compounds undergo a quasi-reversible, one-electron reduction and an irreversible oxidation that show a linear dependence upon the Hammett parameter, sigmap, of the Y group. The cis,trans-(L-N2S2)Mo(V)O(SR) compounds provide a well-defined platform for the systematic investigation of the electronic structures of the Mo(V)OS3 centers and their implications for molybdoenzymes.     

Comparative Genomics and Evolution of Molybdenum Utilization

The trace element molybdenum (Mo) is the catalytic component of important enzymes involved in global nitrogen, sulfur, and carbon metabolism in both prokaryotes and eukaryotes. With the exception of nitrogenase, Mo is complexed by a pterin compound thus forming the biologically active molybdenum cofactor (Moco) at the catalytic sites of molybdoenzymes. The physiological roles and biochemical functions of many molybdoenzymes have been characterized. However, our understanding of the occurrence and evolution of Mo utilization is limited. This article focuses on recent advances in comparative genomics of Mo utilization in the three domains of life. We begin with a brief introduction of Mo transport systems, the Moco biosynthesis pathway, the role of posttranslational modifications, and enzymes that utilize Mo. Then, we proceed to recent computational and comparative genomics studies of Mo utilization, including a discussion on novel Moco-binding proteins that contain the C-terminal domain of the Moco sulfurase and that are suggested to represent a new family of molybdoenzymes. As most molybdoenzymes need additional cofactors for their catalytic activity, we also discuss interactions between Mo metabolism and other trace elements and finish with an analysis of factors that may influence evolution of Mo utilization.     

Molybdenum cofactor biosynthesis in plants and humans

The transition element molybdenum (Mo) needs to be complexed by a special cofactor in order to gain catalytic activity. With the exception of bacterial Mo-nitrogenase, where Mo is a constituent of the FeMo-cofactor, Mo is bound to a pterin, thus forming the molybdenum cofactor Moco, which in different variants is the active compound at the catalytic site of all other Mo-containing enzymes. The biosynthesis of Moco involves the complex interaction of six proteins and is a process of four steps, which also requires reducing equivalents, iron, ATP and probably copper. After its synthesis, Moco is distributed to the apoproteins of Mo-enzymes by Moco–carrier/binding proteins that also participate in Moco-insertion into the cognate apoproteins. A deficiency in the biosynthesis of Moco has lethal consequences for the respective organisms. In humans, Moco deficiency is a severe inherited inborn error in metabolism resulting in severe neurodegeneration in newborns and causing early childhood death. Due to our better understanding of the chemistry of Moco synthesis, a first therapy has been brought to the clinic.     

Structure of the active site of sulfite dehydrogenase from Starkeya novella

In this paper, we report the results of molybdenum K-edge X-ray absorption studies performed on the oxidized and reduced active sites of the sulfite dehydrogenase from Starkeya novella. Our results provide the first direct structural information on the active site of the oxidized form of this enzyme and confirm the conclusions derived from protein crystallography that the molybdenum coordination is analogous to that of the sulfite oxidases. The molybdenum atom of the oxidized enzyme is bound by two Mo=O ligands at 1.73 A and three thiolate Mo-S ligands at 2.42 A, whereas the reduced enzyme has one oxo at 1.74 A, one long oxygen at 2.19 A (characteristic of Mo-OH2), and three Mo-S ligands at 2.40 A.     

Molybdenum oxo-imido aryloxide complexes: oxo analogues of olefin metathesis catalysts

The novel 16-electron molybdenum oxo-imido bis(aryloxide) complexes [Mo(NtBu)(O)(2,6-Me2C6H3O)2(py)] (1) and [Mo(NtBu)(O)(2,6-iPr2C6H3O)2(py)] (2) have been prepared by the salt elimination reactions of [Mo(NtBu)(O)Cl2(DME)] with the appropriate lithium aryloxide and from the cycloaddition reactions of tert-butyl isocyanate with the appropriate molybdenum dioxo bis(aryloxide) complex [Mo(O)2(OAr)2(py)n]. Complexes 1 and 2 are the first isolable and crystallographically characterized molybdenum oxo-imido aryloxide complexes. The geometry around the metal in complexes 1 and 2 is best described as a distorted trigonal bipyramid, with the imido and pyridine ligands occupying the axial positions and the oxo and aryloxide ligands in the equatorial plane. X-ray and IR data have confirmed that the imido ligand is the dominant pi donor in the complexes, resulting in an Mo-O bond order of less than 2.5. Reaction of [Mo(NtBu)(O)Cl2(DME)] with Li(OCH2tBu) instead gave the novel complex [Mo(NtBu)(OCH2tBu)3Cl(py)] (3).