Oxomolybdenum Complexes Research Articles

Toward modeling the high chloride, low pH form of sulfite oxidase: Ka-band ESEEM of equatorial chloro ligands in oxomolybdenum(V) complexes

Two oxomolybdenum(V) complexes, (dttd)MoOCl and [(bdt)MoOCl(2)](-) (where dttd=2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane and bdt=1,2-benzenedithiolate), which contain one or two equatorial chloro ligands, respectively, were studied by electron spin echo envelope modulation (ESEEM) spectroscopy in the microwave K(a)-band (approximately 29GHz). The ESEEM amplitude from the chloro ligands in both compounds is significantly greater than that tentatively attributed to chloride in the vicinity of the oxomolybdenum active site in the high chloride, low-pH (lpH) form of sulfite oxidase (SO). Thus, these ESEEM results rule out equatorial coordination of chloride in the enzyme, although the possibility for a weakly bound chloride in the trans axial position or nearby non-coordinated chloride(s) remains for lpH SO in solution.

Synthesis of novel molybdenum(V) complexes: Structural characterization of two thiosemicarbazonato complexes [MoOCl 2{C 6H 4(O)CH:NNHC:SNHC 6H 5}] and [MoOCl 2{C 10H 6(O)CH:NNHC:SNHC 6H 5}] · CH 3CN, and two oxohalomolybdates NH 4[MoOCl 4(CH 3CN)] and [C 5H 5NH] 2[MoOCl 5] · CH 2Cl 2

Abstract Mononuclear oxomolybdenum(V) complexes with thiosemicarbazonato ligands as condensation products of aldehydes (salicylaldehyde, 2-hydroxynaphthaldehyde) with thiosemicarbazides (H2NNHC:SNHR; R = H, CH3, C6H5) were prepared. The [MoOCl2HL] complexes (HL = thiosemicarbazonato ligand) were characterized by chemical analysis, IR spectroscopy, thermogravimetry and two of them, [MoOCl2{C6H4(O)CH:NNHC:SNHC6H5}] and [MoOCl2{C10H6(O)CH:NNHC:SNHC6H5}] · CH3CN, by single-crystal X-ray structural analysis. In both complexes the MoO3+ core is coordinated by a monobasic tridentate ONS thiosemicarbazonato ligand and two chloro ligands. The coordination around the molybdenum atom is a distorted octahedron. NH4[MoOCl4(CH3CN)] and unstable (C5H5NH)2[MoOCl5] · CH2Cl2 were isolated from the acetonitrile solution of (NH4)2[MoOCl5] and from the acetonitrile and dichloromethane (1:1) solution of (C5H5NH)2[MoOCl5], respectively. Isolation and structural characterization of NH4[MoOCl4(CH3CN)] from the acetonitrile solution suggests the [MoOCl4(CH3CN)]− anion to be the reaction intermediate in the synthesis of molybdenum(V) thiosemicarbazonato complexes. (C5H5NH)2[MoOCl5] · CH2Cl2 is a rare example of a structurally characterized simple oxohalomolybdate(V) with a six-coordinate octahedral [MoOCl5]2− anion.

Oxomolybdenum(VI) and (IV) complexes of pyrazole derived ONO donor ligands – synthesis, crystal structure studies and spectroelectrochemical correlation

Abstract Four cis-dioxomolybdenum complexes of general formula [MoO2(Ln)EtOH] (n = 1–4) and one oxomolybdenum(IV) complex [MoO(L4)EtOH], with potentially tridentate Schiff bases derived from 5-methyl pyrazole-3-carbohydrazide and salicylaldehyde/substituted salicylaldehyde/o-hydroxy acetophenone have been prepared. The Mo(IV) complex is derived from the Mo(VI) dioxo complex by oxotransfer reaction with PPh3. The complexes are characterized by elemental analysis, electronic spectra, IR, 1H NMR, and by cyclic voltammetry. All the Mo(VI) species are crystallographically characterized. The complexes have a distorted octahedral structure in which the ligand behaves as a binegative donor one, leaving the pyrazole –N uncoordinated towards the metal center. It is also revealed from the crystal structure that the Mo(VI) center enjoys an NO5 donor environment.

What is the best theoretical method to study molybdenum dithiolene complexes?

Abstract The reaction sequence of sulfoxide reduction by molybdenum complexes containing two enedithiolate and one oxide ligand is used to evaluate different density functional methods. Relative energies are computed for several stationary points: a pentacoordinate reduced complex, its adducts with water and with the sulfoxide substrate, the transition state for oxygen atom transfer to the metal center, a van der Waals complex of the reduced sulfide with the molybdenum oxo complex and the isolated oxo complex itself. Root mean square deviations from CCSD results or values extrapolated (depending on the size of the ligands used) serve to assess the quality of various popular and promising modern DFT methods available in Gaussian03 (in combination with the SDD basis set augmented by additional polarization functions) and ADF2005 (TZP basis set). B3LYP, B1B95, B97-1 and B98 form the group of best functionals provided by Gaussian according to our criteria. PBE1PBE, B1LYP, MPW1PW91 and B97-2 have increasingly larger deviations but describe the considered reaction sequence qualitatively correct. The reaction profiles produced by HCTH147, BP86, HCTH407, BLYP, G96LYP and especially by OLYP, HCTH93, BH and H and VSXC have severe flaws. These functionals are not recommended for the treatment of oxygen atom transfer reactions involving transition metals. DFT functionals used with ADF derived energies for the BP/TZP density. The relative energies show larger RMS, which can be slightly reduced by inclusion of ZORA scalar relativistic effects and in the case of hybrid functionals further by ZORA spin-orbit relativistic effects. Geometries obtained with BP/TZP produce RMS deviations comparable to the best methods identified when relative energies are derived from single point energies computed by one of these methods.

A Single‐Source‐Precursor Approach to Late Transition Metal Molybdate Materials: The Structural Role of Chelating Ligands in the Formation of Heterometallic Heteroleptic Alkoxide Complexes

AbstractThe synthesis and structural determination of three new heterometallic molybdenum complexes, one with cobalt and two with nickel and two of these with β‐diketonate ligands and one with amino alcohol ligands, are presented. The reaction of cobalt acetylacetonate with [MoO(OMe)4] provides [Co2Mo2O2(acac)2(OMe)10] (1) and [MoO(acac)(OMe)3] (4),and the reaction of nickel acetylacetonate with [MoO(OMe)4]provides [Ni2Mo2O2(acac)2(OMe)10] (2) and 4. The reaction of [Ni(ORN)2] (RN = CHMeCH2NMe2) with [MoO(OMe)4] yields [Ni2Mo2O2(ORN)2(OMe)10] (3). The two new oxomolybdenum complexes undergo ether elimination upon storage to give the corresponding dioxo complexes [MoO2(acac)(OMe)]2 (5) and [MoO2(ORN)(OMe)]2 (6). Compounds 3 and 4 could also be obtained from the reaction of stoichiometric amounts of Hacac with [MoO(OMe)4] and [MoO2(OMe)2], respectively. The local structure around the nickel atom in compound 2 in solution and compound 3 in the solid state and in toluene/hexane solution has been determined by means of EXAFS spectroscopy. The complexes are intended to be used as single‐source precursors, which are attractive in coatings and for the preparation of mesoporous materials; its application for the synthesis of nickel molybdate by sol–gel processing is therefore reported. The oxide material obtained from 3 displays a uniform grain size and a large surface area. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis and characterization of layered double hydroxides intercalated by an oxomolybdenum complex

Abstract Zn,Al-layered double hydroxides (LDHs) intercalated by an anionic oxomolybdenum guest were prepared by ion-exchange of precursor Zn,Al-NO 3 LDHs with the complex [NMe 4 ] 2 [MoO 2 (3,4-dhb) 2 ] (3,4-H 2 dhb=3,4-dihydroxybenzoic acid). Powder X-ray diffraction (XRD) showed that the basal spacing increased from 8.7 to 8.9 A for the precursor materials to 15.0 A for the pillared hybrids. Elemental analysis and IR/Raman spectroscopy indicated that most of the interlayer nitrate anions were displaced by [MoO 2 (3,4-dhb) 2 ] 2− anions. Powder XRD diagrams of Zn,Al-[MoO 2 (3,4-dhb) 2 ], recorded at increasing temperatures in the range 25–450 °C, show that the 15 A phase is stable up to about 160 °C. Slightly above this temperature, partial dehydroxylation of the double hydroxide layers occurs, and the characteristic (00 l ) reflections of the pillared phase are lost. The material is X-ray amorphous in the temperature range 230–450 °C.

Synthesis, structure, and reactivity of some new dipyridyl and diamine-bridged dinuclear oxomolybdenum(VI) complexes

Abstract Some new diimine and diamine-bridged dinuclear Mo(VI) complexes of the general formula [(MoO2L)2(μ-N–N)] [(μ-N–N) = 4,4′-bipyridine (4,4′-bpy) (2); trans-1,2-bis(4-pyridyl) ethene (bpe) (3); trans-1,2-bis(4-pyridyl) ethane (bpa) (4) and p-phenylenediamine (ppda) (5)] along with their common mononuclear precursor [MoO2L(C2H5OH)] (1) have been synthesized, using 2-hydroxy benzoylhydrazone of 2-hydroxybenzaldehyde as tridentate ONO donor Ligand (LH2), and structurally characterized. Structures of all the three dinuclear complexes 2, 3 and 5 are centrosymmetric with both the Mo(VI) acceptor centers existing in the same coordination environment. One 2 from the four dinuclear complexes has taken part in oxo-transfer to the substrate PPh3 forming the corresponding dinuclear Mo(IV)–Mo(IV) complex [(MoOL)2(μ-N–N)] (2a).

Polynuclear oxomolybdenum(VI) complexes of dihydroxybenzoic acids: Synthesis, spectroscopic and structure characterization of a tetranuclear catecholato-type coordinated 2,3-dihydroxybenzoate and a novel tridentate salicylato-type coordinated 2,5-dihydroxybenzoate trinuclear complex

Abstract In order to study the reaction of molybdenum(VI) with dihydroxybenzoic acids (DHBAs), compounds (PPh4)2 · [Mo4O6(μ-O)4(μ3-OCH3)2(2,3-DHBA)2] (1) and (PPh4)2 · [Mo3O6(μ-O)2(2,5-DHBA)2] · CH3CN (2) were isolated and determined by X-ray crystallography. The structure of complex 1 consists of four distorted MoO6 octahedra with two cis- MoO 2 2 + units and two molybdenum atoms having each ligand coordinated and one terminal oxygen atom, while two triple bridged methanol molecules are present. The ligand 2,3-dihydroxybenzoic acid (2,3-DHBA) prefers the catecholate-type coordination with strong hydrogen bonding between the carboxylic hydrogen and the ortho-phenolic oxygen. The structure of complex 2 consists of three distorted MoO6 octahedra. The 2,5-dihydroxybenzoic acid (2,5-DHBA) acts as a tridentate ligand using both carboxyl oxygen atoms, giving thus an unusual trinuclear molybdenum complex with a unique coordination mode of the ligand. The compounds were further investigated using IR, UV–Vis, ESR and NMR spectroscopy, elemental and thermogravimetric analysis, as well as cyclic voltammetry.

Synthesis and characterization of thiocyanato and chlorodioxo tungsten(VI) compounds: Comparative oxygen atom transfer capability with molybdenum analogs

Four new tungsten-oxo(VI) complexes have been synthesized, characterized spectroscopically and their molecular structure established by X-ray diffraction analysis. These bear identical environment as previously reported molybdenum-oxo(VI) complexes, which allowed direct comparison of their spectroscopic properties. Their capability as oxygen atom transfer agents was found to be significantly lower than their molybdenum analogs.

Dimeric Dioxomolybdenum(VI) and Oxomolybdenum(V) Complexes with Citrate at Very Low pH and Neutral Conditions

A novel dimeric dioxomolybdenum(VI) citrate complex, K[(MoO2)2-(OH)(H2cit)2].4H2O (1), with weak coordination of beta-carboxylic acid groups and the first structural example of an oxomolybdenum(V) citrate complex, (NH4)6[Mo2O4(cit)2].3H2O (2) (H4cit = citric acid), are isolated in a very acidic solution (pH 0.5-1.0) and neutral conditions (pH 7.0-8.0), respectively. Complex 1 displays strong double hydrogen bonds through beta-carboxyl and beta-carboxylic acid groups [2.621(9) A]. Transformations of the dimeric molybdenum(VI) citrate show that protonation of a carboxyl group will weaken the coordination of molybdenum(VI) citrate. There are obvious dissociations of molybdenum(VI/V) citrate complexes based on 13C NMR observations in solution.

Sol–gel synthesis, characterization and catalytic property of silicas modified with oxomolybdenum complexes

Abstract Active solid oxidation catalysts can be obtained by incorporating dioxomolybdenum(VI) species derived from MoO 2 Cl 2 (OPMePh 2 ) 2 into silica matrices via a sol–gel method. The supported catalyst has been characterized by elemental analysis, XRD, N 2 -physisorption, FT-IR, FT-Raman, UV–vis and solid-state MAS NMR spectroscopy, and a pathway for the reaction between MoO 2 Cl 2 (OPMePh 2 ) 2 and tetraethoxysilane (TEOS) is suggested. The molybdenum catalyst was tested in the epoxidation of cyclohexene in the presence of tert -butyl hydroperoxide (TBHP). The conversion of cyclohexene and the selectivity to the epoxide were very high and increased further on raising the reaction temperature.

Synthesis, characterization and electrochemical behavior of oxo-bridged (arylimido)[tris(3,5-dimethylpyrazolyl)borato] molybdenum(V) complexes

Abstract Reaction of the oxo-molybdenum(V) precursor [MoTp*(O)Cl 2 ] [Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate] with H 2 NC 6 H 4 R-4 (R = OEt; OPr) in refluxing toluene in the presence of Et 3 N afforded the binuclear oxo-bridged oxo(arylimido) molybdenum(V) complexes [Tp*Mo(O)Cl](μ-O)[Tp*Mo(NC 6 H 4 OR-4)Cl]. Surprisingly, a similar reaction between [MoTp*(O)Cl 2 ] and C 6 H 5 NH 2 yielded the previously reported compound [{MoTp*(O)Cl} 2 (μ-O)] as the only product. The new compounds were characterized by microanalytical data, mass spectrometry, IR and 1 H NMR spectroscopy. Cyclic voltammetric studies of the new compounds, of the previously reported compounds [Tp*Mo(O)Cl](μ-O)[Tp*Mo(NAr)Cl] (Ar = C 6 H 4 OMe-4, C 6 H 4 F-3, C 6 H 4 Cl-4, C 6 H 4 Br-4, and C 6 H 4 I-3), and of [{MoTp*(O)Cl} 2 (μ-O)] revealed a reversible one-electron oxidation process that is little affected by the nature of the substituent on the aryl group, whereas it is greatly affected by replacement of the imido ligand with an oxo ligand. The [{MoTp*(O)Cl} 2 (μ-O)] compound also shows a one-electron reduction process.

Oxygen Atom Transfer in Models for Molybdenum Enzymes: Isolation and Structural, Spectroscopic, and Computational Studies of Intermediates in Oxygen Atom Transfer from Molybdenum(VI) to Phosphorus(III)

Intermediates in the oxygen atom transfer from Mo(VI) to P(III), [Tp(iPr)MoOX(OPR3)] (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate; X = Cl-, phenolates, thiolates), have been isolated from the reactions of [Tp(iPr)MoO2X] with phosphines (PEt3, PMePh2, PPh3). The green, diamagnetic oxomolybdenum(IV) complexes possess local C(1) symmetry (by NMR spectroscopy) and exhibit IR bands assigned to nu(Mo==O) (approximately 950 cm(-1)) and nu(P==O) (1140-1083 cm(-1)) vibrations. The X-ray crystal structures of [Tp(iPr)MoOX(OPEt3)] (X = OC6H4-2-sBu, SnBu), [Tp(iPr)MoO(OPh)(OPMePh2)], and [Tp(iPr)MoOCl(OPPh3)] have been determined. The monomeric complexes exhibit distorted octahedral geometries, with coordination spheres composed of tridentate fac-Tp(iPr) and mutually cis monodentate terminal oxo, phosphoryl (phosphine oxide), and monoanionic X ligands. The electronic structures and stabilities of the complexes have been probed by computational methods, with the three-dimensional energy surfaces confirming the existence of a low-energy steric pocket that restricts the conformational freedom of the phosphoryl ligand and inhibits complete oxygen atom transfer. The reactivity of the complexes is also briefly described.

Mononuclear and Binuclear Cyclopentadienyl Oxo Molybdenum and Tungsten Complexes: Syntheses and Applications in Olefin Epoxidation Catalysis

New half-sandwich molybdenum complexes [Mo(CpBz)Cl2(O)] (1), [{Mo(CpBz)(O)2}2(μ-Ο)] (5), and [Mo(CpBz)Cl2]2 (6) (CpBz = C5(CH2Ph)5), the tungsten derivative [W(CpBz)Cl(O)2] (3), and a high yield synthesis of [Mo(CpBz)Cl(O)2] (2) are described. The molecular structures and cyclic voltammetry measurements of 1 and 2 and a comparative study of various molybdenum dioxo complexes as olefin epoxidation catalyst precursors are also presented. The performance of [Mo(CpPri4)Cl(O)2] (9) as cyclooctene epoxidation catalyst using TBHP as oxidant was determined and compared with other [MoCp‘Cl(O)2] complexes (Cp‘ = C5H5 (7), C5Me5 (8), CpBz (2)) to assess the activity dependence on the ring substituents. Under the same experimental conditions, bimetallic compounds [{MoCp‘(O)2}2(μ-Ο)] (Cp‘ = CpBz (5), Cp* (10), CpBut3 (11), CpPri4 (12)) have been tested and [CpMoO2]2O was checked in aqueous solution, with H2O2 and TBHP as oxidant. Finally, [Mo(CpBz)(CO)3CH3] (4) was used as a catalyst precursor for cyclooctene epoxidation in the presence of TBHP. The analogy between the behaviors displayed by this complex and complex 2 is discussed.

Oxomolybdenum(III, IV and V) complexes of thiofunctional ligands

Reaction of [MoO2(acac)(2)] with Li2NS2S'(2), (S is a thioether, S' a thiophenolate function) yielded the compound Li-7(thf)(17){MoO}(8)center dot 10thf center dot hexane, where {MoO}(8) represents one [(MoO)-O-IV(NS2S'(2))] 1, three [(MoO)-O-III(NS2S'(2))](-) (2, linked, via the oxo group, to [Li(thf)3](+)) and two [(MoO)-O-V(S'(2))(2)](2-)(2) .A mixed-valent variant of 3, [{(MoO)-O-IV(S'(2))} {(MoO)-O-V(S'(2))}](-) (3b, with an additional [Li(thf)3](+) attached to S'), was also identified. The compounds model features pertinent to oxo-transferases containing the molybdopterin cofactor.

Synthesis and characterization of molybdenum oxo complexes of two tripodal ligands: reactivity studies of a functional model for molybdenum oxotransferases

Reaction of the tetradentate ligand N-(2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (L-OH) with MoO2Cl2 in methanol in the presence of NaOMe and PF6- results in the formation of [MoO2(L-O)]PF6. Similarly, the reaction of N-(2-mercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine (L-SH) with MoO2(acac)2 leads to the formation of [MoO2(L-S)]+. The dioxo-molybdenum complex [MoO2(L-O)]+ reacts with phosphines in methanol to afford phosphine oxides and an air-sensitive molybdenum complex, tentatively identified as [Mo(IV)O(L-O)(OCH3)]. The latter complex is capable of reducing biological oxygen donors such as DMSO or nitrate, thereby mimicking the activity of DMSO reductase and nitrate reductase. Reaction of [MoO2(L-O)]PF6 with PPh3 in other solvents than methanol leads to the formation of the Mo(V) dimer [(L-O)OMo(micro-O)MoO(L-O)](PF6)2. The crystal structures of [MoO2(L-O)]PF6 and the micro-oxo bridged dimer are presented.

Incorporation of a (Cyclopentadienyl)molybdenum Oxo Complex in MCM‐41 and Its Use as a Catalyst for Olefin Epoxidation

AbstractThe tricarbonyl complex [(η5‐C5H4‐COOMe)Mo(CO)3Cl] was prepared from the reaction of sodium (methoxycarbonyl)cyclopentadienide, (C5H4‐CO2Me)Na, with (Bu4N)[Mo(CO)5I]. Heating the ester with 3‐(triethoxysilyl)propylamine gave the amide derivative {[η5‐C5H4‐CONH‐C3H6Si(OEt)3]Mo(CO)3Cl}. The functionalised tricarbonyl complex was immobilised in the ordered mesoporous silica MCM‐41 with a loading of 13 wt.‐% Mo (1.4 mmol·g−1) by carrying out a grafting reaction in dichloromethane. Powder X‐ray diffraction and nitrogen adsorption−desorption analysis indicated that the structural integrity of the support was preserved during the grafting and that the channels remained accessible, despite significant reductions in surface area, pore volume and pore size. The success of the coupling reaction was confirmed by 29Si and 13C (CP) MAS NMR spectroscopy. A supported dioxo complex of the type [(η5‐C5H4R)MoO2Cl] was subsequently prepared by oxidative decarbonylation of the tethered tricarbonyl complex using tert‐butyl hydroperoxide (TBHP). The oxidised material is an active catalyst for the liquid phase epoxidation of cyclooctene with TBHP as the oxygen source. Similar catalytic results were obtained using the tethered tricarbonyl complex directly as a pre‐catalyst since fast oxidative decarbonylation occurs under the reaction conditions used. For both systems, the desired epoxide was the only product and the initial activities were about 13 mol·molMo−1·h−1. The solid catalysts were recycled several times. Some activity was lost between the first and second runs but thereafter tended to stabilise. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Nitrosyl and Oxo Complexes of Molybdenum

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural Studies of Mono‐ and Dimetallic MoVI Complexes − A New Mechanistic Contribution in Catalytic Olefin Epoxidation Provided by Oxazoline Ligands

AbstractNew seven‐coordinate oxo(peroxo)molybdenum(VI) complexes with bidentate chiral oxazolines, anionic κ2‐N,O (V) and neutral κ2‐N,N (VI), have been prepared and fully characterised by NMR spectroscopy and single‐crystal X‐ray diffraction studies. Dimetallic dioxo(μ‐oxo)molybdenum(VI) compounds containing oxazolinylpyridine ligands II−IV have also been synthesised. NMR studies showed the presence of several isomers (up to six species) due to the Mo−O−Mo bridge angle and the unsymmetrical nature of the N,N ligand. The roles of mono‐ (I, V and VI) and dimetallic (II) oxomolybdenum(VI) complexes in the catalytic epoxidation of cyclooctene, norbornene and (R)‐limonene have been studied. The high activity of the oxomono(peroxo) precursor V containing the hemilabile oxazolinylphenolate, in contrast to the unexpected low activity for the oxobis(peroxo) species VI, could be justified by the inertness observed for the oxazolinylpyridine ligand. In addition, the dimetallic system II afforded high selectivity towards limonene epoxide formation (trans/cis‐10 ratio up to 9:1). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Modulation of Molybdenum-Centered Redox Potentials and Electron-Transfer Rates by Sulfur versus Oxygen Ligation

Temperature-dependent measurements of potential, E degrees', and electron-transfer rate constant, k(s,h), are reported for electrochemical reduction (in 0.3 M TBAPF(6)/CH(3)CN) of a series of oxomolybdenum(V) complexes, [(Tp)MoO(X,Y)], where Tp = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X,Y is a series of bidentate 1,2-disubstituted aliphatic or aromatic ligands in which oxygen donors are replaced sequentially by sulfur. E degrees' values shift in the positive direction, and k(s,h) values increase as O is replaced by S and as the framework of the ligand is changed from aliphatic to aromatic. The electrochemical enthalpy of activation, measured under conditions of zero driving force as DeltaH= -R partial differential[ln(k(s,h))]/ partial differential(1/T) and corrected for an outer-shell component by the mean spherical approximation, is approximately 10 kJ mol(-1) larger for complexes with O versus S donors and with an aliphatic versus aromatic ligand framework. Thus, the rate of Mo(V/IV) electron transfer is modulated primarily by differences in inner-shell reorganization. Following a recent description of electronic structure contributions to electron-transfer reactivity (Kennepohl, P.; Solomon, E. I. Inorg. Chem. 2003, 42, 679 ff), it is concluded that more effective charge distribution over the entire molecular structure, as mediated by electronic relaxation in S versus O and aromatic versus aliphatic systems, is responsible for the influence of ligand structure on the kinetics and thermodynamics of Mo-centered electron transfer. There is no evidence, based on experimentally measured pre-exponential factors, that sulfur donors or an aromatic ligand framework are more effective than their structural counterparts in facilitating electronic coupling between the electrode and the Mo d(xy) redox orbital.