Ring Contraction of a Tungstacyclopentane Supported on Silica: Direct Conversion of Ethylene to Propylene.

The reaction of W(NAr)(13C4H8)(OSiPh3)2 (1) (NAr = 2,6-diisopropylphenylimido) with silica partially dehydroxylated at 700 °C (SiO2-700) is highly dependent on the reaction conditions. The primary product of this reaction is W(NAr)(13C4H8)(OSiPh3)(OSi(O-)3) (2) when the reaction is carried out in the dark. Grafting 1 onto SiO2-700 in ambient lab light results in the formation of 2, W(NAr)(13CH213CH2)(OSiPh3)(OSi(O-)3) (4), and one isomer of square-pyramidal W(NAr)(13CH213CH(13Me)13CH2)(OSiPh3)(OSi(O-)3) (3). Heating 2 to 85 °C for 6 h results in the formation of 3, 4, W(NAr)(13CH(13Me)13CH213CH2)(OSiPh3)(OSi(O-)3) (5), and W(NAr)((13CH2)213CH(13Me)(13CH2)2)(OSiPh3)(OSi(O-)3) (6). Photolysis of 2 with blue LEDs (λmax = 450 nm) produces 4, both isomers of 3, 5, and free ethylene. In the presence of excess ethylene and blue LED irradiation at 85 °C, 1/SiO2-700 catalyzes the direct conversion of ethylene to propylene.

Tungstacyclopentane Ring Contraction Yields Olefin Metathesis Catalysts.

Exposure of a solution of the square pyramidal tungstacyclopentane complex W(NAr)(OSiPh3)2(C4H8) (Ar = 2,6-i-Pr2C6H3) to ethylene at 22 °C in ambient (fluorescent) light slowly leads to the formation of propylene and the square pyramidal tungstacyclobutane complex W(NAr)(OSiPh3)2(C3H6). No reaction takes place in the dark, but the reaction is >90% complete in ∼15 min under blue LED light (∼450 nm λmax). The intermediates are proposed to be (first) an α methyl tungstacyclobutane complex (W(NAr)(OSiPh3)2(αMeC3H5)), and then from it, a ß methyl version. The TBP versions of each can lose propylene and form a methylene complex, and in the presence of ethylene, the unsubstituted tungstacyclobutane complex W(NAr)(OSiPh3)2(C3H6). The W-Cα bond in an unobservable TBP W(NAr)(OSiPh3)2(C4H8) isomer in which the C4H8 ring is equatorial is proposed to be cleaved homolytically by light. A hydrogen atom moves or is moved from C3 to the terminal C4 carbon in the butyl chain as the bond between W and C3 forms to give the TBP α methyl tungstacyclobutane complex. Essentially, the same behavior is observed for W(NCPh3)(OSiPh3)2(C4H8) as for W(NAr)(OSiPh3)2(C4H8), except that the rate of consumption of W(NCPh3)(OSiPh3)2(C4H8) is about half that of W(NAr)(OSiPh3)2(C4H8). In this case, an α methyl-substituted tungstacyclobutane intermediate is observed, and the overall rate of formation of W(NCPh3)(OSiPh3)2(C3H6) and propylene from W(NCPh3)(OSiPh3)2(C4H8) is ∼20 times slower than in the NAr system. These results constitute the first experimentally documented examples of forming a metallacyclobutane ring from a metallacyclopentane ring (ring contraction) and establish how metathesis-active methylene and metallacyclobutane complexes can be formed and reformed in the presence of ethylene. They also raise the possibility that ambient light could play a role in some metathesis reactions that involve ethylene and tungsten-based imido alkylidene olefin metathesis catalysts, if not others.

E- and Z-trisubstituted macrocyclic alkenes for natural product synthesis and skeletal editing.

Many therapeutic agents are macrocyclic trisubstituted alkenes but preparation of these structures is typically inefficient and non-selective. A possible solution would entail catalytic macrocyclic ring-closing metathesis, but these transformations require high catalyst loading, conformationally rigid precursors and are often low yielding and/or non-stereoselective. Here we introduce a ring-closing metathesis strategy for synthesis of trisubstituted macrocyclic olefins in either stereoisomeric form, regardless of the level of entropic assistance. The goal was achieved by addressing several unexpected difficulties, including complications arising from pre-ring-closing metathesis alkene isomerization. The power of the method is highlighted by two examples. The first is the near-complete reversal of substrate-controlled selectivity in the formation of a macrolactam related to an antifungal natural product. The other is a late-stage stereoselective generation of an E-trisubstituted alkene in a 24-membered ring, en route to the cytotoxic natural product dolabelide C.

Stereodefined alkenes with a fluoro-chloro terminus as a uniquely enabling compound class.

Trisubstituted alkenyl fluorides are important compounds for drug discovery, agrochemical development and materials science. Despite notable progress, however, many stereochemically defined trisubstituted fluoroalkenes either cannot be prepared efficiently or can only be accessed in one isomeric form. Here we outline a general solution to this problem by first unveiling a practical, widely applicable and catalytic strategy for stereodivergent synthesis of olefins bearing a fluoro-chloro terminus. This has been accomplished by cross-metathesis between two trisubstituted olefins, one of which is a purchasable but scarcely utilized trihaloalkene. Subsequent cross-coupling can then be used to generate an assortment of trisubstituted alkenyl fluorides. The importance of the advance is highlighted by syntheses of, among others, a fluoronematic liquid-crystal component, peptide analogues bearing an E- or a Z-amide bond mimic, and all four stereoisomers of difluororumenic ester (an anti-cancer compound).

Interconversion of Molybdenum or Tungsten d2 Styrene Complexes with d0 1-Phenethylidene Analogues.

Upon addition of 5-15% PhNMe2H+X- (X = B(3,5-(CF3)2C6H3)4 or B(C6F5)4) to Mo(NAr)(styrene)(OSiPh3)2 (Ar = N-2,6-i-Pr2C6H3) in C6D6 an equilibrium mixture of Mo(NAr)(styrene)(OSiPh3)2 and Mo(NAr)(CMePh)(OSiPh3)2 is formed over 36 h at 45 °C (Keq = 0.36). A plausible intermediate in the interconversion of the styrene and 1-phenethylidene complexes is the 1-phenethyl cation, [Mo(NAr)(CHMePh)(OSiPh3)2]+, which can be generated using [(Et2O)2H][B(C6F5)4] as the acid. The interconversion can be modeled as two equilibria involving protonation of Mo(NAr)(styrene)(OSiPh3)2 or Mo(NAr)(CMePh)(OSiPh3)2 and deprotonation of the α or ß phenethyl carbon atom in [Mo(NAr)(CHMePh)(OSiPh3)2]+. The ratio of the rate of deprotonation of [Mo(NAr)(CHMePh)(OSiPh3)2]+ by PhNMe2 in the α position versus the ß position is ∼10, or ∼30 per Hß. The slow step is protonation of Mo(NAr)(styrene)(OSiPh3)2 (k1 = 0.158(4) L/(mol·min)). Proton sources such as (CF3)3COH or Ph3SiOH do not catalyze the interconversion of Mo(NAr)(styrene)(OSiPh3)2 and Mo(NAr)(CMePh)(OSiPh3)2, while the reaction of Mo(NAr)(styrene)(OSiPh3)2 with pyridinium salts generates only a trace (∼2%) of Mo(NAr)(CMePh)(OSiPh3)2 and forms a monopyridine adduct, [Mo(NAr)(CHMePh)(OSiPh3)2(py)]+ (two diastereomers). The structure of [Mo(NAr)(CHMePh)(OSiPh3)2]+ has been confirmed in an X-ray study; there is no structural indication that a ß proton is activated through a CHß interaction with the metal. W(NAr)(CMePh)(OSiPh3)2 is also converted into a mixture of W(NAr)(CMePh)(OSiPh3)2 and W(NAr)(styrene)(OSiPh3)2 (Keq = 0.47 at 45 °C in favor of the styrene complex) with 10% [PhNMe2H][B(C6F5)4] as the catalyst; the time required to reach equilibrium is approximately the same as in the Mo system.

Stereochemical Control Yields Mucin Mimetic Polymers.

All animals except sponges produce mucus. Across the animal kingdom, this hydrogel mediates surface wetting, viscosity, and protection against microbes. The primary components of mucus hydrogels are mucins-high molecular weight O-glycoproteins that adopt extended linear structures. Glycosylation is integral to mucin function, but other characteristics that give rise to their advantageous biological activities are unknown. We postulated that the extended conformation of mucins is critical for their ability to block microbial virulence phenotypes. To test this hypothesis, we developed synthetic mucin mimics that recapitulate the dense display of glycans and morphology of mucin. We varied the catalyst in a ring-opening metathesis polymerization (ROMP) to generate substituted norbornene-derived glycopolymers containing either cis- or trans-alkenes. Conformational analysis of the polymers based on allylic strain suggested that cis- rather than trans-poly(norbornene) glycopolymers would adopt linear structures that mimic mucins. High-resolution atomic force micrographs of our polymers and natively purified Muc2, Muc5AC, and Muc5B mucins revealed that cis-polymers adopt extended, mucin-like structures. The cis-polymers retained this structure in solution and were more water-soluble than their trans-analogs. Consistent with mucin's linear morphology, cis-glycopolymers were more potent binders of a bacterial virulence factor, cholera toxin. Our findings highlight the importance of the polymer backbone in mucin surrogate design and underscore the significance of the extended mucin backbone for inhibiting virulence.

Boosting the Metathesis Activity of Molybdenum Oxo Alkylidenes by Tuning the Anionic Ligand σ Donation.

The catalytic performances of molecular and silica-supported molybdenum oxo alkylidene species bearing anionic O ligands [ORF9, OTPP, OHMT - where ORF9 = OC(CF3)3, OTPP = 2,3,5,6-tetraphenylphenoxy, OHMT = hexamethylterphenoxy] with different σ-donation abilities and sizes are evaluated in the metathesis of both internal and terminal olefins. Here, we show that the presence of the anionic nonafluoro-tert-butoxy X ligand in Mo(O){═CH-4-(MeO)C6H4}(THF)2{X}2 (1; X = ORF9) significantly increases the catalytic performances in the metathesis of both terminal and internal olefins. Its silica-supported equivalent displays slightly lower activity, albeit with improved stability. In sharp contrast, the molecular complexes with large aryloxy anionic X ligands show little activity, whereas the activity of the corresponding silica-supported systems is greatly improved, illustrating that surface siloxy groups are significantly smaller anionic ligands. Of all of the systems, compound 1 stands out because of its unique high activity for both terminal and internal olefins. Density functional theory modeling indicates that the ORF9 ligand is ideal in this series because of its weak σ-donating ability, avoiding overstabilization of the metallacyclobutane intermediates while keeping low barriers for [2 + 2] cycloaddition and turnstile isomerization.

Silica-Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis.

Grafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well-defined silica-supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo-based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off-cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica-supported Mo and W imido analogues.

E- and Z-, di- and tri-substituted alkenyl nitriles through catalytic cross-metathesis.

Nitriles are found in many bioactive compounds, and are among the most versatile functional groups in organic chemistry. Despite many notable recent advances, however, there are no approaches that may be used for the preparation of di- or tri-substituted alkenyl nitriles. Related approaches that are broad in scope and can deliver the desired products in high stereoisomeric purity are especially scarce. Here, we describe the development of several efficient catalytic cross-metathesis strategies, which provide direct access to a considerable range of Z- or E-di-substituted cyano-substituted alkenes or their corresponding tri-substituted variants. Depending on the reaction type, a molybdenum-based monoaryloxide pyrrolide or chloride (MAC) complex may be the optimal choice. The utility of the approach, enhanced by an easy to apply protocol for utilization of substrates bearing an alcohol or a carboxylic acid moiety, is highlighted in the context of applications to the synthesis of biologically active compounds.

Traceless Protection for More Broadly Applicable Olefin Metathesis.

An operationally simple in situ protection/deprotection strategy that significantly expands the scope of kinetically controlled catalytic Z- and E-selective olefin metathesis is introduced. Prior to the addition of a sensitive Mo- or Ru-based complex, treatment of a hydroxy- or a carboxylic-acid-containing olefin with commercially available HB(pin) or readily accessible HB(trip)2 (pin=pinacolato, trip=2,4,6-tri(isopropyl)phenyl) for 15 min is sufficient for efficient generation of a desired product. Routine workup leads to quantitative deprotection. A range of stereochemically defined Z- and E-alkenyl chlorides, bromides, fluorides, and boronates or Z-trifluoromethyl-substituted alkenes with a hydroxy or carboxylic acid group were thus prepared in 51-97 % yield with 93 to >98 % stereoselectivity. We also show that, regardless of whether a polar functional unit is present or not, a small amount of HB(pin) may be used to remove residual water, significantly enhancing efficiency.

Protonation Studies of Molybdenum(VI) Nitride Complexes That Contain the [2,6-(ArNCH2)2NC5H3]2- Ligand (Ar = 2,6-Diisopropylphenyl).

[Ar2N3]Mo(N)(O- t-Bu) (1), which contains the conformationally rigid pyridine-based diamido ligand [2,6-(ArNCH2)2NC5H3]2- (Ar = 2,6-diisopropylphenyl), is a catalyst for the reduction of dinitrogen with protons and electrons. Various acids have been added in order to explore where and how the first proton adds to the complex. The addition of adamantol to 1 produces a five-coordinate bis(adamantoxide), [HAr2N3]Mo(N)(OAd)2 (2a), in which one of the amido nitrogens in the ligand has been protonated and the resulting aniline nitrogen in the [HAr2N3]- ligand is not bound to the metal. The addition of [Ph2NH2][OTf] to 1 produces {[HAr2N3]Mo(N)(O- t-Bu)}(OTf) (3), in which an amido nitrogen has been protonated, but the aniline in the [HAr2N3]- ligand remains bound to the metal. Last, the addition of (2,6-lutidinium)BArF4 (BArF4 = {B(3,5-(CF3)2C6H3)4}-) to 1 yields {[Ar2N3]Mo(N)(LutH)(O- t-Bu)}BArF4, in which LutH+ is hydrogen-bonded to the nitride in the solid state and in dichloromethane with Keq = 412 ± 94 and Δ G = -3.6 ± 0.8 kcal at 22 °C. A similar hydrogen-bonded adduct was formed through the addition of (2-methylpyridinium)BArF4 to 1, but the addition of (pyridinium)BArF4 to 1 leads to the formation of (inter alia) {[HAr2N3]Mo(N)(O- t-Bu)}(BArF4), in which the amide nitrogen has been protonated. The addition of cobaltocene to 3 or {[Ar2N3]Mo(N)(LutH)(O- t-Bu)}(BArF4) leads only to the re-formation of 1. X-ray structural studies were carried out on 2a, 3, and {[Ar2N3]Mo(N)(LutH)(O- t-Bu)}(BArF4).

Molybdenum Complexes that Contain a Calix[6]azacryptand Ligand as Catalysts for Reduction of N2 to Ammonia.

[CAC(OMe)6]Mo(N) (3, where [CAC]3- is a calix[6]azacryptand ligand derived from a [6]calixarene) has been prepared in a reaction between Li3[CAC(OMe)6] and ( t-BuO)3Mo(N). An X-ray structural study showed 3 to have a structure similar to that of [HIPTN3N]Mo(N) (where [HIPTN3N]3- is [(3,5-(2,4,6-triisopropylphenyl)2C6H3NCH2CH2)3N]3-). The relatively rigid [CAC(OMe)6]3- ligand in 3 forms a bowl-shaped cavity defined by a 24-atom macrocyclic ring. The Mo-Namido-Cipso angles are ∼8° smaller in 3 than they are in [HIPTN3N]Mo(N). Methoxides on the three linking units point into the cavity above the nitride in 3, whereas the three methoxides on phenyl rings attached to the amido nitrogen atoms point away from the cavity. An analogous [CAC(OMe)3(H)3]Mo(N) complex (9) was prepared in which the three methoxides pointing into the cavity in 3 have been replaced by protons. Its structure differs little from that of 3. The nitride could be protonated in 3 to give {[CAC(OMe)6]Mo(NH)}+, which could be reduced (reversibly) to [CAC(OMe)6]Mo(NH). Catalytic reduction of molecular nitrogen under a variety of conditions with either Ph2NH2OTf or HBArf (BArf- = {B[3,5(CF3)2C5H3]4}-) as the acid and a Co metallocene or KC8 as the reducing agent between -78 and 22 °C in diethyl ether shows that 1.20-1.34 equivalents of ammonia are formed starting with either [CAC(OMe)6]Mo(N) (50% 15N) or [CAC(OMe)3(H)3]Mo(N) (50% 15N).

Syntheses of Molybdenum Oxo Benzylidene Complexes.

The reaction between Mo(O)(CHAro)(ORF6)2(PMe3) (Aro = ortho-methoxyphenyl, ORF6 = OCMe(CF3)2) and 2 equiv of LiOHMT (OHMT = O-2,6-(2,4,6-Me3C6H2)2C6H3) leads to Mo(O)(CHAro)(OHMT)2, an X-ray structure of which shows it to be a trigonal bipyramidal anti benzylidene complex in which the o-methoxy oxygen is coordinated to the metal trans to the apical oxo ligand. Addition of 1 equiv of water (in THF) to the benzylidyne complex, Mo(CArp)(OR)3(THF)2 (Arp = para-methoxyphenyl, OR = ORF6 or OC(CF3)3 (ORF9)) leads to formation of {Mo(CArp)(OR)2(µ-OH)(THF)}2(µ-THF) complexes. Addition of 1 equiv of a phosphine (L) to Mo(CArp)(ORF9)3(THF)2 in THF, followed by addition of 1 equiv of water, all at room temperature, yields Mo(O)(CHArp)(ORF9)2(L) complexes in good yields for several phosphines (e.g., PMe2Ph (69% by NMR), PMePh2 (59%), PEt3 (69%), or P( i-Pr)3 (65%)). The reaction between Mo(O)(CHArp)(ORF9)2(PEt3) and 2 equiv of LiOHMT proceeds smoothly at 90 °C in toluene to give Mo(O)(CHArp)(OHMT)2, a four-coordinate syn alkylidene complex. Mo(O)(CHArp)(OHMT)2 reacts with ethylene (1 atm in C6D6) to give (in solution) a mixture of Mo(O)(CHArp)(OHMT)2, Mo(O)(CH2)(OHMT)2, and an unsubstituted square pyramidal metallacyclobutane complex, Mo(O)(CH2CH2CH2)(OHMT)2, along with ethylene and ArpCH═CH2. Mo(O)(CHArp)(OHMT)2 also reacts with 2,3-dicarbomethoxynorbornadiene to yield syn and anti isomers of the "first-insertion" products that contain a cis C═C bond.

Beyond fossil fuel-driven nitrogen transformations.

Nitrogen is fundamental to all of life and many industrial processes. The interchange of nitrogen oxidation states in the industrial production of ammonia, nitric acid, and other commodity chemicals is largely powered by fossil fuels. A key goal of contemporary research in the field of nitrogen chemistry is to minimize the use of fossil fuels by developing more efficient heterogeneous, homogeneous, photo-, and electrocatalytic processes or by adapting the enzymatic processes underlying the natural nitrogen cycle. These approaches, as well as the challenges involved, are discussed in this Review.

Syntheses of Molybdenum Oxo Alkylidene Complexes through Addition of Water to an Alkylidyne Complex.

Addition of one equiv of water to Mo(CAr)[OCMe(CF3)2]3(1,2-dimethoxyethane) (2, Ar = o-(OMe)C6H4) in the presence of PPhMe2 leads to formation of Mo(O)(CHAr)[OCMe(CF3)2]2(PPhMe2) (3(PPhMe2)) in 34% yield. Addition of one equiv of water alone to 2 produces the dimeric alkylidyne hydroxide complex, {Mo(CAr)[OCMe(CF3)2]2(µ-OH)}2(dme) (4(dme)) in which each bridging hydroxide proton points toward an oxygen atom in an arylmethoxy group. Addition of PMe3 to 4(dme) gives the alkylidene oxo complex, (3(PMe3)), an analogue of 3(PPhMe2) (95% conversion, 66% isolated). Treatment of 3(PMe3) with two equiv of HCl gave Mo(O)(CHAr)Cl2(PMe3) (5), which upon addition of LiO-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (LiOHIPT) gave Mo(O)(CHAr)(OHIPT)Cl(PMe3) (6). Compound 6 in the presence of B(C6F5)3 will initiate the ring-opening metathesis polymerization of cyclooctene, 5,6-dicarbomethoxynorbornadiene (DCMNBD), and rac-5,6-dicarbomethoxynorbornene (DCMNBE), and the homocoupling of 1-decene to 9-octadecene. The poly(DCMNBD) has a cis,syndiotactic structure, whereas poly(DCMNBE) has a cis,syndiotactic,alt structure. X-ray structures were obtained for 3(PPhMe2), 4(dme), and 6.

Synthesis of High-Oxidation-State Mo═CHX Complexes, Where X = Cl, CF3, Phosphonium, CN.

Reactions between Z-XCH=CHX where X = Cl, CF3, or CN and Mo(N-t-Bu)(CH-t-Bu)(OHIPT)Cl(PPh2Me) (OHIPT = O-2,6-(2,4,6-i-Pr3C6H2)2C6H3) produce Mo(N-t-Bu)(CHX)(OHIPT)Cl(PPh2Me) complexes. Addition of 2,2'-bipyridyl (Bipy) yields Mo(N-t-Bu)(CHX)(OHIPT)Cl(Bipy) complexes, which could be isolated and structurally characterized. The reaction between Mo(N-t-Bu)(CH-t-Bu)(OHMT)Cl(PPh2Me) (OHMT = O-2,6-(2,4,6-Me3C6H2)2C6H3) and Z-ClCH=CHCl in the presence of Bipy produces a mixture that contains both Mo(N-t-Bu)(CHCl)(OHMT)Cl(PPh2Me) and Mo(N-t-Bu)(CHCl)(OHMT)Cl(Bipy), but the relatively insoluble product that crystallizes from toluene-d 8 is the phosphoniomethylidene complex, [Mo(N-t-Bu)(CHPPh2Me)(OHMT)(Cl)(Bipy)]Cl. The Mo(N-t-Bu)(CHX)(OHIPT)Cl(PPh2Me) complexes (X = Cl or CF3) were confirmed to initiate the stereoselective cross-metathesis between Z-5-decene and Z-XCH=CHX.

Reduction of Dinitrogen to Ammonia Catalyzed by Molybdenum Diamido Complexes.

[Ar2N3]Mo(N)(O-t-Bu), which contains the conformationally rigid pyridine-based diamido ligand, [2,6-(ArNCH2)2NC5H3]2- (Ar = 2,6-diisopropylphenyl), can be prepared from H2[Ar2N3], butyllithium, and (t-BuO)3Mo(N). [Ar2N3]Mo(N)(O-t-Bu) serves as a catalyst or precursor for the catalytic reduction of molecular nitrogen to ammonia in diethyl ether between -78 and 22 °C in a batchwise manner with CoCp*2 as the electron source and Ph2NH2OTf as the proton source. Up to ∼10 equiv of ammonia can be formed per Mo with a maximum efficiency in electrons of ∼43%.

EPR/ENDOR and Theoretical Study of the Jahn-Teller-Active [HIPTN3N]MoVL Complexes (L = N-, NH).

The molybdenum trisamidoamine (TAA) complex [Mo] {[3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2N]Mo} carries out catalytic reduction of N2 to ammonia (NH3) by protons and electrons at room temperature. A key intermediate in the proposed [Mo] nitrogen reduction cycle is nitridomolybdenum(VI), [Mo(VI)]N. The addition of [e-/H+] to [Mo(VI)]N to generate [Mo(V)]NH might, in principle, follow one of three possible pathways: direct proton-coupled electron transfer; H+ first and then e-; e- and then H+. In this study, the paramagnetic Mo(V) intermediate {[Mo]N}- and the [Mo]NH transfer product were generated by irradiating the diamagnetic [Mo]N and {[Mo]NH}+ Mo(VI) complexes, respectively, with γ-rays at 77 K, and their electronic and geometric structures were characterized by electron paramagnetic resonance and electron nuclear double resonance spectroscopies, combined with quantum-chemical computations. In combination with previous X-ray studies, this creates the rare situation in which each one of the four possible states of [e-/H+] delivery has been characterized. Because of the degeneracy of the electronic ground states of both {[Mo(V)]N}- and [Mo(V)]NH, only multireference-based methods such as the complete active-space self-consistent field (CASSCF) and related methods provide a qualitatively correct description of the electronic ground state and vibronic coupling. The molecular g values of {[Mo]N}- and [Mo]NH exhibit large deviations from the free-electron value ge. Their actual values reflect the relative strengths of vibronic and spin-orbit coupling. In the course of the computational treatment, the utility and limitations of a formal two-state model that describes this competition between couplings are illustrated, and the implications of our results for the chemical reactivity of these states are discussed.