Mo Analogue Research Articles

N2 Reduction versus H2 Evolution in a Molybdenum- or Tungsten-Based Small-Molecule Model System of Nitrogenase.

Molybdenum dinitrogen complexes have played a major role as catalytic model systems of nitrogenase. In comparison, analogous tungsten complexes have in most cases found to be catalytically inactive. Herein, a tungsten complex was shown to be supported by a pentadentate tetrapodal (pentaPod) phosphine ligand, under conditions of N2 fixation, primarily catalyzes the hydrogen evolution reaction (HER), in contrast to its Mo analogue, which catalytically mediates the nitrogen-reduction reaction (N2 RR). DFT calculations were employed to evaluate possible mechanisms and identify the most likely pathways of N2 RR and HER activities exhibited by Mo- and W-pentaPod complexes. Two mechanisms for N2 RR by PCET are considered, starting from neutral (M(0) cycle) and cationic (M(I) cycle) dinitrogen complexes (M=Mo, W). The latter was found to be energetically more favorable. For HER three scenarios are treated; that is, through bimolecular reactions of early M-Nx Hy intermediates, pure hydride intermediates or mixed M(H)(Nx Hy ) species.

Isolation and characterization of thiolato complexes of [CpCr(SBz)]2S, isomeric trans [CpMo(CO)(SBz)]2 and [CpMo(SBz)2]2 (Cp = (η5-C5H5), Bz = PhCH2[sbnd]) from the reactivity of bibenzyl disulfide towards [CpM(CO)3]2 (M = Cr, Mo)

The reactions of [CpM(CO)3]2 (M=Cr (1), Mo (3)) with Bz2S2 have been investigated. 1 underwent a facile reaction at ambient temperature which led to the isolation of [CpCr(CO)2]S (5), [CpCr(CO)2(SBz)]2 (6) and [CpCr(SBz)]2S (7) in 28.8, 42.6 and 11.8% yield, respectively. A similar reaction at 110°C with its congener, [CpCr(CO)2]2 (CrCr) (2) yielded only thermolytic products of 5 (40.3%) and Cp4Cr4S4 (8) (10.6%). With the Mo analogue 3, only the decarbonylated [CpMo(CO)2]2 (MoMo) (4) reacts at 70°C to give an isomeric pair of trans syn and anti [CpMo(CO)(SBz)]2 (9a &9b), [CpMo(SBz)(S)]2 (10) and [CpMo(CO)(SBz)]2S (11) in 43.3, 34.1, 5.5 and 3.1% yield, respectively. A totally decarbonylated new complex of [CpMo(SBz)2]2 (12) was obtained in 33.2% yield from the co-thermolysis of Bz2S2 with 9a at 110°C. All products were fully characterized with NMR and IR spectroscopy and C, H analysis. The molecular structures of 7, 9b and 12 have been elucidated by single crystal X-ray diffraction. Plausible mechanistic pathways leading to the formation of 7, 9 and 12 were discussed.

A comparative study on seniority-based MO and VB calculations of the singlet and triplet energy gaps of open-shell molecules

The recently proposed truncated configuration interaction (CI) approach based on the seniority number (Ω), which is defined as the number of singly occupied orbitals in a Slater determinant, is applied to study the adiabatic singlet-triplet excitation energies of some open-shell non-Kekulé diradical molecules, including: trimethylenemethane (TMM), tetramethyleneethane (TME) and meta-quinodimethane (m-QDM). Inasmuch as the singlet state exhibits strongly correlated electronic structure but the triplet state does not, adequate evaluation of the excitation energies requires balanced treatment of both states. It is found that within the active space comprised with the π-electrons/orbitals, the seniority-based VB approach is able to describe both the lowest singlet and triplet states in balance, but the MO analogue shows inharmonious convergence behavior toward the CASSCF limit. As a result, the seniority-based VB approach, superior to the MO analogue, can produce good singlet-triplet energy gap even at lower levels.

Reaction of [Cp ∗2W 2O 5] with mercaptocarboxylic acids: Addition rather than reduction. Isolation and characterization of Cp ∗WO 2(SCH 2CH 2COOH)

The reaction of Cp ∗ 2W 2O 5 with HS(CH 2) nCOOH (n = 1, 2) in MeOH or in CH 2Cl 2 solutions at room temperature proceeds in slightly different ways depending on the value of n. For n = 2, it selectively yields compound Cp ∗WO 2(SCH 2CH 2CO 2H), which has been isolated and characterized by elemental analysis, NMR and single crystal X-ray diffraction. The reaction is equilibrated, being shifted to the product by absorption of water by anhydrous Na 2SO 4 in CH 2Cl 2, and to the reactants by addition of water. Contrary to the Mo analogue, no products resulting from metal reduction are obtained. The corresponding reaction for n = 1 occurs similarly at low substrate/W ratios (<0.5), but proceeds further to several uncharacterized products for greater substrate amounts. The primary product could not be isolated, but its 1H NMR spectrum suggests a different, asymmetric structure.

ChemInform Abstract: Gas-Phase Oxidation of Group 6 Metal Carbonyl Anions

Oxidation of Cr(CO) 5 - , Mo(CO) 5 - , and W(CO) 5 - was examined in a low-pressure (∼5×10 -8 mbar) and a high-pressure external(∼3.5 mbar) environment using a Fourier-transform ion cyclotron resonance spectrometer equipped with a high-pressure external ion source. Although the oxidation chemistry displayed by the Cr and W systems was fairly similar, the Mo analogue had several unique features. In addition, remarkable differences were observed between high- and low-pressure oxidation

Olefin Epoxidation by H2O2/MeCN Catalysed by Cyclopentadienyloxidotungsten(VI) and Molybdenum(VI) Complexes: Experiments and Computations

Compounds [Cp*(2)M(2)O(5)] (M = Mo, 1; W, 2) are efficient pre-catalysts for cyclooctene (COE) epoxidation by aqueous H(2)O(2) in acetonitrile/toluene. The reaction is quantitative, selective and takes place approximately 50 times faster for the W system (k(obs) = 4.32(9)x10(-4) s(-1) at 55 degrees C and 3x10(-3) M concentration for the dinuclear complex, vs. 1.06(7)x10(-5) s(-1) for the Mo system). The rate law is first order in catalyst and COE substrate (k = 0.138(7) M(-1) s(-1) for the W system at 55 degrees C), whereas increasing the concentration of H(2)O(2) slows down the reaction because of an inhibiting effect of the greater amount of water. The activation parameters for the more active W systems (DeltaH(double dagger) = 10.2(6) kcal mol(-1); DeltaS(double dagger) = -32(2) cal mol(-1) K(-1)) were obtained from an Eyring study in the 25-55 degrees C temperature range. The H(2)O(2)urea adduct was less efficient as an oxidant than the aqueous H(2)O(2) solution. Replacement of toluene with diethyl ether did not significantly affect the catalyst efficiency, whereas replacement with THF slowed down the process. The epoxidation of ethylene as a model olefin, catalysed by the [Cp*MO(2)Cl] systems (M = W, Mo) in the presence of H(2)O(2) as oxidant and acetonitrile as solvent, has been investigated by DFT calculations with the use of the conductor-like polarisable continuum model (CPCM). For both metal systems, the rate-limiting step is the transfer of the hydroperoxido O(alpha) atom to the olefin, in accordance with the first-order dependence on the substrate and the zero-order dependence on H(2)O(2) found experimentally in the catalytic data. The activation barrier corresponding to the rate-limiting step is 4 kcal lower for the W complex than for the corresponding Mo analogue (32.3 vs. 28.3 kcal mol(-1)). This result reproduces well the higher catalytic activity of the W species. The different catalytic behaviour between the two systems is rationalised by a natural bond orbital (NBO) study and natural population analyses (NPA). Compared to Mo, the W(VI) centre withdraws more electron density from the sigma bonding [O-O] orbital and favours, as a consequence, the nucleophilic attack of the external olefin on the sigma*[O-O] orbital.

WITHDRAWN: A crystal engineered unprecedented Rh(V) complex in [RhOBr5]2- crystallized with 2,2’- bipyridinium cation and its Mo analogue: Comparison with Density Functional Theory calculations

WITHDRAWN: A crystal engineered unprecedented Rh(V) complex in [RhOBr5]2- crystallized with 2,2’- bipyridinium cation and its Mo analogue: Comparison with Density Functional Theory calculations

Bonding Nature of Open-Lantern-type Dinuclear Cr(II) Complexes. Theoretical Study with the MRMP2 Method

Open-lantern-type dinuclear Cr(II) complex, [Cr(R(1)NC(R(2))NR(3))(2)](2) (R(1) = Et, R(2) = Me, and R(3) = (t)Bu), was theoretically investigated with DFT, CASSCF, and MRMP2 methods. The DFT-optimized Cr-Cr distance (1.757 A) is too short compared to the experimental value (1.960 A). The CASSCF method does not present the minimum in the range of the Cr-Cr distance from 1.75 to 2.05 A. The MRMP2 method presents the optimized Cr-Cr distance of 1.851 A, which is a little shorter than the experimental value. These results suggest that both nondynamical and dynamical correlations are considerably large in this complex. The Cr-Cr bond order is evaluated to be 2.40 with the CASSCF method, which is much smaller than the formal bond order of 4. In the Mo analogue, on the other hand, the DFT, CASSCF, and MRMP2 methods present almost the same Mo-Mo distance (2.151 A). The Mo-Mo bond order is evaluated to be 3.41, which is somewhat smaller than the formal value but much larger than the Cr-Cr bond order. These differences arise from the much larger d-d overlap integral of the Mo-Mo pair than that of the Cr-Cr pair. Though nondynamical correlation effect is very large in this dinuclear Cr(II) complex, the Cr-Cr distance of this complex was experimentally discussed to be short, based on formal shortness ratio (FSR). We wish to propose here orbital shortness ratio (OSR) based on the distance providing maximum overlap integral to discuss the M-M bond distance. According to the OSR, we understand that the Cr-Cr distance of 1.960 A is long but the Mo-Mo distance of 2.151 A is short. This understanding is consistent with much larger nondynamical correlation in the dinuclear Cr(II) complex than in the Mo(II) analogue. Interesting differences are also observed between M-M and Si-Si multiple bonds. The differences are discussed in terms of sigma- and pi-type overlap integrals and the participation of Si 3s orbital in the sigma-bonding orbital.

Structural, Spectroscopic, and Kinetic Investigation of the Molybdenum Dialkylhydrazido Complexes [MoBr(NNC5H10)(dppe)2]Br and [Mo(NNC5H10)(dppe)2]: Activation Parameters and Revised Mechanism for N−N Cleavage†

The reaction of [Mo(NNC(5)H(10))(dppe)(2)] (B(Mo)) with an excess of acid, HNEt(3)BPh(4), is investigated applying temperature-dependent stopped-flow measurements. The kinetic data indicate a biphasic process with rate constants k(obs(1)) and k(obs(2)) which are both slower than the single rate constant reported by Henderson et al. (Henderson, R. A.; Leigh, G. J.; Pickett, C. J., J. Chem. Soc., Dalton Trans. 1989, 425-430). Moreover, both rate constants exhibit a linear dependence on the acid concentration with a large intercept which is attributed to acid-dependent and acid-independent components of each reaction phase, respectively. All four reaction channels exhibit temperature-dependent reaction rates. Furthermore, B(Mo) and its Mo(IV) analogue [MoBr(NNC(5)H(10))(dppe)(2)]Br (A(Mo)) are characterized structurally and spectroscopically. Density-functional theory calculations are performed to locate possible barriers in the overall reaction scheme and determine their energies, providing additional information for the formulation of a mechanism. The temperature-dependent rate of N-N cleavage is explained by a revised mechanism which involves an alpha-protonated intermediate that is inert with respect to N-N cleavage and is generated from its beta-protonated counterpart by a rapid 1,2-proton shift. The implications of these results with respect to N(2) reduction in the Chatt cycle and the enzyme nitrogenase are discussed.

Effect of the Nature of the Metal Atom on Hydrogen Bonding and Proton Transfer to [Cp*MH3(dppe)]: Tungsten versus Molybdenum

The hydrogen-bonding and proton-transfer pathway to complex [Cp*W(dppe)H(3)] (Cp*=eta(5)-C(5)Me(5); dppe=Ph(2)PCH(2)CH(2)PPh(2)) was investigated experimentally by IR, NMR, UV/Vis spectroscopy in the presence of fluorinated alcohols, p-nitrophenol, and HBF(4), and by using DFT calculations for the [CpW(dhpe)H(3)] model (Cp=eta(5)-C(5)H(5); dhpe=H(2)PCH(2)CH(2)PH(2)) and for the real system. A study of the interaction with weak acids (CH(2)FCH(2)OH, CF(3)CH(2)OH, (CF(3))(2)CHOH) allowed the determination of the basicity factor, E(j)=1.73+/-0.01, making this compound the most basic hydride complex reported to date. A computational investigation revealed several minima for the [CpW(dhpe)H(3)] adducts with CF(3)CH(2)OH, (CF(3))(2)CHOH, and 2(CF(3))(2)CHOH and confirms that these interactions are stronger than those established by the Mo analogue. Their geometries and relative energies are closely related to those of the homologous Mo systems, with the most stable adducts corresponding to H bonding with M-H sites, however, the geometric and electronic parameters reveal that the metal center plays a greater role in the tungsten systems. Proton-transfer equilibria are observed with the weaker proton donors, the proton-transfer step for the system [Cp*W(dppe)H(3)]/HOCH(CF(3))(2) in toluene having DeltaH=(-3.9+/-0.3) kcal mol(-1) and DeltaS=(-17+/-2) cal mol(-1) K(-1). The thermodynamic stability of the proton-transfer product is greater for W than for Mo. Contrary to the Mo system, the protonation of the [Cp*W(dppe)H(3)] appears to involve a direct proton transfer to the metal center without a nonclassical intermediate, although assistance is provided by a hydride ligand in the transition state.

Oxidation of Thiophene Derivatives with H2O2 in Acetonitrile Catalyzed by [Cp*2M2O5] (M = Mo, W): A Kinetic Study

The oxidation of benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (DMDBT) by H2O2 to the corresponding sulfoxides and sulfones has been studied under homogeneous conditions in MeCN with the compounds [Cp*2M2O5] (M = Mo (1), W (2)) as precatalysts. The W system is ca. 100 times more efficient than the Mo analogue, while the relative reactivity of the thiophene substrates is approximately DBT/DMDBT/BT ≈ 10/5/1. For all reactions rate constants for both steps (thiophene derivative to sulfoxide, k1; sulfoxide to sulfone, k2) were measured. While k1 ≈ k2 for DBT and DMDBT, k1 << k2 for BT, independent of catalyst. Activation parameters for the stepwise oxidations of thiophene derivative to sulfoxide (BT to BTO, ΔH⧧ = 11.4(5) kcal mol−1 and ΔS⧧ = –26.1(1.6) eu; DBT to DBTO, ΔH⧧ = 7.7(6) kcal mol−1 and ΔS⧧ = –33(2) eu) and sulfoxide to sulfone (BTO to BTO2, ΔH⧧ = 10.8(5) kcal mol−1 and ΔS⧧ = –21.8(1.6) eu; DBTO to DBTO2, ΔH⧧ = 10.3(9) kcal mol−1 and ΔS⧧ = –25(3) eu) were calculated from variable temperature studies using [Cp*2W2O5]. DFT calculations suggest that the greater reactivity of DBT relative to BT is not caused by ground-state effects but rather by a transition-state effect associated with the greater thermodynamic gain in DBT oxidation.

First example of photomagnetic effects in ionic pairs [Ni(bipy) 3] 2[Mo(CN) 8] · 12H 2O

Ionic triads formed by [Ni II(bipy) 3] 2+ (bipy = 2,2′-bipyridyl) and diamagnetic [M IV(CN) 8] 4− (M = Mo and W) were prepared and structurally characterized. The two compounds are isostructural and their structure consists of a three-dimensional hydrogen-bonded framework where cation–anion interactions occur through short contacts M–CN⋯H–C(bipy). Before irradiation, the Mo analogue behaves as paramagnet with small intermolecular interactions between the [Ni II(bipy) 3] 2+ cations. Upon irradiation with visible light, it exhibits a reversible photomagnetic effect, which is interpreted in terms of the formation of paramagnetic [Mo V(CN) 8] 3− and [Ni II(bipy) 2(bipy − )] + due to the outer-sphere electron transfer.

Bis(methyl-ammonium) tetra-sulfido-tungstate(VI).

The title compound, (CH6N)2[WS4], was synthesized by the reaction of ammonium tetra­sulfidotungstate(VI) with aqueous methyl­amine. The title compound is isotypic with the corresponding Mo analogue (CH6N)2[MoS4], and its structure consists of a slightly distorted tetra­hedral [WS4]2− dianion and two crystallographically independent methyl­ammonium (MeNH3) cations, all of which are located on crystallographic mirror planes. The tetra­sulfidotungstate anions are linked to the organic cations via hydrogen-bonding inter­actions.

Bis[benzyl(methyl)ammonium] tetrasulfidotungstate(VI)

The title compound, (C8H12N)2[WS4], was synthesized by the aqueous reaction of ammonium tetra­sulfidotungstate(VI) with benz­yl(meth­yl)amine in a 1:2 molar ratio. The compound is isotypic with the corresponding Mo analogue, (C8H12N)2[MoS4], and its structure consists of a slightly distorted tetra­hedral [WS4]2− dianion and two crystallographically independent benz­yl(meth­yl)ammonium cations, with all atoms located in general positions. The cations and anion are linked by weak N—H⋯S and C—H⋯S inter­actions, the strength and number of which can explain the observed W—S bond distances.

Synthesis and solid state characterization of two insoluble tetrathiometalates

The aqueous reaction of ammonium tetrathiometalates (NH4)2[MS4] (M = Mo or W) with (dbtmen)Br2 · 2H2O (dbtmen = N,N′-dibenzyl-N,N,N ′, N′-tetramethylethylenediammonium dication) results in the formation of the highly insoluble compounds (dbtmen)[MoS4] (1) and (dbtmen)[WS4] (2) in near quantitative yields. Compounds (1) and (2) have been characterized by elemental analysis, spectroscopic methods, X-ray powder diffraction and TG–DTA. Both compounds exhibit nearly identical IR spectra and X-ray powder patterns. The compounds exhibit a single strong signal for the asymmetric M–S stretching vibration at 475 cm−1 in (1) and at 457 cm−1 in (2). Complex (2) is thermally more stable than the corresponding Mo analogue (1). Thermal decomposition products of (1) and (2) are carbon contaminated amorphous metal disulfides and are formulated as MoS1.99C2.06N0.07 and WS1.75C3.02 based on elemental analysis of the residue.

Reduction Pathway of End-On Terminally Coordinated Dinitrogen. V. N−N Bond Cleavage in Mo/W Hydrazidium Complexes with Diphosphine Coligands. Comparison with Triamidoamine Systems

N-N cleavage of the dialkylhydrazido complex [W(dppe)2(NNC5H10)] (B(W)) upon treatment with acid, leading to the nitrido/imido complex and piperidine, is investigated experimentally and theoretically. In acetonitrile and at room temperature, B(W) reacts orders of magnitude more rapidly with HNEt3BPh4 than its Mo analogue, [Mo(dppe)2(NNC5H10)] (B(Mo)). A stopped-flow experiment performed for the reaction of B(W) with HNEt3BPh4 in propionitrile at -70 degrees C indicates that protonation of B(W) is completed within the dead time of the stopped-flow apparatus, leading to the primary protonated intermediate B(W)H+. Propionitrile coordination to this species proceeds with a rate constant k(obs(1)) of 1.5 +/- 0.4 s(-1), generating intermediate RCN-B(W)H+ (R = Et) that rapidly adds a further proton at Nbeta and then mediates N-N bond splitting in a slower reaction (k(obs(2)) = 0.35 +/- 0.08 s(-1), 6 equiv of acid). k(obs(1)) and k(obs(2)) are found to be independent of the acid concentration. The experimentally observed reactivities of B(Mo) or B(W) with acids in nitrile solvents are reproduced by DFT calculations. In particular, geometry optimization of models of solvent-coordinated, Nbeta-protonated intermediates is found to lead spontaneously to separation into the nitrido/imido complexes and piperidine/piperidinium, corresponding to activationless heterolytic N-N bond cleavage processes. Moreover, DFT indicates a spontaneous cleavage of nonsolvated B(W) protonated at Nbeta. In the second part of this article, a theoretical analysis of the N-N cleavage reaction in the Mo(III) triamidoamine complex [HIPTN3N]Mo(N2) is presented (HIPTN3N = hexaisopropylterphenyltriamidoamine). To this end, DFT calculations of the Mo(III)N2)triamidoamine complex and its protonated and reduced derivatives are performed. Calculated structural and spectroscopic parameters are compared to available experimental data. N-N cleavage most likely proceeds by one-electron reduction of the Mo(V) hydrazidium intermediate [HIPTN3N]Mo(NNH3)+, which is predicted to have an extremely elongated N-N bond. From an electronic-structure point of view, this reaction is analogous to that of Mo/W hydrazidium complexes with diphos coligands. The general implications of these results with respect to synthetic N2 fixation are discussed.

Absence of 1,3-metallotropic shifts in M(CO) 5 complexes of pyrimidine and 1,3,5-triazine (M=W, Mo, Cr)

M(CO) 5L complexes with M=W 0, Mo 0 and Cr 0 and L=pyrimidine and 1,3,5-triazine were prepared by photochemical generation from the parent M(CO) 6 complexes in acetone. All six complexes are not fluxional among available N-donor sites from room temperature up to ca. 60°C in CDCl 3, and do not undergo rapid exchange with external excess free ligand. Thus, as in the case of the W(CO) 5L complexes, a one-bond migration between N sites in a diazine occurs readily for pyridazine (e.g. a 1,2-metallotropic shift has been reported), but two-bond migrations for a 1,3-metallotropic shift is not seen with pyrimidine or triazine, just as the three-bond migration for a 1,4-metallotropic shift was not observed previously for the pyrazine complexes. The decomposition processes of the M(CO) 5L complexes in CDCl 3 with pyrazines is known to follow the order Cr (minutes lifetime)⪢Mo (several hours)>W (days to weeks duration), but for the pyrimidine and 1,3,5-triazine complexes Cr and W are of comparable stability (stable for 1–2 weeks), and more so than the Mo analogue (stable for about 7–8 h). 1H NMR data on all six complexes show that protons α to the site of metallation are shifted downfield by 0.27–0.36 ppm compared to the free ligand. The magnitude of Δ δ for the effect of metals on the chemical shift follow the order W>Mo>Cr. The more remote H4 protons of pyrimidine or triazine exhibit the smallest downfield shift upon metallation by the M(CO) 5 moiety. For the Cr(CO) 5 derivative the H4 proton of pyrimidine and H5 of triazine are shifted upfield. The upfield shift effect for the H5, near absence of a net effect for H4 of pyrimidine, and the upfield shift observed for H4 of the triazine complex of the Cr series are attributed mainly to a strong resonance component with dπ back-donation for the Cr derivative, as well as a possible TIP influence on the chemical shift parameter, contributions that are attenuated for the Mo and W complexes with higher energy excited states. No evidence for η 2-type coordination for these d 6 complexes was obtained in contrast to Ru(II) d 6 chemical relatives which adopt η 2 coordination to pyrimidines.

Synthesen und Strukturen von neuen 1,3,5‐Triphospha‐2,4,6‐trisilacyclohexanen und Untersuchung ihrer Ligandeneigenschaften gegenüber [M(CO)3]‐Komplexfragmenten (M = Cr, Mo)

Syntheses and Structures of New 1,3,5‐Triphospha‐2,4,6‐trisilacyclohexanes and Investigation of Their Ligand Properties Toward [M(CO)3] Complex Fragments (M = Cr, Mo)*The synthesis of the new P3Si3‐heterocyclohexanes 1b‐1e are reported. They are easily accessible by salt elimination reaction of the corresponding dichloro organosilanes with lithium cyclohexylphosphanide. Their NMR‐spectroscopic properties, structural features and reactivity toward [M(CO)3(NCMe)3] are compared to those of the previous described derivative 1a. The structure of 1b has been established by X‐ray crystallography, showing that the Si3P3six‐membered ring has a slightly distorted chair‐like conformation and the organo substituents at silicon and phosphorus are located in the equatorial positions. The phosphorus atoms are pyramidal surrounded [sum of bond angles 302.1 (P1), 304.8 (P2) and 303.1° (P3)] and the Si‐P distances are identical with those in 1a and in other silacyclophosphanes. The complexation of the triphosphane‐coronands 1b‐e with [M(CO)3(NCMe)3] (M = Cr, Mo) gives the desired triphos‐phane crown‐like complexes 2b‐e and 3e in 38–53% yield. The X‐ray crystal structures of the Cr complexes 2b and 2e exhibit that the organo groups at silicon, surprisingly, being axial oriented and the cyclohexyl groups being in equatorial positions, whereas in case of the Mo analogue of 2e, that is 3e, all organo substituents prefer equatorial sites. The different configuration of the free ligand 1b compared with that in the complex 2b and the different configuration of the ligand 1e in the complexes 2e and 3e is due to the steric congestion (bulky aryl and [M(CO)3] groups) in these molecules. The change of configurations can be achieved by phosphorus inversion during the complexation process; the ease of inversion is rationalized by a intrinsic low inversion barriere at phosphorus (10–20 kcal mol−1) in silacyclophosphanes. Another complex type has been observed for the previous reported unusual Cr complex 2a, in which one Si‐H bond ousts a phosphorus atom in the coordination to the Cr center. The different structures of the Cr complexes 2b‐d compared to the structure of 2a clearly proves that the formation of a Si, H, Cr three‐center two‐electron bond in 2a is determined by the second ortho‐methyl group at the aryl substituent (mesityl) and the tiny metal center (Cr).

The first triangular V 3S 72+ complex. Synthesis and structure of (Et 4N) [V 3S 7(Me 2dtc) 3] · CH 3CN

A triangular V 38 7 2+ complex (Et 4N)[V 3S 7(Me 2dtc) 3]·CH 3CN has been obtained from a complicated reaction system containing (NH 4) 3VS 4 as a starting material. The complex crystallizes in the triclinic, space group P1̄ with crystal data: a=13.027(2), b=13.180(5), c=12.554(4) Å, a=97.00(3), β=116.21(2), Γ=95.28(3)°, V=1893.5(5) Å 3, Z=2, R=0.043 and R w=0.054 for 4501 reflections with I> 3σ( I). The anion [V 3S 7(Me 2dtc) 3] − is isostructural (exclusive of R groups in the ligand), but not isoelectronic with its Mo analogue [Mo 3S 7(Et 2dtc) 3] +, and has a lower mean oxidation state (+3 1 3 ) of vanadium than that observed in [V 3S 4(edt) 3] 3−. The structural parameters show the identity of three V atoms, implying an electronic delocalization. The 1H NMR spectrum exhibits two types of methyl resonances of the ligand at 10.69 and 8.72 ppm with an intensity ratio of 1:1.

Gas-phase oxidation of Group 6 metal carbonyl anions

Oxidation of Cr(CO) 5 - , Mo(CO) 5 - , and W(CO) 5 - was examined in a low-pressure (∼5×10 -8 mbar) and a high-pressure external(∼3.5 mbar) environment using a Fourier-transform ion cyclotron resonance spectrometer equipped with a high-pressure external ion source. Although the oxidation chemistry displayed by the Cr and W systems was fairly similar, the Mo analogue had several unique features. In addition, remarkable differences were observed between high- and low-pressure oxidation