Molybdenum, and Tungsten

Introduction.—The spectrum of Mo has been reanalysed and the ionization potential of this ion estimated as 144.0 + 1.0eV.  

The organometallic chemistry of molybdenum has been reviewed and papers published in 1971 dealing with this area of chemistry for molybdenum and tungsten have been surveyed. Reviews discussing aspects of catalysis by tungsten compounds and the geochemical cycle of molybdenum in the environment have been published. Coupled proton-electron transfer mechanisms have been proposed for the action of molybdenum in several enzymes to explain many experimental observations.  

Billings and J. L. Sonderegger, Amer. Chem Soc. Div Water. Air Waste Client. Gen Paper. 1971,11, 166 (Chem Abs., 1973,78, 149850v), 

A new type of electron-deficient metal cluster system, e.g. (61) and (62), has been identified by X-ray crystallography following the preparation of these compounds from [(t -Cp)2MoH2] and an excess of AljMe in toluene. Their ease of formation suggests that a considerable range of electron-deficient transition-metal compounds of this type should be isolable.  

Carbonyl Complexes.—As indicated for chromiums only a selection of the material reported this year is presented. 

Chromium Molybdenum, and Tungsten. Triple metal-metal bonded complexes of the types [MaLe] (M = Mo or W, L = NRj, OR, or R) and [ M(CO)aCp a] have been reviewed. Although MO calculations suggest the structures of the former should be eclipsed, the known examples, which contain bulky terminal ligands, have ethane-like, staggered conformations. Attempts to determine the mechanism by which [MaLg] complexes are formed have, as yet, been unsuccessful although it is clear that metal-metal metathesis reactions between [MOaLe] and [WaLe] occur far less readily than do those between [ Mo(CO)aCp a] and [ W(CO)aCp a].  

Oxidation of [Mo(CO 4L] affords low yields of [Mo2(CO) L2] (L = diaza-butadiene), which is thought to contain a double metal-metal bond bridged by ligands L acting as r,jr-donors. [W(CO) ] and AW -di-3,5-xylylforraamidine 

The molecular structures of the triple metal-metal bonded complexes [ M-(CO)2Cp 2] (M = Cr or Mo ) are strikingly different the former contains a non-linear arrangement of the MjCpg skeleton, whereas that of the latter is linear with semi-bridging carbonyls. Based on MO calculations arguments are given to account for the difference, and it is sug ted that conjugation between the metal-metal and carbon-carbon Ji-systems occurs in the dimolybdenum complex.  

Allenes react with [ M(CO)2Cp 2] (M = Mo or W) to give 1 1 adducts in which the allene bridges the triple metal-metal bond,  

A critical evaluation of the data obtained by X-tSLy and neutron diffraction studies on [CM-H)Mo2(CO)4(At-PMe2)Cp2] suggests that for closed, bent M—H—M bonds the X-ray results will give shorter M—H distances and larger M—H—M bond angles.  

Chromium, Molybdenum, and Tungsten. - It is virtually impossible to obtain Cr, Mo, and W in the metallic state at the surface of any support via the usual impregnation technique, because the reduction of salts is incomplete and, at the most, a low-valence state of the metal is obtained (e.g., Cr in the case of chromium). Few papers deal with Cr or Mo either as evaporated films or as monocrystals. The reader is, however, referred to the review by Sheppard. 

Contact of NO with metal subcarbonyls invariably yields dinitrosylic species where the central atom has been oxidized by NO itself. In the case of Cr, valence states of two or three are likely to be attainedin the case of Mo and W the tetravalent state is favoured. The equivalence of the NO oscillators has been proved (in the case of Mo complexes) by isotopic exchange. Millman and Hall have studied the same system and assigned the observed bands to dimeric dinitrosyl species. 

The interaction of Cr(CO)e with silica has also been studied by our group and the basic findings by Howe have been confirmed. In addition, it has been found that at 773 K an oxidative interaction takes place yielding Cr These ions can further interact with Cr(CO)6, giving rise to weakly chemisorbed species (CO)sCr-COCr , where a terminal CO group interacts via a dative bond with a Cr ion acting as a Lewis centre. Such a structure is analogous to that proposed by Kazusaka and Howe for Mo(CO)6 on Al . 

All three carbonyls behave similarly towards thoroughly outgassed MgO. Ni(CO)4 and Fe(CO)s show vide infra) similar behaviour. The interaction takes place with ionic pairs Mg O in low co-ordination giving species of the type (1). The lowering of the symmetry in both the M(C0)6 structures formed on AI2O3 and MgO causes the appearance of new carbonylic modes as expected. Between ambient temperature and 200 °C, dimeric and trimeric species are also formed above 250°C irreversible and complete decarbonylation occurs as also monitored by the 

Chromium, Molybdenum, and Tungsten.- The radical anions [CpM(CO)2(NO)] (M = Cr, Mo) have been prepared by electrochemical 

Deprotonatlon of cations of the type (2 M = Mo, W n = 1,2) affords the corresponding cyclic ester complexes (3) resulting from intramolecular reaction the analogues with n = 3 give only products of Intermolecular reaction treatment of the complexes (3 n = 2) 

RC=CR proceed by Insertion of the alkyne Into a W-C bond and ligand 

26 reported. Interaction of W vapour with a mixture of cyclopentane 

Chromium, Molybdenum, and Tungsten. Neutron diffraction studies on [Crj-(CO)io(a -D)] at 17 K provide evidence for a four-fold site distribution of the deuterium. A more simple preparation of homo- and hetero-dinuclear complexes from the combination of 17-e species [M(CO)3Cp, (M=Cr, Mo, W) Mn(CO)s Fe(CO)2Cp Co(CO)4 Ni(CO)Cp] has been described. Heating [Cr(CO)2( -C6He)(NCMe)] results in the formation of the Cr Cr complex, [Cr2( -C6H6)2(/M-CO)3], and the antiferromagnetic complexes, [CraCpgCw-S)-(jM-EPh)2], (E = S, Se), have been reported. The preparation of a variety of nitrosyl dimers of chromium, molybdenum, and tungsten has been reported  

Bianchi, G. Menchi, F. Francalanci, F. Piacenti, U. Matteoli, P. Frediani, and C. Botteghi, y. Organomet. Chem., 1980,188, 109. 

Levisalles, F. Rose-Munch, H. Rudler, J. C. Daran, Y. Dromzee, and Y. Jeannin, /. Ghem. Soc., Ghem. Gommun., 1980, 685. 

Fischer, W. Kellerer, B. Zimmer-Gasser, and U. Schubert, /. Organomet. Ghem., 1980, 199, C24. 

Chrraniiim, Molybdenum, and Tungsten.—Little has been reported of reactions involving Group VI transition metals. Chelating phosphine ligands of the 

Manganese, Tecimetinin, and Rhenium.— Photolysis of pentacarbonyltri-fluoromethylmanganese in an argon matrix at 17 K results in carbon monoxide insertion into the Mn—CFj bond.  

Trimethyl- and dimethyl-tin hydrides react photochemically with penta-carbonyltrifluorovinylrhenium to give reduced vinyl complexes (88)— (90) the rrawj ifluorovinyl complex (90) is the major product, and a free-radical addition-elimination mechanism has been postulated. Hydrido-pentacarbonylmanganese reacts with bis(trifluoromethyl)diazomethane to form the complex (CF3)2CH Mn(CO)j. The reaction of HMnCCX)) with 

The most abundant ion in the mass spectra of complexes of the type RFMn(CO)j is [MnF]+, while some derivatives of alicyclic fluorocarbons show fragmentations involving loss of MnF. In similar rhenium complexes, the tendency to form metal-free fluorocarbon ions is much reduced.  

Group VI.—Chromium, Molybdenum, and Tungsten. Further evidence for cis labilization in substituted octahedral complexes of metal carbonyls is found in the reactions of [M(CO)4(amine)2] with which give complexes c/s-[M(CO)4- 

Few dinitrogen complexes of chromium are known. Complex 8 decomposes at 20 C with loss of both N2 and PMes. Treatment of complex 9 with acid produces small quantities of ammonia (7%) and hydrazine (1%). Reaction of the polynuclear complex 10 with water regenerates the starting rhenium complex tran5-[ReCl(N2)(PMe2Ph)4], and with dioxygen produces the cation [ReCl(N2)(PMe2Ph)4].  

Following the discovery of rrans-Mo(N2)2(dppe)2, 15, by Hidai in 1969, a series of four landmark reactions in the area of the chemistry of coordinated dinitrogen were reported by Chatt and co-workers over a period of three years. These are (i) the reaction of 33 with acyl halides to form nitrogen-carbon bonds (Equation (ii) protonation of 15 and 

33 to form nitrogen-hydrogen bonds (Equation (iii) alkylation of 

15 and 33 to form nitrogen-carbon bonds using alkyl halides (Equation 12) and (iv) protonolysis to produce ammonia (and hydrazine) (Equation 13)  

This latter reaction only occurs when there is at least one monodentate phosphine coordinated to the metal. For the purpose of discussion, the remainder of this section will be divided into reactions that (i) form nitrogen-carbon bonds (ii) form nitrogen-hydrogen bonds and (iii) reactions of polynuclear complexes. 

In terms of the reaction catalyzed, molybdopterin-containing enzymes can be divided in two groups those that mediate oxygen atom transfer, such as dimethyl sulfoxide (DMSO) reductase and sulfite oxidase (SO), and those that catalyze hydroxylation reactions of aromatic heterocyclic compounds and aldehydes [116], for instance xanthine oxidoreductase (XOR) and aldehyde oxidoreductase (AOR). However, this functional classification does not coincide with structural properties that suggest that the enzymes should be grouped into five families, whose most representative members are (1) DMSO reductase (2) XOR (3) SO (4) aldehyde- 

The chemistry of ditungsten paddlewheel compounds is substantially less developed than the periodically related dichromium and dimolybdenum analogs. This is in part due to the increased susceptibility of the W2 core to reaction with Oj and in part to the increased difficulty in synthesizing the tetracarhoxylate analogs. One of the most commonly used methods is the reaction of WCl and the sodium carhoxylate with Na/Hg in THF (Eq. (6.2)) [12]. While this route works for a number of carboxylates, Wj(02CCH3)4 has to be prepared by metathesis of 

In addition to homoleptic compounds, species with more than one ligand type can be prepared. For example, the reaction of M2(T PB)4 (M = Mo, W T PB = 2,4,6-triisopropylbenzoate) with 2equiv. of 2-thienylcarboxylic acid (02C-fh) yields fraMs-M2(T PB)2(02C-Th)2 [15]. Mixed carboxylate/amidinate compounds of form fraKs-M2(02CR)2(R NC R NR )2 can also be prepared by the reaction of dimetal tetracarboxylate with the alkali metal salt of the ligand [16]. 

The potential of the M2 redox couple can be tuned by over 1V by changing the met2d and axi2d or equatorial ligands. For example, the oxidation potential of a series of compounds of form Mo2(EE C=CPh)4 (EE = NPhjj, (NPh 0, NPh S, and OO) can be tuned by 0.9 V with simple N for 

O for S substitution [22], This large change in redox potential can alter the reactivity of the MOj core the dimolybdenum tetracarboxylate Mo2(T PB)4 can act as a catalyst for radical addition reactions of polyhaloalkanes to 1-alkenes, whereas amidinate and guanidinate analogs are catalysts for the radical polymerization of methyl methacrylate [23]. 

The axial sites of dimolybdenum and ditungsten compounds are weak Lewis acids that can coordinate to a variety of neutral or anionic ligands. It has been found that the Lewis acidity of one axial site can be enhanced by axial coordination of a lithium ion to the opposite axial site an increase in the affinity of M02 for Cl was found in LiMo2(monothiosuccinimide)4Cl [24]. The Lewis acid activation results from a polarization of the Mo-Mo bond, induced by the lithium cation coordinated to the other axial site. 

Introduction.—Reviews on the molecular structure of co-ordination compounds and organometallic compounds containing molybdenum and tungsten as determined by X-Tdiy diffraction have appeared. Structural aspects of Mo , Mo, and Mo complexes have been examined and molybdenum complexes themselves have also been specifically reviewed. A discussion of molybdenum and tungsten dialkyl-amides and disilylamides has been included in a recent review and the co-ordination chemistry of aryldiazonium cations (ArNj) of molybdenum and tungsten has been discussed.  

N o attempt has been made to report on the organometallic chemistry of molybdenum and tungsten as this is adequately reviewed in the appropriate Specialist Periodical Report and elsewhere. Only a small selection of catalytic, kinetic, and electrochemical applications are reported and the relevant Specialist Periodical Report should be consulted for a comprehensive account. 

The function of molybdenum in enzymic mechanisms has been reviewed and the role of molybdenum in nitrogen fixation examined.  

Thermodynamic data, chemical equilibria, and standard potentials, which should provide a useful source of information for the Group VI elements is contained in an excellent review. Another review of thermodynamic data has appeared which lists the heats of reaction and formation of molybdenum and tungsten oxides and halides, and some molybdates and tungstates.  

Manojlovic-Muir, Molecular Structure by Diffraction Methods , ed. L. E. Sutton and G. A. Sim., (Specialist Periodical Reports), The Chemical Society, London, 1975, Vol 3, p. 263. 

Introduction.— The papers presented at the conference on The Chemistry and Uses of Molybdenum have now been published262 and recent developments in the synthetic2 and organometallic1 chemistry of molybdenum and tungsten have been reviewed. 

Baldea and S. Schoen, Stud. Univ. Babes-Bolyai, Ser. Chem., 1973,18, 47 Chem. Abs.. 1973. 79. 136207). 

Less-Common Metals, 1974, 36, 1-536. The papers presented at the Conference on the Chemistry and Uses of Molybdenum. Reading, 1973. 

Sherrill, J. H. Nibert, and J. Selbin, Inorg. Nuclear Chem. Letters. 1974, 10, 845. 

The half-life of 181W has been re-investigated and a value of 120.95 + 0.02 days determined, which differs significantly from the currently accepted value 272 The u.v. absorption spectra of molybdenum atoms isolated in rare-gas matrices at 14 K have been correlated with similar gas-phase spectral data and assigned in spherical symmetry. Diffusion of the metal atoms in an Ar matrix was also studied and some tentative evidence obtained for dimer formation.273 The standard heat of vapourization of molybdenum has been determined274 as 689.3 kJ (g atom)-1. 

Introduction.—A review discussing the properties of the molybdenum-containing enzymes, in particular the e.s.r. characteristics of xanthine oxidase, and a text describing the analytical chemistry of molybdenum and tungsten, have been published. 

The seventh ionization potential of molybdenum has been estimated as 129.04 and 125.66 eV from spectral studies. 

Sala-Pala and J. E. Guerchais, Bull. Soc. chim. France, 1971, 3178. 

760 -p iwell and D. F. Wood, Analytical Chemistry of Molybednum and Tungsten , Pergamon, Elsmford, New York, 1971. 

The chemical transport of M0O2 with iodine in a temperature gradient suggests the existence of M0O2I2 in the vapour phase. The remarkable elimination-stabilized alkyl WMe has been obtained by the interaction of WClg with LiMe in ether and shown to be reasonably inert to chemical attack.  

Perhaps the most striking feature of the complexes described in this section is the great variety of the structural types exhibited. Co-ordination numbers of the metal atoms vary from three to eight, and a wide range of different co-ordination polyhedra is thereby displayed. With one exception, the inorganic complexes of chromium have the metal atom in the tripositive state, but for molybdenum and tungsten the higher oxidation states, from three- to six-positive, are all well represented. Comparative studies of related molecules are few. 

Interest in the molecular structures of molybdenum and tungsten complexes, particularly in those containing N-donor ligands, is enhanced by the occurrence of molybdenum in the enzyme nitrogenase. It is possible that the stereochemistry of these metals may be of importance in understanding processes of in vivo and in vitro nitrogen-fixation. 

The abbreviations used for various ligands in this chapter are shown in Table 1. 

The [Cr KNPr a)s] molecule contains a chromium atom co-ordinated by a trigonal-planar array of nitrogen atoms, and is one of only rarely found examples of this type of co-ordination geometry. The Cr(NC2)a unit has Z 3 symmetry. The Cr-N distances of ca. 1.87 A and the planarity of the CrNCa system for each of the amide ligands are considered indicative of ligand-to-metal w-bonding. 

The absolute configurations of two [Cr LJ complexes, where L represents a bidentate ligand, have been determined as A(+)Bg,-[Cr(malonate)3P and A(+)s -[Cr(ox)3] -. The configuration of the first complex is opposite to that deduced from spectroscopic measurements. 

It has previously been shown that recombination of the fragments formed on photolysis of Cr(CO)e in methane matrices may be induced by irradiation in the 

Flash photolysis of Cr(CO)e, Mo(CO)6, W(CO)e, or Fe(CO)B in the gas phase produced the respective metal atoms and also species assigned as Cr2 and Mo2.190 In the presence of 02 the corresponding monoxides were detected, and CrH and CrD were formed on photolysis of Cr(CO)e in a hydrogen or deuterium atmosphere. 

In a study of the light-induced exchange of CO from (norbornadiene)M(CO)4 (M = Cr, Mo, or W) using 13CO, it was found that the axial ligand is preferentially displaced (equation 62). Secondary reactions then cause scrambling 

On photolysis (21), which is itself prepared photochemically, forms (22) and (23).1 4 

Recent publications dealing with (arene)chromium compounds include an investigation of the photodecomposition of (benzene)Cr(CO 3 in cyclohexane 

Manganese. Low temperature NMR spectroscopy has been used to study alkyl halide coordination to [(ri ri -C5H4CH2CH2Br)Mn(CO)2]. The lability of H2 in [Mn(H2)(CO)3(PCy3)2] has been investigated.  

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