Metal carbonyls molybdenum hexacarbonyl

The mononuclear metal carbonyls contain only one metal atom, and they have comparatively simple structures. For example, nickel tetracarbonyl is tetrahedral. The pentacarbonyls of iron, ruthenium, and osmium are trigonal bipyramidal, whereas the hexacarbonyls of vanadium, chromium, molybdenum, and tungsten are octahedral. These structures are shown in Figure 21.1. 

In the course of a study of the formation of fluorophosphoranes from chloro-phosphines (22) we observed one exception in the compound chloromethyldi-chlorophosphine, which reacted smoothly with antimony trifluoride to give the flammable fluorophosphine, C1CH2PF2, under conditions where many other chloro-phosphines were invariably converted into fluorophosphoranes. As this fluorophosphine is readily available, its interaction with a metal carbonyl derivative was studied, and cycloheptatriene molybdenum tricarbonyl, obtained from the reaction of molybdenum hexacarbonyl with cycloheptatriene (I, 2), was chosen as a starting compound. 

The reaction, of metal carbonyls with 1,3-diketones generally results in a complete displacement of carbon monoxide accompanied by oxidation of the metal to yield 1,3-diketonato complexes. For example, iron pentacarbonyl, chromium hexa-carbonyl, and molybdenum hexacarbonyl afford FefCgHjOOs,1 Cr(CsHr02)8,2 and Mo(CgH702)s,2,s respectively, when allowed to react with 2,4-pentanedione. 

Aside from the carbonyl group, the Cp ligand and its analogs are probably the most common ligand systems in organometaUic chemistry. This is also reflected in the large number of cyclopentadienyl metal carbonyl dimers and their derivatives that have been studied. For molybdenum, direct reaction of molybdenum hexacarbonyl and dicyclopentadiene is one of several routes to [CpMo(CO)3]2 (equation 14). 

Tungsten or molybdenum hexacarbonyls were also found by us to be readily prepared from WClf, or M0CI5, respectively, with iron pentacar-bonyl. The yield was 75% with V(CO)g 210) and 30% with Mo(CO)5 211), higher pressures of CO being necessary in the latter case. As far as I know, our discoveries of 1939 were a starting point for the industrial preparation of metal carbonyls through the carbonylation of metal chlorides accomplished in various countries. 

Besides the conventional methods, the metallo-carbene route to access cyclic compounds has become a versatile tool in sugar chemistry. Synthesis of stavudine 112, an antiviral nucleoside, from an allyl alcohol [101] is realized by a Mo(CO)5-mediated cyclization reaction (O Scheme 26). Molybdenum hexacarbonyl smoothly reacts with the triple bond of 113 to generate the intermediate Mo-carbene, which undergoes a clean cyclorearrangement to yield the furanoid glycal 114. Alkynol isomerization is effected by group-6 transition metal carbonyl complexes [102]. 

Disulfides. The reaction of sulfonyl chlorides (alkyl or aryl) with molybdenum hexacarbonyl in anhydrous tetramethylurea at 70° (N2) provides a convenient synthesis of disulfides. The metal carbonyl does not function as a catalyst ... 

Molybdenum hexacarbonyl is a highly toxic substance. It is listed in Toxic Substance Control Act. Acute symptoms include headache, dizziness, nausea, vomiting, and fever. Because of its low vapor pressure, the risk of exposure of this compound, however, by inhalation is lower than that from the volatile tetra- or pentacoordinated metal carbonyls. Ingestion of the compound can cause death. Absorption of its solution through skin may cause severe poisoning, manifesting the same effects as those from other routes of exposure. 

Caution. Chromium, molybdenum and tungsten hexacarbonyls are volatile solids (Cr > Mo > W) and like all metal carbonyl compounds should be considered to be toxic. tert-Butylisocyanide has a pungent odor, and although many isocyanides are reported to exhibit no appreciable toxicity to mammals, it should still be handled with care. Carbon monoxide is evolved in these reactions and being an odorless, toxic gas, care should be exercised to carry out the reactions in an efficient ventilation hood with the apparatus venting into a well-ventilated region of the hood. 

The role of transition-metal carbonyls and particularly those of the Group 6 metals in homogeneous photocatalytic and catalytic processes is a matter of considerable interest [1]. UV irradiation especially provides a simple and convenient method for generation of thermally active co-ordinately unsaturated catalyst for alkenes or alkynes transformation. By using tungsten and molybdenum carbonyl compounds as catalysts, alkenes and alkynes can be metathesized, isomerised and polymerized. Photocatalytic isomerization of alkenes in the presence of molybdenum hexacarbonyl was observed by Wringhton thirty years ago [2]. Carbonyl complexes of molybdenum catalyze not only... 

It is apparent that if the Lewis base is charged rather than neutral, the substituted metal carbonyl will have the same charge as the Lewis base. In this manner certain anionic metal carbonyls can be synthesized by the displacement of carbonyl groups with anionic Lewis bases. Frequently used for this type of reaction are the halide ions and cyanide ion. Many of these reactions have been carried out on the relatively stable hexacarbonyls of chromium, molybdenum, and tungsten, their derivatives or other related... 

The hexacarbonyls of chromium, molybdenum, and tungsten react with a variety of Lewis bases to form very stable also hexacoordinate substitution products. If the Lewis base is anionic a metal carbonyl anion will result as discussed above. 

While metallocene-derived complexes 103 deserve interest mainly because of their redox properties and applications resulting therefrom, half-sandwich complexes 104 and 105 show more diverse chemical reactions. Based on their synthesis of the fulvalene dianion 102, Smart et al. [99] prepared some molybdenum complexes such as 108 [62% yield based on Na(DME)Cp] by treatment of 102 with molybdenum hexacarbonyl followed by oxidation with bromine. Vollhardt reported the synthesis of a variety of homo- and heterodinuclear fulvalene complexes such as 109-112 by treatment of dihydrofulvalene and respective metal carbonyls. The dichromium complex 110 has some catalytic activity in the hydrogenation of conjugated dienes [110]. The authors report the reaction with iron carbonyls being unsuccessful, whereas the reaction with dicobaltoctacarbonyl resulted in the formation of fulvalene complex 112 lacking a metal-metal bond in 80% yield (Scheme 10.38) [100, 101]. Later, the diiron complex corresponding to 109 was reported [111]. Hermann et al. and Cuenca etal. prepared zirconium(IV) and zirconium(III) fulvalene complexes [112,113]. 

The neutral complexes of chromium, molybdenum, tungsten, and vanadium are six-coordinate and the CO molecules are arranged about the metal in an octahedral configuration as shown in stmcture (3). Vanadium carbonyl possesses an unpaired electron and would be expected to form a metal—metal bond. Steric hindrance may prevent dimerization. The other hexacarbonyls are diamagnetic. 

The photochemical synthesis of the tungsten carbonyl metallocarborane [Eq. (18) ] is also effective in the preparation of the molybdenum carbonyl analog. The resulting air-sensitive complexes show chemical behavior similar to the cyclopentadienyl analogs, C5H5M(CO)3, in that they undergo protonation with anhydrous HOI and met.hylation with CH3I. They also react further with metal hexacarbonyls to afford bimetallic complexes 54) ... 

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