Metal carbonyls tungsten 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. 

Tungsten hexacarbonyl is used to produce tungsten coatings on base metals. This is done by deposition of the carbonyl on the metal surface, which decomposes to leave a tungsten coating. 

Tungsten hexacarbonyl is produced by heating tungsten metal with carbon monoxide at high pressure. Also, carbonyl can be prepared by reducing the tungsten hexachloride by heating with iron powder under carbon monoxide pressure. 

Most of the work is done with carbonyl complexes of group-VIA metals. Tungsten hexacarbonyl reacts at RT with methyllithium in solvents such as diethylether or THF almost quantitatively with addition of the carbanion to the carbon atom of a coordinated CO to give the lithium salt of acetylpentacarbonyl tungstate. This complex can be converted by aqueous tetramethylammonium bromide into the more stable tetramethyl-ammonium salt ... 

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. 

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. 

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) ... 

The simple metal hexacarbonyls of molybdenum and tungsten will catalyze allylic alkylation, with dramatic effects on the regioselectivity. The comparison of Mo(CO)6 with Pd° shows a remarkable contrast in regioselectivity (equation 67). The basis for the selectivities is somewhat mysterious, especially considering another dramatic reversal of selectivity with change in anion, from malonate, using Mo(CO)6 and the same conditions. This regioselectivity of Mo carbonyl catalysts has been used to synthesize (7 )-baclofen, as shown in equation (68). 

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