Metal carbonyls Pentacarbonyl chromium

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. 

The Group VI metals would not be expected to form binary metal carbonyl cations, but they do form some substituted cations with nitrogen and phosphorus ligands. The paramagnetic monomeric and dimeric chromium pentacarbonyl iodides react in liquid ammonia with iodide expulsion. 

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. 

Such aspects of metal carbonyl structure may be explained by consideration of the coordination number of the central metal atom as an important factor in determining the stability of metal carbonyls. As is the case with other transition metal derivatives such as the ammines, octahedral hexa-coordinate metal carbonyl derivatives seem to be especially favored. Thus, hexacoordinate chromium hexacarbonyl is obviously more stable and less reactive than pentacoordinate iron pentacarbonyl or tetracoordinate nickel tetracarbonyl. Moreover, hexacoordinate methylmanganese pentacarbonyl is indefinitely stable at room temperature (93) whereas pentacoordinate methylcobalt tetracarbonyl (55) rapidly decomposes at room temperature and heptacoordinate methylvanadium hexacarbonyl has never been reported, despite the availability of obvious starting materials for its preparation. 

In 1962 and 1963 a pair of papers by Stolz, Dobson and Sheline [23,24] looked at the IR spectra of the then suspected pentacarbonyl intermediates of the group 6 metals chromium, molybdenum and tungsten. These papers provided early support for the idea that one CO ligand is initially lost in the first step of the photoreactions of these carbonyls. Analysis of the CO stretching vibrations managed to rule out other possible species such as the W2(CO)xq anion and the W(CO)5 anion. Analogous results were found for the pentacarbonyls of chromium and molybdenum. The chromium result is particularly relevant for the photochemistry discussed later. 

Conjugate reduction of enones. The alkali metal carbonylchromates reduce a, -unsaturated carbonyl compounds to the corresponding saturated carbonyl compounds in 4d-807o yield. They are comparable to potassium hydridotetra-carbonylferrate (6, 483-486), but are simpler to prepare because chromium hexacarbonyl is a stable solid and less toxic than iron pentacarbonyl. Examples ... 

Chromium carbenes can also be prepared by the so-called Semmelhack-Hegedus route. Chromium hexacarbonyl is first reduced to a nucleophilic pentacarbonyl dichromate dianion 23 with sodium naphthalenide or potassium carbide. Reaction of this dianion species with an acid chloride gives a metal alkoxide that can be quenched with an electrophile to provide the desired chromate ester 24. Alternatively, the dianion can be added to an amide carbonyl to give a tetrahedral intermediated which collapses to the chromate amide 25 on treatment with trimethylsilyl chloride. 

Diazonium salts also readily react with nickel carbonyl, yielding mainly carboxylic acids and ketones in the presence of water and hydrochloric add (26, 27). Iron pentacarbonyl and dicobalt octacarbonyl with diazonium salts behave similarly, but the hexacarbonyls of chromium and molybdenum are virtually ineffective. This reaction may be considered as a transition metal-catalyzed carbonylation of aryl radicals, and is closely related to the Meer-wein reaction (26). 

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