Chromium hexacarbonyl octahedral structure

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. 

The structural studies have suggested an octahedral geometry for chromium hexacarbonyl. The Ci C distance is found to be 1.92 A, while the C—O bond length is 1.16 A. The molecule is also found to be diamagnetic. 

The interpretation of the structures of the simpler carbonyls was also aided by infrared spectroscopy. Chromium, molybdenum, and tungsten form hexacarbonyls of the type M(CO)6. Electron-diffraction and x-ray studies have indicated a regular octahedral structure Infrared measurements have confirmed this. 

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. 

Chromium forms a white solid, hexacarbonyl, Cr(CO)6, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [Cr(CO)5I] . 

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