Chromium hexacarbonyl determination

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. 

Figure 2 A breakdown diagram of chromium hexacarbonyl ions obtained by PEPICO. This ion dissociates by a sequential mechanism. The heats of formation of the various ions can be determined from the energy onsets of their formation. Reproduced with permission from Das PR, Nishimura T and Meisels GG (1985) Fragmentation of energy selected hexacarbonylchromium ion. Journal of Physical Chemistry S9 2808-2812.

Chromium hexacarbonyl reacts with 1,6-methanocyclodecapentaene forming the complex, 5.12h [97h], whose crystal structure has been determined by X-ray diffraction [97c]. In this molecule the chromium lies below one side of the clerical hat configuration of the olefin. In order that the chromium achieves the 18-electron configuration it is necessary to postulate that it is bonded, albeit weakly to the C, and C, carbons. 

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