Comparisons With Heavier Group 6 Organometallics
The heavier group 6 elements molybdenum and tungsten form organometallic compounds similar to those for chromium but also with differences. Whereas Cr(III) aquo alkyl compounds are well studied, the corresponding Mo(III) and W(III) compounds are not. Whereas chromocene is a stable compound, the related molybdenocene and tungstenocene are highly reactive. On the other hand, Mo and W readily form derivatives of the type Cp2MX2, whereas the smaller Cr does not form such clamshell compounds. Homoleptic alkyl and aryl complexes of the type R4M are rare, and hexamethyltungsten has no analogue in Cr chemistry.
Similar are the carbonyls such as molybdenum hexacarbonyl and tungsten hexacarbonyl and the related carbene and carbyne complexes. Compounds of the type 2 are known for all three metals, e.g. Cyclopentadienylmolybdenum tricarbonyl dimer. The chromium compound is however prone to homolysis of the Cr-Cr bond owing to steric crowding.
In the Kauffmann olefination, molybdenum(III) chloride and methyllithium form an organometallic complex capable of carbonyl olefination.
Read more about this topic: Organochromium Chemistry
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