Related Compounds of Vanadium, Niobium, and Tantalum

Recent investigations directed toward the synthesis of homoleptic vanadium alkyls (10) have not been successful for VR5 species, and only 

and VR3O compounds have been obtained (38), even when the promising novel ligand systems R = (CH3)3SiCH2, (CH3)3CCH2, or n(CH3) 3Si]2CH were employed. 

It might have been expected that the reactions could lead to vanadium ylides instead of the pentaalkyls, but this was obviously not the case. From the literature it is not clear whether or not true vanadium (Y) compounds have been used as starting materials in at least some of the reactions to warrant the appropriate oxidation state. The problems associated with the vanadium pentahalides (except for the fluoride) and other VX6 compounds may well be the reason for the difficulties arising in the syntheses. If these difficulties can be overcome, there should be a chance for a successful preparative procedure according to one of the following pathways  

Considering the presently available knowledge on vanadium(V) compounds, reaction (58b) seems to be more attractive, as compounds of types (RO)3VO, (RO)3V=NR, etc., are well documented for both R = alkyl and R = silyl (4, 81), whereas only very few VX6 species are known. The pertinent chemistry of the congeners niobium and tantalum offers examples for pentacoordination as well as for ylide formation. 

It was only in a 1974 report (83) that some evidence was provided for the possible existence of pentamethylniobium through the isolation of its addition compounds with a ditertiary phosphine and with methyllithium. These compounds were synthesized according to the following scheme  

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