Pentaalkyls and Alkylidene Trialkyls of the Group V Elements

The chapter focuses on the pentaalkyls and alkyidene trialkyls of the group V elements. The revived interest in this area led to new efforts for improved methods of synthesis for pure alkylidene trialkyls (ylides), for the detection and possible characterization of a pentaalkylphosphorane species, and for a better understanding of the state of structure and bonding in ylides and pentaalkyls. This recent work is summarized in this chapter with the main focus directed to some of the very simple compounds and their silicon derivatives, which have turned out to be of special significance. These derivatives have not only provided new synthetic pathways but also have shown improved thermal stability (as in the case of arsenic ylides) and a modified pattern of chemical reactivity. The broad scope of phosphorus ylides and pentaorganophosphorane chemistry was covered in the leading multivolume series on organophosphorus chemistry. The recent success of efforts to synthesize pentaalkyls [Nb (CH,)₅ ] and even ylides ([ ( CH₃)₃ CCH₂]₃TaCHC( CH₃)₃ ) of the subgroup metals vanadium, niobium, and tantalum add a new dimension to the scope of this chapter. Alkylidene triorganophosphoranes (phosphorus ylides) are known with a large variety of substituents both at the phosphonium center and at the carbanionic function. The chapter also describes the biochemistry of arsenic ylides and pentaalkylarsoranes.