BORON-PHOSPHORUS COMPOUNDS AND MULTIPLE BONDING

Boron‐phosphorus compounds have not been as thoroughly studied as their boron‐nitrogen counterparts. Until recently many classes of B‐P compounds that had been well established in B‐N chemistry were either unknown or poorly characterized. This statement is particularly true for compounds involving possible multiple bonding between boron and phosphorus. For example, detailed structural information on simple monomeric phosphino‐boranes, R2BPR′2, did not become available until 1986 even though the isoelectronic SiC double bonded species, the silenes, had already been reported. However, new work has shown that it is possible to prepare and characterize several novel types of boron‐phosphorus compounds with varying degrees of multiple B—P bonding. These include not only monomeric phosphinoboranes but also phosphanediylborates (borylphosphides), three‐ and four‐membered rings (diphosphadi‐boretanes), boron phosphorus analogues of borazine, B‐P skeletal analogues of allyl cations and anions, butadiene and cage compounds. Structural, spectroscopic (mainly NMR) and theoretical studies reveal some important differences between B‐P and B‐N compounds which in many cases can be traced to the presence of a high inversion barrier at phosphorus that reduces the π interaction. This usually causes compounds such as R2BPR′2 to associate through σ bonding between B and P. Supporting evidence for this view comes from species that involve phosphorus and nitrogen in competitive π bonding with a boron p orbital in which the dative interaction between B and N is dominant and the phosphorus center remains pyramidal. Recently published work has shown that steric and electronic factors can be used to favor π bonding and give an approximately planar system. Furthermore, theoretical studies reveal that pp π overlap in a planar B‐P system is of similar efficiency to its B‐N analogue. Good examples are seen in the phosphanediyl borates, the boron‐phosphorus analogues of borazine and the π‐allyl cations, whose molecular configurations and B—P bond lengths support strong boron—phosphorus π bonding. Copyright © 1990 by VCH Verlagsgesellschaft mbH, Germany