MULTIPLE BONDING, PI-BONDING CONTRIBUTIONS AND AROMATIC CHARACTER IN ISOELECTRONIC BORON-PHOSPHORUS, BORON-ARSENIC, ALUMINUM-NITROGEN AND ZINC-SULFUR COMPOUNDS

Variable temperature H-1 NMR studies of various boron-phosphorus (B-P) and boron-arsenic (B-As) compounds have afforded data on the strength of B-P and B-As pi-bonds. For [RPBMes2]- and [RAsBMes2]- ions the energy of the pi-bond can be as high as 24 kcal mol-1 and 21 kcal mol-1, respectively. Inversion barriers near 10 kcal mol-1 were determined for the pnictide center in simple monomeric phosphino or arsino boranes. The effects of substituent size and type on the B-P or B-As multiple bonds were also investigated. Compounds with extended linear arrays of up to five boron or phosphorus atoms were also synthesized and characterized both structurally and spectroscopically. In addition, the aromaticity of six-membered rings comprised of (B-P)3 arrays was investigated. Structural spectroscopic and theoretical data support the view that the unsaturated B3P3 ring system has greater aromatic character than borazine. The range of quasi aromatic rings was also extended to include the isoelectronic and isolobal rings comprised of the arrays (Al-N)3 (alumazene) (:GeN)3, and (ZnS)3. The bonding and the relative aromatic character of these rings is also discussed.