Knowledge Bank

The importance of substituent effects in determining the structure, stability, and reactivity of organic species has received considerable attention. Mononuclear boron compounds are also subject to substantial substituent effects but these influences have received less consideration. The recently synthesized two-and three-coordinite boron cations, along with the well-known neutral three-coordinate substituted borones, provide a useful series for comparison of substituent effects as a function of boron charge and coordination number. We have employed ab inito SCF calculations with the $6-31G\ast$ basis set to compute the structure of three separate molecular series: $R2B+,R2BH$, and $R2H(NH3)+$. for $R=H,CH3.NH2.OH$, and F. The selected R groups span a wide range of $\sigma$ doctor/acceptor abilities as well as $\pi$ donor strengths. The relative importance of these factors in determining the strength of the R - H bonds can be analyzed through variation in molecular structure, charge distribution, and energeties. A spectrum of bonding characteristics exists, with the two-coordinate cations dominated by ligand $\pi$ donor ability, three-coordinate neutrals preferring good $\pi$ acceptors, and the three-coordinate cations occupying a middle ground.