SYNTHESIS AND STRUCTURE OF DIARYLBORYL-SUBSTITUTED HYDRAZINES AND DIPHOSPHANES - ROLE OF SIGMA-ORBITAL HYBRIDIZATION AND PI-ORBITAL OVERLAP IN N-N AND P-P MULTIPLE-BOND LENGTHS

The synthesis and spectroscopic and structural characterization of several substituted diphosphanes and hydrazines have been carried out in order to estimate the relative importance of rehybridization and pi-bonding in N-N and P-P double-bond lengths. The presence of the -BMes2 (Mes = 2,4,6-Me3C6H2) group in the compounds Mes2B(1-Ad)PPPh2 (1) (1-Ad = 1-adamantyl), [Mes2B(1-Ad)P]2 (2), and [Mes2B(Mes)P]2 (3) induces essential planarity at the boryl-substituted phosphorus atoms and dramatic changes in the P-P bond lengths. Normally, single P-P bonds are about 2.22 angstrom long, as is observed in [Mes(I)P]2 (4), whereas in 1 the P-P bond length is near 2.17 angstrom and in 2 and 3 it is near 2.11 angstrom. The latter distance is about halfway between those of normal single and double P-P bonds. The use of the -BMes2 group as a substituent on hydrazines also results in planarity at the boryl-substituted nitrogen and in the shortening of the N-N bonds. In this case, however, the percentage change in the N-N bond length is not as great as that seen in the phosphorus compounds. For example, the N-N bond length in Mes2B(Ph) NN(H)Ph (5) is near 1.44 angstrom and in Mes2B(H)NN(H)BMes2 (6) it is 1.41 angstrom whereas in hydrazine the N-N distance is 1.46 angstrom (cf. N-N double bond almost-equal-to 1.25 angstrom). Oddly, the N-N bond length in 1,2-diphenylhydrazine (7) is 1.39 angstrom. In this case, however, the nitrogen is almost planar and the N-C bond is short. The larger changes observed in the diphosphanes have been interpreted on the basis of the relative weakness of the P-P pi-bond and the fact that the hybridization changes in phosphorus are significantly greater than those observed for nitrogen. Similarly, comparison of the data for 1-7 with the normal bond lengths for P-P and N-N double bonds, which are 2.02 and 1.25 angstrom long, respectively, leads to the conclusion that p-p pi-bonds account for only about half of the shortening in P-P double bonds whereas they account for approximately 70-75% of the contraction in the case of N-N double bonds.