STRUCTURAL EVIDENCE FOR P(PI)-P(PI) CONJUGATION IN AMINOSUBSTITUTED PHOSPHAALKYNES

The molecular structure of 2,2,6,6-tetramethylpiperindinophosphaalkyne was determined by the X-ray structural method. The main geometrical p arameters are as follows: P=C 1.559(2), N-C(sp) 1.316(2) angstrom, P=C -N 178.9(1)-degrees, with an almost planar trigonal bond configuration for the N atom and the chair conformation of the piperidine ring. Str uctural evidence for the nitrogen lone pair conjugation with the pi-sy stem of the triple bond was found to be different in phosphaalkynes P= C-NR2 and nitriles N=C-NR2. Quantum-chemical ab initio calculations (H F/6-31G) showed that this is caused by a different character of polar ization of the P=C and N=C triple bonds.