EXPERIMENTAL AND THEORETICAL-STUDY OF PI-EFFECTS IN P-COORDINATED (DIPHENYLPHOSPHINO)ALKYNES

In order to study the nature of pi-effects on phosphinoalkynes, we hav e carried out a combined experimental and theoretical study on [(X)A(2 )PC=CR](+) systems where X = H, CH3, Fp (Fp = CpFe(CO)(2)) and A = H, Ph. The P-coordinated (diphenylphosphino)alkyne metal complexes [(Fp)P h(2)PC=CR][BF4] have been prepared and characterized by microanalysis and IR, H-1, C-13, and P-31 NMR spectroscopy. The crystal structure of [(Fp)Ph(2)PC=CPh](+) has been determined by X-ray diffraction. This c ompound crystallizes in the triclinic space group P-1 with unit cell p arameters a = 9.777(1) Angstrom, b = 10.351(1) Angstrom, 12.857(5) Ang strom, alpha = 92.38(2)degrees, beta = 103.07(2)degrees, gamma = 100.0 2(1)degrees, D-c = 1.469 g cm(-3), Z = 2. Least-squares refinement usi ng all 4068 independent reflections led to a final R value of 0.049 (a ll data), Ab initio calculations with geometry optimization have been performed in related model systems. A natural population and natural b ond orbital analysis of the wave functions has been performed. The exp erimental difference between the C-13 NMR chemical shifts of acetyleni c carbons and the calculated difference between NPA. atomic charges is linearly correlated. A pi-electron transfer from the filled pi (C=C) orbitals to the empty phosphorus d orbitals has not been observed. Whe n X = H, CH3 a strong polarization of the pi(C=C) bond is detected, wh en X = Fp the polarization is reduced and pi-back-donation from metal d orbitals to the empty sigma(P-A) orbitals is found.