In order to study the nature of π-effects on phosphinoalkynes, we have carried out a combined experimental and theoretical study on [(X)A2PC≡CR]+ systems where X = H, CH3, Fp (Fp = CpFe(CO)2) and A = H, Ph. The P-coordinated (diphenylphosphino)alkyne metal complexes [(Fp)Ph2PC≡CR][BF4] have been prepared and characterized by microanalysis and IR, 1H, 13C, and 31P NMR spectroscopy. The crystal structure of [(Fp)Ph2PC≡CPh]+ has been determined by X-ray diffraction. This compound crystallizes in the triclinic space group P-1 with unit cell parameters a = 9.777(1) Å, b = 10.351(1) Å, c = 12.857(5) Å, α = 92.38(2)°, β = 103.07(2)°, γ = 100.02(1)°, Dc= 1.469 g cm-3, Z = 2. Least-squares refinement using all 4068 independent reflections led to a final R value of 0.049 (all data). Ab initio calculations with geometry optimization have been performed in related model systems. A natural population and natural bond orbital analysis of the wave functions has been performed. The experimental difference between the 13C NMR chemical shifts of acetylenic carbons and the calculated difference between NPA atomic charges is linearly correlated. A π-electron transfer from the filled π (C≡C) orbitals to the empty phosphorusd orbitals has not been observed. When X = H, CH3 a strong polarization of the π(C≡C) bond is detected, when X = Fp the polarization is reduced and π-back-donation from metald orbitals to the empty σ*P-A orbitals is found. © 1995, American Chemical Society. All rights reserved.