Electrophilic attack with methyl trifluoromethanesulfonate or tetrafluoroboric acid, to new alkynyl rhodium complexes containing alkenylphosphanes, lead's to butenynyl coupling products or to the unprecedented rhodaphosphacycle complexes [Rh(eta(5)-C5Me5){kappa(4)-P,C,C,C)-iPr(2)PCH(2)C (=CH2)C(CH2R)C=C(R)}] [BF4] (R = Ph (11a), p-tol (11b)). These complexes 11a,b can be explained as a result of the coupling of three organic fragments in the molecule, the alkynyl, the vinylidene, generated in situ by reaction with HBF4 (A), and the C C double bond from the alkenylphosphane. DFT computational Studies on the formation of complex 11a suggest the [2 + 2] intramolecular cycloaddition between the double bond of the allylphosphane and the C alpha-C beta of the vinylidene A as the most plausible pathway for this reaction.