Pentamethylcyclopentadienylruthenium(II) half-sandwich complexes containing the alkenyl phosphine ligand allyldiphenylphosphine (ADPP) are described. ADPP reacts with [(eta(5)-C5Me5)RuCl2](2) to give the disubstituted ruthenium(II) compound [(eta(5)-C5Me5)Ru(eta(3)-ADPP)(eta(1)-ADPP)] [PF6] (1). The dynamic behavior of compound 1 has been studied by variable temperature H-1 and P-31{H-1} NMR spectroscopy. The hemilabile properties of the novel eta(3)-ADPP ligand, a neutral monometallic homophosphaallyl, are illustrated by reactions of compound 1 with sodium thiocyanate, carbon monoxide, and terminal alkynes. Compound 1 reacts with NaNCS, CO, HC=C (C6H5), HC=CCH2OH, and HC=CCH2CH2OH to form [(eta(5)-C5Me5)Ru(eta(1)-ADPP)(2)(NCs)] (2), [(eta(5)-C5Me5)Ru-(eta(1)-ADPP)(2)(CO)][PF6] (3), [(eta(5)-C5Me5)Ru(eta(1)-ADPP)(2){C=C(H)(C6H5)}][PF6] (4), unstable [(eta(5)-C5Me5)Ru(eta(1)-ADPP)2{ C=C(H)(CH2OH)}][PF6] (5), and [(eta(5)-C5Me5)Ru(eta(3)-ADPP){C(CH2)(3)O}][PF6] (6), respectively. Compound 6 is unusual because its formation is accompanied by the loss of one ADPP ligand, unlike the other substitution products 2-5. Compound 6 is dynamic and readily epimerizes in solution. The characteristic H-1, C-13{H-1}, and P-31{H-1} NMR spectroscopic features of all compounds are described. Crystal data for 6 : monoclinic space group P2(1)/n, a = 9.711(1) Angstrom, b = 12.418(2) Angstrom, c = 24.363(7) Angstrom, beta = 99.60(2)degrees, V = 2896.9(10) Angstrom(3) Z = 4, D-c = 1.554 g cm(-3), R = 0.0336.