The reaction between monosubstituted hydrazines and Ru(II) vinylidene complexes [RuCl(P∩N)2-(=C=CH(R))]BPh4 (where R = Ph, n-Bu and P∩N = 3,5-dimethyl[1-((2-diphenylphosphino)ethyl)]pyrazole (Me 2PyP)) afforded metallacyclobutene derivatives of the general formula [RuCl(κ1P-Me2PyP)(κ2P,N-Me 2PyP)(κ2N,C-NH2N′(R 1)C(CH2R2))]BPh4 (where R 1, R2 = Me, Ph, n-Bu). The new metallacyclic ruthenium complexes contain a four-membered ring (-Ru=C-N-N-) and one pendant P,N-donor ligand, Me2PyP, bound through the P-donor atom. The solid-state and solution-state structures of these complexes were confirmed through X-ray crystallographic analysis and extensive 2D NMR spectroscopic studies. The reactivity of [RuCl(Me2PyP)2(=C=CH(R))]BPh4 toward amines and disubstituted hydrazines was also investigated. The Me 2PyP ligand exhibited hemilabile behavior on reaction of [RuCl(Me2PyP)2-(=C=CH(R))]BPh4 with aniline. The mechanism of the formation of the four-membered metallacycle was investigated, and it was found that the formation of N-C bonds from hydrazines and vinylidene was only possible when facilitated by displacement of the N-donor atom of the P,N-donor ligand Me2PyP by the -NH2 group of the monosubstituted hydrazines.