A number of novel phosphinohydrazines, rPr(2)P-NPh-NPh-H (1), iPr(2)P-NH-NH-PiPr(2) (2), iPr(2)P-NMe-NH-PiPr(2) (3), and H-NMe-NH-PiPr2 (4), were prepared and characterized. The interaction of 1 with 1 equiv of n-BuLi afforded a complex compound [Li(DME)(3)][Li{(NPh-NPh-PiPr(2))-kappa N-2](5). The reaction of 5 with NiBr2 resulted in the formation of the first stable transition metal phosphinohydrazide [Ni{(NPh-NPh-PiPr(2))-kappa(2) N,P}(2)] (6). Similarly, the cobalt(II) derivative [Co{(NPh-NPh-PiPr(2))-kappa N-2,P}(2)] (7) was. prepared by the reaction of 1 with Co[N(SiMe3)(2)](2). n X-ray study reveals formation of the complexes containing elongated N-N bonds (1.443(1), 1.466(2), and 1.470(2) A for 5, 6, and 7, respectively) as compared with the starting material 1 (1.407(1) angstrom). Nickel phosphinohydrazide 6 has a square-planar cis configuration; the cobalt complex 7 possesses a square-planar centrosymmetric trans configuration. The half-sandwich nickel(II) complex [CpNi{(NPh-NPh-PiPr(2))-kappa N-2,P}] (8) was prepared by prolonged heating of phosphinohydrazine, 1 with NiCp2 in toluene. The lithiation of 3 with n-BuLi resulted in the formation of an iminophosphoranate [LiN=PiPr(2)-NMe-PiPr(2)] (13) (in situ), which is the product of insertion of a PiPr(2) group into the nitrogen-nitrogen bond. The hydrolysis of 13 followed by the addition Of CoCl2 gave the phosphino-iminophosphoranato complex [CoCl2{(HN=PiPr(2)-NMe-PiPr(2))-kappa N-2,P}] (15) according to X-ray investigation. The phosphinohydrazine 3 reacted with FeX2 in toluene to form adducts (1:1) [FeX2((PiPr(2)-NMe-NH-PiPr(2))-kappa P-2,P'] (X = Cl (9), Br (10)), while CoCl2 gave the complex salt [(Co(PiPr(2)-NMe-NH-PiPr(2))-kappa P-2,P')(2)(mu-Cl)(3)][CoCl3(THF)] (11). A THF solution of complex 11 shows thermochromic behavior.