The synthesis and characterization of two 1,2-cyclopentyl-bridged diiminophosphine proligands, (CY5)[NPN]H-DMP(2) (CY5 = cyclopentylidene; DMP = 2,6-Me2C6H3) and (CY5)[NPN]H-DIPP(2) (DIPP = 2,6-(Pr2C6H3)-Pr-i), are presented, and tautomerization to the corresponding 1,2-cyclopentenyl-bridged enamineimine phosphine precursors is reported. These two new proligands are obtained by deprotonation of N-DMP- or N-DIPP-cydopentylideneimine (N-DMP, 2,6-dimethylphenyl; N-DIPP, 2,6-diisopropylphenyl) and the subsequent addition of 0.5 equiv of dichlorophenylphosphine. Each ligand precursor exists as a mixture of isomers that consist of the diimine, enamineimine, and dienamine tautomers and corresponding stereoisomers, each of which could be identified. The bis(dimethilamido)zirconium complexes (CY5)[NPN]Zr-DMP(NMe2)(2) and (CY5)[NPN](PZr)-P-DIP(NMe2)(2) were prepared directly from the neutral proligands and Zr(NMe2)(4) via protonolysis. Exchange of the dimethylamido ligands in the latter complexes for chlorides and iodides takes place upon reaction with excess Me3SiCl and Me3SiI, respectively. A dinuclear zirconium-dinitrogen complex, {(CY5)[NPN]Zr-DMP(THP)}(2)(mu-eta(2):eta(2)-N-2), was obtained via KC8 reduction of (CY5)[NPN](ZrCl2)-Zr-DMP under 4 atm of N-2. On the basis of single-crystal X-ray analysis, N-2 has been reduced to a side-on-bound hydrazido (mu-eta(2):eta(2)-N-2(4-)) unit. This dinitrogen complex is thermally unstable and decomposes in solution.