The nature of the heteroatom X incorporated in the five-membered PXP-diphosphine bridging chain was found to play a primary unit role both in the overall stability and in the stereochemical arrangement of nitrido-containing [M(N)(PXP)](2+) metal fragments (M = Tc, Re). Thus, by mixing PXP ligands with labile [Re(N)Cl-4](-) and Tc(N)Cl-2-(PPh3)2 nitrido precursors in CH2Cl2/MeOH mixtures, a series of neutral M(N)Cl-2(PXP) complexes (M = Tc, 1-5; M = Re, 8, 9) was collected. In the resulting distorted octahedrons, PXP adopted facial or meridional coordination, and combination with halide co-ligands produced three different stereochemical arrangements, that is, fac,cis, mercis, and mer,trans, depending primarily on the nature of the diphosphine heteroatom X. When X = NH, mer,cis-Tc(N)Cl-2(PNP1), 1, was the only isomer formed. Alternatively, when a tertiary amine nitrogen (X = NR; R = CH3, CH2CH2OCH3) was introduced in the bridging chain, fac,cis-M(N)Cl-2(PN(R)P) complexes (M = Tc, 2, 3; M = Re, 8f) were obtained. Isomerization into the mer,cis-Re(N)Cl-2(PN(R)P), 8m, species was observed only in the case of rhenium when the tertiary amine group carried the less encumbering methyl substituent. fac,cis-Tc(N)Cl-2(PSP), 4f, was isolated in the solid state when X = S, but a mixture of fac,cis-Tc(N)Cl-2(PSP) and mertrans-Tc(N)Cl-2(PSP), 4m, isomers was found in equilibrium in the solution state. A similar equilibrium between fac,cis-M(N)Cl-2(POP) (M = Tc, 5f; M = Re, 9f) and mer,trans-M(N)Cl-2(POP) (M = Tc, 5m; M = Re, 9m) species was detected in POP-containing complexes. The molecular structure of all of these complexes was assessed by means of conventional physicochemical techniques including multinuclear NMR spectroscopy and X-ray diffraction analysis of representative mer,cis-Tc(N)Cl-2(PN(H)P), 1, fac,cis-Tc(N)Cl-2(PSP), 4f, and mer, cis- Re(N)Cl-2(PN(Me) P), 8m, compounds.