Chemistry of the strong electrophilic metal fragment [(99)Tc(N)(PXP)](2+) (PXP = diphosphine ligand). A novel tool for the selective labeling of small molecules.

Monosubstituted [M(N)Cl(2)(POP)] [M = Tc, 1; Re, 2] and [M(N)Cl(2)(PNP)] [M = Tc, 3; Re, 4] complexes were prepared by reaction of the precursors [M(N)Cl(4)](-) and [M(N)Cl(2)(PPh(3))(2)] (M = Tc, Re) with the diphosphine ligands bis(2-diphenylphosphinoethyl)ether (POP) and bis(2-diphenylphosphinoethyl)methoxyethylamine (PNP) in refluxing dichloromethane/methanol solutions. In these compounds, the diphosphine acted as a chelating ligand bound to the metal center through the two phosphorus atoms. Considering also the weak interaction of the heteroatom (N or O) located in the middle of the carbon backbone connecting the two P atoms, we found that the coordination arrangement of the diphosphine ligand could be viewed as either meridional (m) or facial (f), and the resulting geometry as pseudooctahedral.