Facial vs. Meridional Coordination Modes in Re-I Tricarbonyl Complexes with a Carbodiphosphorane-based Tridentate Ligand

The reactivity of the novel cationic ligand precursor [(dppm)(2)CH](PF6) (1) with a weakly coordinating anion and [ReBr(CO)(5)] is reported. The initial coordination results in the formation of the facially coordinated dicationic complex fac-[({dppm}(2)CH)Re(CO)(3)]X-2 (fac-2, X=Br, PF6). The facial coordination mode is retained in fac-[({dppm}(2)C)Re(CO)(3)](PF6) (fac-3) upon deprotonation of the central cationic donor group. Quantum chemical investigations indicate that for both complexes, 2 and 3, the meridional coordination mode is thermodynamically favored. In line with these findings, the isomerization of the facially coordinated complex fac-3 to the meridionally coordinated complex mer-[({dppm}(2)C)Re(CO)(3)](PF6) (mer-3) is observed under irradiation with UV-light.