The reaction between di-2-pyridyl ketone 4-aminobenzoylhydrazone hydrate (dpk4abh.H 2 O) and [Re(CO) 5 Cl] in refluxing toluene gave fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl]. Hydrolysis of fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] to form fac-[Re(CO) 3 (j 3 -N,O,N-dpkO,OH)] occurs in wet solvents. The identity of fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] was established from the results of its elemental analysis, spectroscopic measurements, and electrochemical properties. The infrared spectra of fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] show the facial coordination of the carbonyl (C"O) groups, the coordination of dpkabh and none coordination of the carbonyl (C@O) group. 1 H-NMR measurements on fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] in d 6 -dmso and d 7 -dmf confirmed the coordination of dpkabh, and demonstrate sensitivity of fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] to its surroundings as manifested by the chemical shift variations observed in different solvents. Variable temperature 1 H-NMR studies showed stronger interaction between d 7 -dmf and fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] compared to d 6 -dmso and the amine protons of coordinated dpk4abh undergo faster exchanges with solvated water protons compared to the amide proton. The electronic absorption spectra of fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] showed significant solvent dependence. In CH 3 CN, a single intra-ligand charge transfer (ILCT) transition appeared at 332 nm and assigned to p ? p à of dpk followed by dpk ? hydrazone charge transfer. In a protophilic solvent, such as dmf, two intraligand charge transfer (ILCT) transitions assigned to fac-[Re(CO) 3 (j 2 -N,N-dpk4abh-H)Cl] and fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] appeared at 464 and 350 nm, respectively. Reversible interconversion between fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] and its conjugate base fac-[Re(CO) 3 (j 2 -N,N-dpk4abh-H)Cl] was established in protophilic solvents by using a base to shift the equilibrium to the conjugate base and an acid to shift the equilibrium or conjugate base to the neutral form. Acid-base titrations of fac-[Re(CO) 3 (j 2 -N,N-dpk4abh-H)Cl] or fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] show acids or bases in concentrations as low as 1x10 À6 M can be detected and determined using protophilic solutions of fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl]. Electrochemical measurements on fac-[Re(CO) 3 (j 2 -N,N-dpk4abh)Cl] in CH 3 CN and CH 2 Cl 2 showed sequential irreversible redox processes consistent with the electrochemical reduction or oxidation of the hydrazone backbone.