Abstract The reaction between [Mn(CO) 5Br] and di-2-pyridylketone- p-nitrophenylhydrazone (dpknph) in diethyl ether under ultrasonic conditions gave fac-[Mn(CO) 3(dpknph)Br] in good yield. Optical and thermodynamic measurements on fac-[Mn(CO) 3(dpknph)Br] in non-aqueous polar solvents revealed reversible interconversion between two intense charge transfer absorption bands due to π–π* (dpk), followed by dpk → nitro intraligand charge transfer transition (ILCT), mixed with metal ligand charge transfer transition (MLCT) due to d π ( Mn ) → π ( dpk ) ∗ . In non-polar solvents, a single absorption band appeared. Extinction coefficients of 46 200 ± 2000 and 28 400 ± 2000 M −1 cm −1 were calculated in DMSO for the low- and high-energy electronic states of fac-[Mn(CO) 3(dpknph)Br] using excess NaBF 4. Changes in enthalpy (Δ H ø) of +14.0 and −12.1 kJ mol −1, entropy (Δ S ø) of +28.65 and −64.30 J mol −1 K −1, and free energy (Δ G ø) of +5.48 and +7.08 kJ mol −1 at 298 K were calculated for the interconversion between the high and low energy electronic states of fac-[Mn(CO) 3(dpknph)Br]. These results allow for the use of these systems ( fac-[Mn(CO) 3(dpknph)Br] and surrounding solvent or solute molecules) as optical sensors for a variety of physical and chemical stimuli that include metal ions. Group 12 metal ions in concentrations as low as 1.00 × 10 −9 M can be detected and determined using fac-[Mn(CO) 3(dpknph)Br] in dmso in the presence and absence of NaBH 4.