Scheme 2. Reaction Pathway of [Re(3,3′-DHBPY)(CO)₃Cl].

Involving facile chemical deprotonation of one hydroxyl group at 3,3′-DHBPY by addition of excess KOH to yield [Re(3,3′-DHBPY-H⁺)(CO)₃Cl]⁻ stabilized by intraligand hydrogen bonding. Exposure to the light of λ > 400 nm triggers the “roll-over” isomerization, resulting in the Re–N∩O^– metallacycle. Addition of HCl at any stage will reverse the process.