DFT Insights on Ligand Photodissociation Pathways in Ruthenium–Terpyridine Complexes: ³ MLCT- or ³ MC-Triggered?

We computationally elucidated the mechanism of acetonitrile (ACN) photodissociation in Ru-terpyridine PACT complexes from Turro's series, [Ru(tpy)(L)(ACN)]n+, where tpy = 2,2':6',2″-terpyridine and L = 4,4'-dimethyl-2,2'-dipyridylbipyridine (1), acetylacetonate (2), or 1,3-diphenylpropane-1,3-dione (3). DFT reproduced the experimental ³MLCT-³MC energy-gap trends and revealed an atypical dissociative ³MC minimum dominated by tpy distortion rather than pronounced Ru-ACN elongation. From this distorted ³MC state, we located a transition structure for ACN loss leading to a pentacoordinate intermediate (³RuP), demonstrating that photosubstitution can proceed without a canonical elongated-bond ³MC geometry. For 1, the ³MC → ³RuP dissociation barrier exceeded the ³MLCT → ³MC internal-conversion barrier, whereas for 2, the two processes were competitive, rationalizing experimental photodissociation quantum yields. Relaxed scans suggest that both ³MC- and ³MLCT-mediated pathways for ACN loss are plausible and competitive across the series. The strongest support for ³MLCT-mediated dissociation is found for complex 1, where a transition state has been localized. Finally, sterically tuned tpy derivatives bearing ortho- or meta-methyl substituents (4, 5) biased ³MLCT-³MC conversion by stabilizing the distorted ³MC state while leaving the ACN-loss barrier largely unchanged, providing a design strategy to tune photosubstitution.