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Luminescence lifetimes of Ruthenium (II) polypyridine compounds is thought tobe controlled by the barrier to conversion of triplet metal-ligand chargetransfer ($^{3}$MLCT) state to a non-luminescent triplet metal-centered($^{3}$MC) state. This work builds on earlier work [J. Photochem. Photobiol. A276, 8 (2014)] and [J. Photochem. Photobiol. A 348, 305 (2017)] that derivedseveral orbital-based luminescence indices of which the third (LI3) was basedupon frontier-molecular-orbital-like ideas and correlated linearly with valuesof $E_{ave}$. $E_{ave}$ is a large underestimate of the true$^3$MLCT$\rightarrow$ $^3$MC TS barrier height in the case of the trisbipyridine ruthenium(II) cation {[Ru(bpy)$_3]^{2+}$}, but accurate TS barrierheights are difficult to obtain experimentally, it was judged useful to verifythe ideas used to derive the LI3 index by calculating the energetics of thegas-phase $^{3}$MLCT $\rightarrow$ $^{3}$MC reaction for four complexes$\{$[Ru(N$^\wedge$N)$_3$]$^{2+}$ with N$^\wedge$N = bpy ({6}), 4,4'-dm-bpy({70}), 4,4'-dph-bpy ({73}), and 4,4'-DTB-bpy ({74}) $\}$ using the samedensity functional and basis sets used in calculating LI3. We examine the transdissociation mechanism in detail at the B3LYP/6-31G+LANLDZ(Ru) level anduncover a two-part mechanism. In the first part, the electron is transferred toa single ligand rather than symmetrically to all three ligands. It is the twoRu-N bonds to this ligand which are equally elongated in the transition state.The intrinsic reaction coordinate then continues down a ridge in hyperspace andbifurcates into one of two symmetry-equivalent $^3$MC structures with elongatedtrans bonds. Interestingly, no significant difference is found for the TSbarriers for the four complexes treated here. Instead, LI3 is linearlycorrelated with the energy difference $\Delta$ E = E($^{3}$MLCT) - E($^{3}$MC).

Test of the Orbital-Based LI3 Index as a Predictor of the Height of the  $^3$MLCT $\rightarrow$ $^3$MC Transition-State Barrier for Gas-Phase  [Ru(N$^\wedge$N)$_3$]$^{2+}$ Polypyridine Complexes