Spectroscopic and Computational Investigations of the Temperature-Dependent Emission Behavior of [Re(CNx)<sub>5</sub>Cl] and [Re(CNx)<sub>6</sub>]<sup>+</sup> Complexes

[Re(CNx)<sub>5</sub>Cl] and [Re(CNx)<sub>6</sub>](PF<sub>6</sub>) complexes form with the isocyanide ligand, 2,6-dimethylphenylisocyanide (CNx). [Re(CNx)<sub>5</sub>Cl] crystallizes in the space group <i>P</i>(2)1/<i>c</i> with a Re−Cl bond length of 2.5278(16) Å, a Re−C bond length <i>trans</i> to Cl of 1.937(6) Å, and Re−C bond lengths in the equatorial plane ranging from 1.998(6) to 2.034(6) Å. The complexes are highly emissive at 77 K with emission lifetimes of 5.1 and 4.6 μs, respectively, but are nonemissive at room temperature. Density functional theory (DFT) geometry optimizations of the ground and the triplet metal-ligand-to-ligand charge transfer (<sup>3</sup>MLLCT) states of the complexes in the gas phase place the energy of the <sup>3</sup>MLLCT state for [Re(CNx)<sub>5</sub>Cl] 200 cm<sup>-1</sup> higher than the experimental emission peak at 77 K. The energies of the singlet excited states of the two complexes calculated in ethanol using time-dependent density functional theory (TDDFT) and conductor-like polarizable continuum model (CPCM) deviate by less than 600 cm<sup>-1</sup> from the corresponding UV−vis peaks in the same solvent. The TDDFT/CPCM calculation confirms the existence of <sup>3</sup>d<i>−</i>d states that provide a nonradiative relaxation pathway accounting for loss of emission from the emitting state at room temperature. Calculations also reveal a singlet to triplet excitation at 35 700 cm<sup>-1</sup> near the absorption of the CNx ligand at 35 200 cm<sup>-1</sup> and a <sup>3</sup>π→π* state of CNx 200 cm<sup>-1</sup> higher than the experimental phosphorescence energy.