Excitation of mer-1, mer-2 and cisfac- [3] 3+ with a blue cw-laser (450 nm) in MeCN solution at room temperature reveals weak emission bands peaking at 1300, 1250 and 1550 nm, with full widths at half maximum (FWHM) of 980, 840 and ca. 1200 cm -1 , respectively (Figure , Table , Figure ). The Stokes shifts of 600-1000 cm -1 indicate a small but significant excited state distortion in all three complexes that is also reflected in the respective DFT-optimized geometries of optimized metal-centered doublet states (Figure , Table ). Hence, we assign the emission to spin-flip phosphorescence from slightly distorted 2 T1 states. Mer-[Cr(ddpd)2] 3+ only showed small Stokes shifts (<100 cm -1 ) and sharper bands (FWHM = 420 cm -1 ) in comparison, which might reflect the higher flexibility of the first coordination sphere around the larger molybdenum(III) ion in mer-1, mer-2 and cisfac- [3] 3+ . In contrast to most Cr III complexes, emission of mer-1, mer-2 and cisfac- [3] 3+ was only detected from their lowest-energy doublet states, although the 2 T1/ 2 E states are most likely thermally equilibrated due to being close in energy. We speculate that the spin-flip transitions from higher-lying doublet states have lower radiative rates, and are therefore difficult to detect. Due to the weak luminescence intensity, we were not able to measure luminescence lifetimes or quantum yields. From our experience with other samples measured in our spectrometer, we estimate the quantum yields of mer-1, mer-2 and cisfac- [3] 3+ in MeCN at room temperature to be below 0.0001 %. Mer-1 and mer-2 emit at lower energies than fac-MoX3(Me3 [9]aneN3) (X = Cl, Br, Me3 [9]aneN3 = 1,4,7-trimethyl-1,4,7-triazacyclononane) with phosphorescence band maxima at 1120 and 1130 nm, respectively. This can be explained by a stronger nephelauxetic effect induced by the 'softer' aromatic ddpd ligand compared to the 'harder' amine-based ligand Me3 [9]aneN3. A similar blue-shift of the emission can be observed for comparable Cr III complexes with pyridine and amine ligands. Cisfac-[3] 3+ is the first example for a luminescent molybdenum(III) complex solely stabilized by polypyridyl ligands, and shows the lowest spinflip emission energy reported so far. This suggests an unexpectedly high covalency between the metal and the ddpd ligands. Emission in this low energy region is very rarely observed from transition metal complexes and more commonly found for ff-transitions of lanthanides. Deactivation of low-energy luminescence can occur via energy transfer to vibrational overtones of the ligand or solvent with a distance dependence of r -6 . Given that the emission bands of mer-1, mer-2 and cisfac- [3] 3+ overlap with NIR absorption bands of acetonitrile at 1170 and 1380 nm originating from vibrational overtones (Figure ), it is surprising that the luminescence is detectable at all, even in solution at room temperature. Possibly, the metalconfined nature of the spin-flip states hinders quenching via this intermolecular pathway. Apart from the solvent, the coordinated ddpd ligand presents C-H oscillators in close proximity to the metal center. The emission bands of mer-1 and mer-2 are located between the first (v 2 ) and second (v 3 ) C-H vibrational overtones of the ligand (1670 and 1136 nm), while the emission band of cisfac- [3] 3+ significantly overlaps with the first overtone v 2 (C-H) (Figure ). Hence, we expect ligand deuteration to strongly enhance phosphorescence lifetimes and quantum yields in mer-1, mer-2 and especially cisfac- [3] 3+ , in analogy to reports on chromium(III) complexes. Work along these lines is currently in progress in our labs.