Research in the field of photochemistry and photophysics of transition metal complexes in the past few decades primarily focused on materials based on precious metals due to their favorable excited state properties. In the past 10 years, scientific curiosity drove researchers to explore earth-abundant metal complexes in more detail. These efforts uncovered photoactive complexes with intriguing excited state properties that can show unique reactivity following novel mechanisms. In particular, photoactive chromium(III) complexes saw a renaissance. It was kicked off by the discovery of design principles that allowed to access materials with bright long-lived excited states. The seed for this revolution was the so-called molecular ruby, mer-[Cr(ddpd)2] 3+ (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine), that shows a dual phosphorescence in the near-infrared (NIR) with a high quantum yield and lifetime of 13.7 % and 1.12 ms, respectively. The emission originates from nested metal-centered doublet states that only differ from the quartet ground state by a flipped spin (spin-flip emission). In the molecular ruby, the ligand ddpd produces a large ligand field splitting that shifts distorted 4 T2 states to higher energy which would otherwise enable non-radiative deactivation of the emissive spin-flip states. Because no antibonding orbitals are occupied in the spin-flip states, they only show negligible excited state distortion and hence sharp emission bands. The energy of these nested excited states depends on the interelectronic repulsion which is determined by the nephelauxetic effect, i.e. the extent of delocalization of the d-electrons on the ligands, which is often expressed using the Racah parameters B and C from ligand field theory. Covalent M-L bonds lead to lowenergy spin-flip states while ionic bonds increase the excited state energy. Phosphorescence from chromium(III) complexes is usually found between 720 and 800 nm. Recently, it was shown that the emission energy can be tuned from the red (709 nm) to NIR-II spectral region (up to 1067 nm) by ligand design. An important deactivation pathway for low-energy luminescence is energy transfer to vibrational overtones of the ligand or the solvent. Statistic deuteration of ddpd boosted the luminescence quantum yield of mer-[Cr(ddpd)2] 3+ to 30 %. Overall, complexes with spin-flip excited states have been applied in optical sensing (p, T, p(O2)), as energy and electron transfer catalysts, as sensitizers for photon upconversion, as qubits and for circularly polarized luminescence. Generally, spin-flip emission is neither limited to chromium(III) nor the d 3 electron configuration. Other metal-centers such as vanadium(III) or manganese(IV) have been explored as well and show dramatically different excited state energies due to their different M-L bond covalency. In contrast to the large body of work covering octahedral chromium(III) complexes, those of the heavier homolog molybdenum(III) have remained largely unexplored. Although both ions share the same d 3 electron configuration, molybdenum(III) complexes are more labile and prone to oxidation and dimerization due to the higher ionic radius of the central metal. Homoleptic Mo III complexes are particularly rare with most of the reported examples being stabilized by halides. Recent reports on mononuclear molybdenum(III) complexes include the investigation of the strongly solvatochromic [Mo(bpy)Cl4] -64 (bpy = 2,2'-bipyridine) and the redox chemistry of [Mo( Me PDP)2] n-(n = 0, 1, 2, 3) with a tridentate pyridinepyrrolate ligand (H2 Me PDP = 2,6-bis(5methyl-3-phenyl-1H-pyrrol-2-yl)pyridine). Detailed investigations of the zero-field splitting (ZFS) of mononuclear molybdenum complexes are limited to few reports. Studies of luminescent molybdenum(III) complexes are equally rare, with only 10 examples covered in three publications, all of which feature at least three (pseudo)halide ligands. Due to the low interelectronic repulsion in the extended 4d orbitals, the reported emission is red-shifted compared to typical chromium(III) complexes and located >1100 nm. A trend which is also expected when considering the Racah parameter B of the free ions (Cr 3+ : 918 cm -1 , Mo 3+ : 610 cm -1 ). Hence, molybdenum(III) complexes provide a great chemical platform to explore the effects of strong spin-orbit coupling and low interelectronic repulsion on spin-flip states. Additionally, their investigation can provide molecular materials of earth-abundant metals with highly desired luminescence in the NIR-II spectral region without the need to rely on rare-earth elements. Here, we present the synthesis and structural characterization of the heteroleptic halido complexes mer-MoX3(ddpd) (X = Cl: mer-1, X=Br: mer-2) and the homoleptic complex cisfac-[Mo(ddpd)2] 3+ (cisfac-[3] 3+ ). We report their electronic absorption spectra including spinforbidden absorptions in the red to NIR spectral region. All three complexes show weak spinflip phosphorescence in the NIR-II in solution at room temperature, with cisfac- [3] 3+ featuring the lowest spin-flip phosphorescence energy reported to date (max = 1550 nm). For cisfac-[3] 3+ , we also provide a detailed investigation of the ground state electronic structure by means of superconducting quantum interface device (SQUID) magnetometry and EPR spectroscopy, and the excited state dynamics by fs-transient absorption (TA) spectroscopy. We support our experimental results with density functional theory (DFT) and ab initio multi-reference quantum chemical calculations.