AbstractThe first crystal structure of a molybdenum complex9with a hydrogenated pterinanda sulfur ligand contributes to the discussion about the active center of molybdenum and tungsten enzymes containing a molybdopterin cofactor. Complex9was synthesized through a redox reaction of [MoVIO2(LN‐S2)] (8; LN‐S2= pyridine‐2, 6‐bis(methanethiolato)) with 5, 6, 7, 8‐tetrahydropterin (7). 2 HCl (H4Ptr.2 HCl). The complex crystallizes, with a non‐coordinating Cl‐atom acting as a counterion, in the monoclinic space group C2/c (No. 15) with cell dimensionsa= 22.900(5),b= 10.716(2),c= 17.551(4) Å, β = 120.36(3)°, andZ= 8. We interpret9as [MoIVO(LN‐S2)(H+‐q‐H2Ptr)]Cl (q = quinonoid; H2Ptr = dihydropterin), i.e., a MoIVmonooxo center coordinated by a pyridine‐2, 6‐bis(methanethiolato) ligand and a protonated dihydropterin. The spectroscopic properties of this new complex are comparable to those of other crystalline molybdenum complexes of hydrogenated pterins without additional S‐coordination. The slightly H2O‐soluble complex9reacts with the natural enzyme substrate DMSO very slowly, possibly due to the lack of easily dissociab