The coordination of alloxazine and pterins to molybdenum(IV) is demonstrated in this study. The synthesis of MoOCl3(pteridineH), where pteridineH is the protonated form of 1,3,7,8-tetramethylalloxazine (tmaz), 2-pivaloyl-6,7-dimethylpterin (piv-dmp), and 6,7-dimethylpterin (dmp), proceeds readily starting from Mo(IV)Cl-4(acetonitrile)(2) and the pteridine ligand in chloroform or methanol. X-ray crystal structures of MoOCl3(tmazH) (1) and MoOCl3(piv-dmpH) (2) show that Mo chelates each pteridine at the carbonyl oxygen and pyrazine nitrogen and that the pteridine ligand is protonated at the other nitrogen in the pyrazine ring. A third X-ray structure for MoOCl3(H(3)dmp) (4) is included in this work since its determination permits the comparison of metrical parameters for the oxidized and reduced forms of a pterin in identical molybdenum coordination environments. The major difference observed in the structures of 2 as compared to 4 is the Mo-NS bond length which is significantly shorter in compound 4 containing the reduced form of the pterin. Pteridine protonation is facilitated by molybdenum(IV) coordination due to partial reduction of the pteridine ring through electronic delocalization from Mo to the pteridine ligand. Electronic spectroscopy monitoring the solution reactivity of 1, 2, and MoOCl3(dmpH) (3) provides evidence to support this idea. Solution conditions favoring deprotonation of the complexes 1-3 promote pteridine dissociation and complex decomposition.