Presence of the hydrogen bonding near a metal center can influence the properties of the complex. Here, we describe changes in redox and spectral properties in discrete dioxo-molybdenum centers coordinated by a single thiolato ligand that can support an intra-ligand hydrogen bond. We have utilized thiophenolato ligands that can harbor hydrogen bonding between the thiophenolato sulfur with an amide functionality creating either a five-or a six-member ring. Methylation of the amide functionality removes the NH⋯S hydrogen bonding thus provides a basis for understanding the effect of hydrogen bonding. These thiophenolato ligands have been used in synthesizing dioxo-Mo VI complexes of type Tp*MoO 2 (S-o-RC 6 H 4), where R = CONHMe (11), CONMe 2 (12), NHCOMe (13), and N(Me)COMe (14). The complexes have been characterized by NMR, infrared, and UV-visible spectroscopy. Spectroscopic data clearly indicate the presence of hydrogen bonding in both 11 and 13, and stronger in 13, where hydrogrn bonding stabilizes a fivemember ring. All complexes exhibit Mo VI /Mo V redox couple and redox potentials are modulated by the nature of H-bonding. Compound 14 with the electron-releasing N(Me)COMe group has the highest reduction potential and is more difficult to reduce. water molecules from SO 3 2− to the oxomolybdenum center has been observed, 2,3,4 which raises questions about the roles of hydrogen bonding in modulating properties of the metal center. Sulfite oxidase is considered to be a prototypical oxotransferase, where water serves

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