page_title: Transition metal oxo complex
section_title: Molybdenum/tungsten oxo species
hierarchical_section_title: Transition metal oxo complex / Molecular oxides / Metalloenzymes / Molybdenum/tungsten oxo species
context_page_description: A transition metal oxo complex is a coordination complex containing an oxo ligand. Formally O²⁻, an oxo ligand can be bound to one or more metal centers, i.e. it can exist as a terminal or as bridging ligands. Oxo ligands stabilize high oxidation states of a metal.
  Oxo ligands are pervasive, comprising the great majority of the Earth's crust. They are also found in several metalloenzymes, e.g. in the molybdenum cofactor and in many iron-containing enzymes. One of the earliest synthetic compounds to incorporate an oxo ligand is sodium ferrate circa 1702.
context_section_description: The oxo ligand (or analogous sulfido ligand) is nearly ubiquitous in molybdenum and tungsten chemistry, appearing in the ores containing these elements, throughout their synthetic chemistry, and also in their biological role (aside from nitrogenase).  The biologically transported species and starting point for biosynthesis is generally accepted to be oxometallates MoO₄⁻² or WO₄⁻².  All Mo/W enzymes, again except nitrogenase, are bound to one or more molybdopterin prosthetic group.  The Mo/W centers generally cycle between hexavalent (M(VI)) and tetravalent (M(IV)) states.  Although there is some variation among these enzymes, members from all three families involve oxygen atom transfer between the Mo/W center and the substrate.  Representative reactions from each of the three structural classes are:
  Sulfite oxidase: SO₃⁻²  +  H₂O   →   SO₄⁻² + 2 H⁺  +  2 e⁻
  DMSO reductase: H₃CS(O)CH₃ (DMSO)  +  2 H⁺  +  2 e⁻   →   H₃CSCH₃ (DMS) + H₂O
  Aldehyde ferredoxin oxidoreductase: RCHO  +  H₂O   →   RCO₂H  +  2 H⁺ +  2 e⁻
  The three different classes of molybdenum cofactors are shown in the Figure.  The biological use of tungsten mirrors that of molybdenum.
caption: Three structural families of molybdenum cofactors: a) xanthine oxidase, b) sulfite oxidase, and c) (DMSO) reductase.  The DMSO reductase features two molybdopterin ligands attached to molybdenum.  They are omitted from the figure for simplicity.  The rest of the heterocycle is similar to what is shown for the other two cofactors.
exif: {}