MoO42

MoO₄²⁻, commonly referred to as the molybdate ion, is an inorganic polyatomic anion composed of one molybdenum atom centrally coordinated by four oxide ligands in a tetrahedral geometry. The ion carries a 2‑ negative charge and is the conjugate base of the weak acid molybdic acid (H₂MoO₄). In solid salts, molybdate typically forms ionic lattices with alkali or alkaline‑earth metal cations, such as sodium molybdate (Na₂MoO₄) and calcium molybdate (CaMoO₄). The tetrahedral MoO₄²⁻ unit is isoelectronic with sulfate (SO₄²⁻) and chromate (CrO₄²⁻), leading to similar chemical behavior, including solubility in water and the ability to form complex anions with transition metals.

Molybdate occurs naturally in a variety of minerals, most notably molybdenite (MoS₂), which oxidizes to release molybdate ions during weathering. In the environment, molybdate is a mobile form of molybdenum, an essential trace element for many organisms. It functions as a cofactor in enzymes such as nitrate reductase and aldehyde oxidase, where it participates in redox reactions.

Industrial applications of molybdate include corrosion inhibition, as molybdate salts are used in water treatment and metal‑working fluids. They also serve as pigments (e.g., ammonium molybdate) and as catalysts in petroleum refining. Laboratory use often involves the colorimetric determination of phosphates, where molybdate reacts to form a blue complex (the phosphomolybdate assay). While generally of low toxicity, high concentrations can be harmful to aquatic life, prompting regulated disposal practices.