Transition metal tetraperoxide complexes are coordination complexes with the formula <sup>n-</sup>. They all feature 8-coordinate metal centers with four side-bonded peroxide ligands. The O-O distances, which are near 150 picometers, indicate O<sub>2</sub><sup>2-</sup>. They are prepared by reaction of the metal oxides with an alkaline solution of hydrogen peroxide.
Titanium-peroxide complexes have long been the basis of a colorimetric test for peroxide. The orange salts are known for M' = Li, Na, K.
Tetraperoxides of the vanadium triad are well established; all feature metals in their highest oxidation states. Salts of vanadium(V) tetraperoxide trianion, which is blue, have long been known since the 19th century following investigations of the reaction of hydrogen peroxide with vanadium pentoxide. The salt and (colorless) have also been verified by X-ray crystallography.
Turning to the tetraperoxides of the group VI metals, the most distinctive complex is , which features the d<sup>1</sup> ion Cr(V). The metals centers in all other tetraperoxides have the d<sup>0</sup> electronic configuration. Many salts are known. Early evidence for polyperoxides of Mo and W was obtained by Raman spectroscopy, which is sensitive for the O-O vibration near 800 cm<sup>-1</sup>. This work was followed by crystallographic confirmation of the alkali metal salts of , which are red and yellow, respectively.