Electron-rich polyoxometalates (POMs), known since the early discovery and development of POM chemistry, are POMs incorporating extra electrons upon reduction and comprise an emergent family of different archetypes, structural flexibility and functionality. Here, we describe synthetic strategies to obtain electron-rich POMs with important catalytic, electronic and magnetic properties and discuss their differences and advantages compared to their fully oxidized analogues. This is the first review summarizing the existing knowledge about polyoxometalate reduction, encompassing a comprehensive description of reduced compounds (over 200 structures are reviewed) and the influence the reduction causes on the structure, function and properties of this molecule class.Polyoxometalates (POMs) are a large group of transformable discrete anionic polynuclear metal-oxo clusters. These compounds contain arrays of corner-and edge-sharing pseudo-octahedrally coordinated MO 6 (M = V, Nb, Mo, W) units, packed to form an ionic core, where the electronic configuration of the metal is usually d 0 or d 1 (metals in their highest oxidation states). 1 These metals are commonly called addenda atoms or peripheral elements and their ionic radii and charge are suitable for O 2coordination. The coordination number of the addenda atoms can be increased from 4 to 6 upon acidification and they are able to form double bonds with unshared terminal oxygens in MO 6 octahedra through p π -d π interactions. One of the most widely accepted classification of POMs divides them into two groups: 1) isopolyanions (IPAs), which consist of only one type of metal (M) atom, [M m O y ] q-, and 2) heteropolyanions (HPAs), with the general formula [X r M m O y ] q-, where X is the so-called heteroatom. POMs have multiple applications in various areas, such as catalysis, 2,3 bio− and nanotechnology, 4 medicine, 5-6 macromolecular crystallography, 7-9 electrochemistry, 10 material sciences 11 and molecular magnetism 12 and many of them are related to their redox properties. POMs are often recognized as electron reservoirs because of their strong capacity to bear and release electrons indicating their redox nature. 13 POMs can be regarded as softLewis bases due to the abundant oxygen atoms that can donate electrons to electron acceptors. However, the addenda metal ions of the polyanion skeletons possess unoccupied orbitals that can accept electrons and thereby act as Lewis acids.2The reduced, also called electron-rich, POMs typically retain the general structure of their parent molecule and are often characteristically deep blue in color comprising a very large group of complexes known as the "poly blues" or "heteropoly blues" (FIG. 1). Their blue color is the result of intense d-d electron transitions and intervalence chargetransfers.14The capacity to reduce a particular POM depends on the charge to nuclearity ratio and for heteropolyanions the kind and oxidation state of the heteroatoms must be taken into consideration. In 1972, Pope 19 divided all POMs into three ...
