Electron‐Rich Half‐Sandwich Complexes—Metal Bases par excellence

Electron‐rich, half‐sandwich complexes of the type C n R n ML 2 or C n R n MLL′ are built up of an aromatic five‐ or six‐membered ring, a d 8 ‐metal, and either a pair of two‐electron donors or an equivalent chelating ligand. Such complexes behave like Lewis bases and react with a wide variety of electrophiles, El or ElX, to form products with a new metal‐element bond. According to their reactivity they are comparable to the Vaska‐type compounds. Certain of the products obtained after addition of the electrophile undergo interesting subsequent reactions in which, for example, metal complexes containing molecules that are unstable in the free state, such as CS, CSe, CH 2 S, CH 2 Se, CH 2 Te, CH 3 CHS, CH 3 CHSe, CH 2 CS, CH 2 CSe, and CH 2 CTe are formed. Moreover, cycloadditions as well as reactions with coordinatively unsaturated transition‐metal compounds which result in formation of heterometal binuclear complexes demonstrate that the metal bases C n R n ML 2 and C n R n MLL′ are valuable synthetic building blocks. Furthermore, very recent investigations have indicated links between metal basicity and the problem of CH activation.