The reactivity of aluminium compounds is dominated by their electron deficiency and consequent electrophilicity; these compounds are archetypal Lewis acids (electron-pair acceptors). The main industrial roles of aluminium, and classical methods of synthesizing aluminium–element bonds (for example, hydroalumination and metathesis), draw on the electron deficiency of species of the type AlR3 and AlCl31,2. Whereas aluminates, [AlR4]−, are well known, the idea of reversing polarity and using an aluminium reagent as the nucleophilic partner in bond-forming substitution reactions is unprecedented, owing to the fact that low-valent aluminium anions analogous to nitrogen-, carbon- and boron-centred reagents of the types [NX2]−, [CX3]− and [BX2]− are unknown3,4,5. Aluminium compounds in the +1 oxidation state are known, but are thermodynamically unstable with respect to disproportionation. Compounds of this type are typically oligomeric6,7,8, although monomeric systems that possess a metal-centred lone pair, such as Al(Nacnac)Dipp (where (Nacnac)Dipp = (NDippCR)2CH and R = tBu, Me; Dipp = 2,6-iPr2C6H3), have also been reported9,10. Coordination of these species, and also of (η5-C5Me5)Al, to a range of Lewis acids has been observed11,12,13, but their primary mode of reactivity involves facile oxidative addition to generate Al(III) species6,7,8,14,15,16. Here we report the synthesis, structure and reaction chemistry of an anionic aluminium(I) nucleophile, the dimethylxanthene-stabilized potassium aluminyl [K{Al(NON)}]2 (NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene). This species displays unprecedented reactivity in the formation of aluminium–element covalent bonds and in the C–H oxidative addition of benzene, suggesting that it could find further use in both metal–carbon and metal–metal bond-forming reactions.