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 AlCl3 (1,2). Whereas aluminates, [AlR4](-), are well known, the idea of reversing polarity and using an aluminium reagent as the nucleophilic partner in bondforming 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 unknown(3-5). Aluminium compounds in the + 1 oxidation state are known, but are thermodynamically unstable with respect to disproportionation. Compounds of this type are typically oligomeric(6-8), although monomeric systems that possess a metal-centred lone pair, such as Al(Nacnac) Dipp (where (Nacnac)(Dipp) = (NDippCR)(2)CH and R = Bu-t, Me; Dipp = 2,6-(Pr2C6H3)-Pr-i), have also been reported(9,10). Coordination of these species, and also of (eta(5)-C5Me5)Al, to a range of Lewis acids has been observed(11-13), but their primary mode of reactivity involves facile oxidative addition to generate Al(iii) species(6-8,14-16). Here we report the synthesis, structure and reaction chemistry of an anionic aluminium(i) nucleophile, the dimethylxanthenestabilized 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.