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 AlR and AlCl. Whereas aluminates, [AlR], 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 [NX], [CX] and [BX] are unknown. Aluminium compounds in the +1 oxidation state are known, but are thermodynamically unstable with respect to disproportionation. Compounds of this type are typically oligomeric, although monomeric systems that possess a metal-centred lone pair, such as Al(Nacnac) (where (Nacnac) = (NDippCR)CH and R = Bu, Me; Dipp = 2,6- PrCH), have also been reported. Coordination of these species, and also of (η-CMe)Al, to a range of Lewis acids has been observed, but their primary mode of reactivity involves facile oxidative addition to generate Al(III) species. Here we report the synthesis, structure and reaction chemistry of an anionic aluminium(I) nucleophile, the dimethylxanthene-stabilized potassium aluminyl [K{Al(NON)}] (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.

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This review focuses on the reactivity of nine-atom deltahedral clusters of group 14, also known as deltahedral Zintl ions, toward various main-group and transition-metal compounds. The redox chemistry of these species and their ability to form oligomers, polymers, and functionalized clusters are discussed, and the cluster geometries, their bonding, and electronic structures are described.

Solid-state auride salts featuring the negatively charged Auion are known to be stable in the presence of alkali metal counter-ions. While such electron-rich species might be expected to be nucleophilic (cf. I -), their instability in solution means that this has not been verified experimentally. Here we report the two-coordinate gold complex (NON)AlAuP t Bu3 (3, NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) synthesised by the reaction of the potassium aluminyl complex [K{Al(NON)}]2 (1) with t Bu3PAuI, and which features a strongly polarized bond, Au(d-)-Al(d+). 3 has been studied computationally, with QTAIM charge analysis implying a charge at gold (-0.82) which is in line with the relative electronegativities of the two metals (Au: 2.54; Al: 1.61 on the Pauling scale). Consistently, 3 is found to act as an unprecedented nucleophilic source of gold, reacting with diisopropylcarbodiimide and CO2 to give the Au-C bonded insertion products (NON)Al(X2C)AuP t Bu3 (X = N i Pr, 4; X = O, 5).Transition elements are known to be able to access multiple oxidation states 1 , a property which underpins widespread application in fields such as small molecule activation and catalysis 2,3 . The vast majority of transition metal complexes however, feature cationic metals in positive oxidation states, ligated by neutral or anionic donors 1,4 . Systems featuring formal negative oxidation states, such as the tetracarbonylferrate 5 or bis(benzene)vanadium 6 anions are much less common, and usually require strong p-acceptor ligands, most frequently CO 7 . In this regard, gold is unique, being the only transition metal to give rise to a stable "naked" monoanion (Au -, auride) in the condensed phase 8 . In part, this is due to relativistic effects which contract the 6s orbital significantly, resulting in an electron affinity of 2.30 eV, the highest of any transition metal 9,10 .This value is more comparable to those of the chalcogens (e.g. S: 2.08 eV; Se: 2.02 eV) than to the lighter group 11 congeners (Cu: 1.23 eV; Ag 1.30 eV) 10 . The 12-electron auride anion is typically generated by the reduction of metallic gold with alkali metals, to give salts such as CsAu and RbAu 11,12 ; the solution chemistry of these salts, however, is restricted to liquid ammonia 13,14 .Reduction of organometallic gold compounds to give systems in low oxidation states (i.e. zero or below) has been attempted, but with limited success [15][16][17][18][19][20] . Thermodynamics typically drive the aggregation of molecular Au(0) systems to clusters of colloidal gold [15][16][17][18] . Recently however, electron-rich gold complexes have been reported by Bertrand and co-workers, by making use of strongly p-accepting cyclic(alkyl)(amino)carbenes (CAAC) ligands (I and II, Figure 1) 19 . In addition, a four-coordinate molecular 'boroauride' was reported by Harman and co-workers last year in which the gold centre is stabilized by a diboraanthracene-based scaffold (III, Figure 1) 20 .Notwithstanding these examples, and even though Auions...

In all likelihood the first synthesis of the phosphaethynolate anion, PCO , was performed in 1894 when NaPH was reacted with CO in an attempt to make Na(CP) accompanied by elimination of water. This reaction was repeated 117 years later when it was discovered that Na(OCP) and H are the products of this remarkable transformation. Li(OCP) was synthesized and fully characterized in 1992 but this salt proved to be too unstable to allow for a detailed investigation of its chemistry. It was not until the heavier analogues of this lithium salt were isolated, Na(OCP) and K(OCP) (both of which are remarkably stable and can be even dissolved in water), that the chemistry of this new functional group could be explored. Here we review the chemistry of the 2-phosphaethynolate anion, a heavier phosphorus-containing analogue of the cyanate anion, and describe the wide breadth of chemical transformations for which it has been thus far employed. Its use as a ligand, in decarbonylative and deoxygenative processes, and as a building block for novel heterocycles is described. In the mere twenty-six years since Becker first reported the isolation of this remarkable anion, it has become a fascinating reagent for the synthesis of a vast library of, often unprecedented, molecules and compounds.