The reactivity of a geometrically constrained phosphorus(III) complex bearing the N,N-bis(3,5-di-tert-butyl-2-phenolate)amide pincer ligand (P(ONO); 1) towards oxidants and reductants is explored. This compound can be readily oxidized to the phosphorus(V) dihalo-derivatives P(ONO)X where X=Cl (2), Br (3) and I (4). Attempts at isolating the analogous difluoride through oxidation of 1 were unsuccessful yielding only the hydrofluoride P(ONO)(H)F (5), however P(ONO)F (6) can be accessed via a halide exchange reaction of 2 with KF. Compound 2 can be employed as a precursor to novel cationic species through chloride ion displacement using strong Lewis bases. Thus, reaction of 2 with two or three molar equivalents of dimethylaminopyridine (DMAP) affords [P(ONO)(Cl)(DMAP) ] (7) and [P(ONO)(DMAP) ] (8). Reaction of 2 with the weaker bidentate base 2,2'-bipyridine (bipy) affords [P(ONO)(Cl)(bipy)] (9), although this species was only accessible upon addition of a halide abstracting agent. The dicationic tris(pyridine) adduct [P(ONO)(py) ] (10) is also accessible by reaction of 4 with pyridine. Oxidation of 1 using oxygen gas proceeds slowly and allows for the observation of two compounds, a mixed valence dimeric phosphorus(III)/phosphorus(V) compound [P(ONO)(μ -O)(μ :κ ,κ -ONO)P] (11) and the fully oxidized species [P(ONO)(μ -O)(μ :κ ,κ -ONO)P(O)] (12). Finally, reaction of 1 using KC results in the dimerization of the putative radical anion [P(ONO)] through formation of a P-P bond to afford [P(ONO)] (13). Reactions with TEMPO result in the formation of the trigonal bipyramidal species P(ONO)(TEMPO) (14).

The ambiphilic nature of geometrically constrained Group 15 complexes bearing the N,N-bis(3,5-di-tert-butyl-2-phenolate)amide pincer ligand (ONO 3À )i se xplored. Despite their differing reactivity towards nucleophilic substrates with polarised element-hydrogen bonds (e.g.,N H 3 ), both the phosphorus(III), P(ONO)( 1a), anda rsenic(III), As(ONO)( 1b), compounds exhibit similar reactivity towards charged nucleophilesa nd electrophiles. Reactions of 1a and 1b with KOtBu or KNPh 2 afford anionicc omplexes in which the nucleophilic anion associates with the pnictogen centreÀ (Pn = P( 3a), As (3b)).C ompound 2a can subsequentlybe reacted with ap rotons ource or benzylbromide to afford the phosphorus(V) compounds (tBuO)HP(O-NO) (4a)a nd (tBuO)BzP(ONO) (5a), respectively,w hereas analogous arsenic(V) compounds are inaccessible.E lectrophilic substrates, such as HOTf and MeOTf, preferentially associate with the nitrogen atom of the ligand backbone of both 1a and 1b,g iving rise to cationic species that can be rationalised as either ammonium salts or as amine-stabilised phospheniumo ra rsenium complexes( [Pn{ON(H)O}] + (Pn = P (6a), As (6b)) and [Pn{ON(Me)O}] + (Pn = P( 7a), As (7b)). Reactiono f1a with an acid bearing an ucleophilic counteranion (such as HCl) gives rise to ap hosphorus(V) compound HPCl(ONO) (8a), whereas the analogous reaction with 1b results in the addition of HCl across one of the AsÀOb onds to afford ClAs{(H)ONO} (8b). Functionalisation at both the pnictogenc entre and the ligand backbone is also possible by reactiono f7a/7b with KOtBu, which affords the neutral species (tBuO)Pn{ON(Me)O} (Pn = P( 9a), As (9b)).T he ambiphilic reactivity of theseg eometrically constrainedc omplexes allowss omei nsight into the mechanism of reactivity of 1a towardssmall molecules, such as ammonia and water.

C e r i u m (I I I)-M e d i a t e d H y d r o c h a l c o g e n a t i o n o f A l k y n e s

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