Metal-Ligand Cooperation as Key in Formation of Dearomatized Ni-II-H Pincer Complexes and in Their Reactivity toward CO and CO2

The unique synthesis and reactivity of [((PNP)-P-R*)NiH] complexes (1a,b), based on metalligand cooperation (MLC), are presented ((PNP)-P-R* = deprotonated PNP ligand, R = Pr-i, Bu-t). Unexpectedly, the dearomatized complexes 1a,b were obtained by reduction of the dicationic complexes [((PNP)-P-R)Ni(MeCN)](BF4)(2) with sodium amalgam or by reaction of the free ligand with Ni-0(COD)(2). Complex 1b reacts with CO via MLC, to give a rare case of a distorted-octahedral PNP-based pincer complex, the Ni(0) complex 3b. Complexes 1a,b also react with CO2 via MLC to form a rare example of eta(1) binding of CO2 to nickel, complexes 4a,b. An unusual CO2 cleavage process by complex 4b, involving C-O and C-P cleavage and C-C bond formation, led to the Ni-C-O complex 3b and to the new complex [(PiPr(2)NC(2)O(2))Ni(P(O)Pr-i(2))] (5b). All complexes have been fully characterized by NMR and X-ray crystallography.