Metal-ligand cooperation (MLC) continues to attract attention as a novel synthetic methodology to expand the role of transition metals in organometallic reactions. The ligand in MLC delivers necessary electron, proton and/or a functional group to assist metal during the chemical reactions. Although various phosphine ligands have been widely utilized in organometallic chemistry, examples of “non-innocent” phosphine ligand are relatively rare. Recently, our group reported a new type of metal-ligand cooperation with an anionic PPP ligand (PPP^– = ^–P[2-PⁱP_r2-C₆H₄]₂). The reversible phosphide/phosphinite interconversion of a PPP ligand coupled with a 2-electron redox change of nickel (II and 0) was observed in this system. Recently a reversible 1-electron redox process of a PPP ligand coupled with P-P bond formation was studied. Divalent nickel amide species with two different amido groups (-NHMes and –NTol2) were prepared to study reversible formation of P-N and P-P bonds at a nickel center supported by a PPP ligand. While the formation of a P-N bond was successfully accomplished by addition of CO(g) to the solution of (PPP)Ni-NTol₂ (Figure 1, left), the formation of a dimeric nickel(0) species, {(PPP)NiCO}2 was observed from the reaction of (PPP)Ni(NHMes) with CO(g) (Figure 1, right). The P-P bond formation was suggested as a radical pathway according to initial mechanistic studies. Activation of P-N and P-P bonds by using various reagents such as H₂ will be discussed.

Figure 1. Crystral structure of (PP^NTol2P)Ni(CO) (left) and {(PPP)NiCO}₂ (right)