Phosphide containing tridentate pincer systems reveal cooperative reactivity in the presence of the 1^st-row transition metal. For example, our group recently reported a unique metal⎯ligand cooperation (MLC) of a PPP (PPP^– = P[2-PⁱP_r2-C₆H₄]₂^–) ligand occurring at a single nickel center. By introducing a π-acidic ligand such as N₂ or CO to a (PPP)nickel(II) complex, a two-electron redox change occurs at the nickel center and this reaction is nicely coupled with the transformation of phosphide to phosphinite via P⎯O/S bond formation. Thus, the group transfer reaction mediated by two-electron MLC can be established with a (PPP)Ni moiety. In order to expand current technology with a PPP system, single-electron MLC reactions are currently under investigations. In group 9, 1^st-row transition metal, cobalt is our first choice to attain such open-shell type reactivity. A (PPP)Co moiety was prepared from the metalation of PPPH with CoBr₂ resulted in a generation of trigonal bipyramidal (PPHP)CoBr₂. Deprotonation of a cobalt(II)bromide complex with trimethylamine, in fact, induced the formation of a dimeric cobalt(I) bromide complex, {(PPP)CoBr}₂ with high yield. The resulting dimeric cobalt(I) complex was generated from two 1-electron transfer reactions coupled with the formation of a P⎯P bond of the two central phosphide moieties. This P⎯P bond can be cleaved by a reduction of dinuclear cobalt(I) complex generating a central phosphide bridged cobalt(I) complex {(PPP)Co}₂. Thus, the P⎯P bond formation and cleavage show a redox active character of the PPP ligand system cooperated with cobalt center. The details of one-electron redox-active processes of a (PPP)Co species through reversible P⎯P formation and cleavage will be presented.

Scheme 1.