Electronic spectra of π-cylopentadienyl(triethyl phosphine) copper(I). Ligand-to-ligand charge transfer

The complex CpCuI(PEt3) with Cp=cyclopentadienyl shows a longest-wavelength absorption at λmax=350 nm (in n-hexane) which is assigned to a Cp−→PEt3 ligand-to-ligand charge transfer (LLCT) transition. An analogy to the ylide Cp−–P+Propyl3 is drawn which displays the Cp− to phosphonium charge transfer (CT) absorption at λmax=267 nm (in n-hexane). Solid CpCu(PEt3) exhibits an emission at λmax=525 nm which is assumed to originate from the LLCT triplet.