Accessibility of 17-electron structures for cyclopentadienylchromium(III) compounds .1. Experimental studies on the dichloride and dimethyl compounds

Adducts of the CpCrX(2) (X = Cl, CH3) fragments with the bidentate ligands Me(2)PCH(2)PMe(2) (dmpm), Me(2)PCH(2)CH(2)PMe(2) (dmpe), and Ph(2)PCH(2)CH(2)PPh(2) (dppe) are described. Depending on the amount of ligand used, dinuclear 2:1 adducts or mononuclear 1:1 adducts are obtained. The dinuclear adducts have a CpX(2)Cr(mu-eta(1):eta(1)-L-L)CrX(2)Cp structure featuring 15-electron Cr centers, as shown by X-ray structural determinations on [CpCrCl(2)](2)(dppe), [CpCrCl(2)](2)(dmpe), and [CpCr(CH3)(2)](2)(dmpe). The reaction between [CpCrCl(2)](2)(L-L) and L-L affords the mononuclear CpCrCl(2)(L-L) derivatives. A P-31-NMR monitoring of this reaction for L-L = dmpm reveals a quantitative reaction, followed by release of free dmpm. This phenomenon is interpreted as a dmpm-induced isomerization of [CpCrCl(2)](2)(dmpm) to [CpCrCl(dmpm)](+)[CpCrCl(3)](-). The mononuclear adducts have a 15-electron configuration CpCrX(2)(eta(1)-L-L) with a dangling phosphine ligand, both in the solid state, as shown by an X-ray crystal structure of CpCrCl(2)(dmpm), and in solution, as shown by P-31-NMR, UV-visible, and EPR spectroscopies. The hypothetical 17-electron configuration where the bidentate L-L ligand adopts a chelating configuration, CpCrX(2)(eta(2)-L-L), is not experimentally observed for any of the systems investigated.