ACCESSIBILITY OF 17-ELECTRON STRUCTURES FOR CYCLOPENTADIENYLCHROMIUM(III) COMPOUNDS .2. CYANIDE DERIVATIVES WITH 15-ELECTRON AND 17-ELECTRON CONFIGURATIONS

Ion metathesis of [(ring)CrCl2](2) (ring = Cp, Cp) with excess-KCN in water affords the complexes [(ring)Cr(CN)(3)](-), which have been iso lated as PPh4+ salts and characterized by elemental (C, H) analysis, H -1 NMR, magnetic suceptibility, and an X-ray investigation for the Cp species. The interaction of these complexes with excess CN- generate equilibrium amounts of the EPR-active, 17-electron [(ring)Cr(CN)(4)](2 -) adducts. The equilibrium constant for this addition process is 2.3 +/- 0.1 M-1 for ring = Cp and (4.4 +/- 0.2) x 10(-3) M-1 for ring = Cp at 298 K. Either treatment of [CpCr(CN)(3)](-) with monodentate (L) or bidentate (L-2) tertiary phosphine ligands (e.g. PMe3, PMe2Ph,.dmpm , dmpe) or reaction between CpCrCl2(eta(1)-L-2) or [CpCrCl2L + L] and excess Me3SiCN leads to solutions containing small equilibrium amounts of EPR-active species which are characterized by hyperfine coupling t o two equivalent P nuclei and are interpreted as the 17-electron CpCr( CN)(2)L-2. The thermal accessibility of these 17-electron Cr(III) cyan ide derivatives, as compared with the inaccessibility of the correpond ing chloride and methyl systems, is analyzed energetically in terms of the ligand steric and electronic effects.