Accessibility of 17-Electron Structures for Cyclopentadienylchromium(III) Compounds. 2. † Cyanide Derivatives with 15- and 17-Electron Configurations
Résumé
Ion metathesis of [(ring)CrCl2]2 (ring = Cp, Cp*) with excess KCN in water affords the complexes [(ring)Cr(CN)3]-, which have been isolated as PPh4+ salts and characterized by elemental (C, H) analysis, 1H 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) × 10-3 M-1 for ring = Cp* at 298 K. Either treatment of [CpCr(CN)3]- with monodentate (L) or bidentate (L2) tertiary phosphine ligands (e.g. PMe3, PMe2Ph, dmpm, dmpe) or reaction between CpCrCl2(η1-L2) or [CpCrCl2L + L] and excess Me3SiCN leads to solutions containing small equilibrium amounts of EPR-active species which are characterized by hyperfine coupling to two equivalent P nuclei and are interpreted as the 17-electron CpCr(CN)2L2. The thermal accessibility of these 17-electron Cr(III) cyanide derivatives, as compared with the inaccessibility of the correponding chloride and methyl systems, is analyzed energetically in terms of the ligand steric and electronic effects.
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