~ Pergamon PII: S0277-5387(97)00496-8 Po@hedron Vol. 17, No. 7, pp. I 115 1119, 1998 '~2 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0277-5387/98 $19.00+0.00 New CpCrCIz(PR3) complexes:physical properties and reduction chemistryt Matthieu Bender-Gresse, a Edmond Collange, ~ Rinaldo Poli °* and Sundeep Mattamana b "Laboratoire de Synth6se et d'Electrosynth6se Organom6tallique, Universit6 de Bourgogne, Facult6 de Sciences "Gabriel", 6 Boulevard Gabriel, 21100 Dijon Cedex ~' Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 (Received 25 September 1997; accepted 24 October 1997) Abstract--Compounds CpCrClzL (L = PMe2Ph, 1 ; PMePh2, 2 ; PPh3, 3) have been prepared. Their magnetic moment is in accord with the presence of three unpaired electron and a variable temperature ~H NMR investigation of 1 shows Curie-Weiss behavior. The ~H NMR, EPR and UV-visible spectra are in agreement with those of other previously reported compounds of the same family. An electrochemical investigation shows the accessibility of an irreversible reduction process. A parallel sodium reduction investigation of complexes CpCrC12L (L = PMe3 and q~-dppe) suggests that the reduction process is followed by immediate chloride loss, and then by a ligand redistribution process to afford chromocene and unstable inorganic phosphine complexes of Cr(II). © 1998 Elsevier Science Ltd. All rights reserved Keywords: CpCrCI2 ; reduction chemistry ; Curie-Weiss behavior. We have investigated in some detail a class of stable organometallic radicals based on the CpMoC12(PR3)2 stoichiometry and have shown that they undergo a dissociative phosphine exchange process [1]. The sup- posed 15-electron CpMoCIz(PR3) intermediate is probably favored by the release of pairing energy, since it is calculated to adopt a spin quartet ground state [2,3], while the associative path is disfavored by sterics and the high energy HOMO of the precursor. Attempts to stabilize and isolate such an intermediate, which is isostructural and isoelectronic with stable Cr(lII) complexes, have been to date unfruitful [4,5]. The relative stability of the 17-electron S = 1/2 struc- ture for CpMo(IIl) complexes and of the 15-electron S -- 3/2 structure for CpCr(lIl) complexes have been rationalized on a theoretical standpoint by differences in M--PR3 bonding and electron pairing energy [2,3]. We have thus become interested in examining the corresponding phosphine exchange process on the 15- electron Cr(III) compounds. For this purpose, a larger ? Herrn Prof. Dr. h.c. mutt. W. A. Herrmann zurn 50. Geburstag geuidmet. * Author to whom correspondence should be addressed. number of complexes was required than previously available and an exchange reaction that would be amenable to a kinetic study. We report here the syn- thesis of these complexes, a study of their physical properties to complement those already available in the literature, and an electrochemical and chemical reduction study. The phosphine exchange kinetic studies are in progress and will be presented in a later contribution. EXPERIMENTAL All operations were carried out under an atmo- sphere of dinitrogen. Solvents were dehydrated by conventional methods and distilled directly from the dehydrating agent prior to use (THF and Et20 from Na/benzophenone, hexane from Na/K, heptane and toluene from Na, and CHzCI2 from P205). NMR spec- tra were recorded on a Bruker AC200 spectrometer; the peak positions are reported with positive shifts downfield of TMS as calculated from the residual solvent peaks (~H) or downfield of external 85% H_~PO4 (3~p). For each 3~P-NMR spectrum, a sealed capillary containing H3PO4 was immersed in the same NMR solvent used for the measurement and this was 1115