Reactions of Monomeric [1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 CeH and CO with or without H 2 : An Experimental and Computational Study. Evan L. Werkema a Laurent Maron* b$ Odile Eisenstein* c Richard. A. Andersen* a a) Chemistry Department and Chemical Sciences Division of Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 b) Laboratoire de Physique Quantique, (UMR 5626-CNRS), IRSAMC, Université Paul Sabatier, 118 Route de Narbonne, 31064 Toulouse Cedex 04, France c) Chimie Théorique Méthodologie Modélisation, Institut Charles Gerhardt (UMR5253), Université Montpellier 2, 34095 Montpellier, France Laurent.Maron@irsamc.ups-tlse.fr Odile.Eisenstein@univ-montp2.fr raandersen@lbl.gov Abstract: Addition of CO to [1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 CeH, Cp’ 2 CeH, in toluene yields the cis-(Cp’ 2 Ce) 2 (μ-OCHCHO), in which the cis-enediolate group bridges the two metallocene fragments. The cis-enediolate quantitatively isomerizes intramolecularly to the trans-enediolate in C 6 D 6 at 100°C over seven months. When the solvent is pentane, Cp’ 2 Ce(OCH 2 )CeCp’ 2 forms, in which the oxomethylene group or the formaldehyde dianion bridges the two metallocene fragments. The cis-enediolate is suggested to form by insertion of CO into the Ce-C bond of 1