The reaction between [(Mo(p-OCOMe),},] and SiMe3CI in the presence of a variety of donor ligands L [tetrahydrofuran (thf), Me2S, Et,S, PEt,, PPh3, PBun3, or Ph2P(CH2),PPh2, where n = 1 or 21 yields binuclear compounds such as [{ M o L C I ( ~-O C O M ~) } ~] or [( MoL2CI,),]. The products may vary according to the stoicheiometry of the reaction. The new compounds described are [{ MoCI(p-OCOMe) (thf)}2], [(MoCI( PPh3) (JJ-OCOM~))~], isomers of [{ MoCl( PEt,) (p-OCOMe)}2], trans-[{MoCI( PBu",) (p-OCOMe)},], and [M0~Cl,(OCOMe)~(dppm)1. Interconversion of some of these compounds by ligand-replacement reactions is also demonstrated. The crystal structure of trans-[{ MoCI( PBu",) (p-OCOMe)},] is described ; the crystals are monoclinic, space group P2, /n, with a = 14.969(2), b = 15.146(2), c = 8.920(2) A, p = 105.14(1)", and Z = 2. Refinement converged with R = 0.048, R' = 0.063, for 1 762 observed reflections [I > 30(1)]. The M o -M o separation is 2.099(1) A. The presence of isomers of the various compounds is demonstrated from the hydrogen-1 n.m.r. spectra. The dimer [{Mo(p-OCOMe),},] (1) has been shown to act as a precursor for the formation of molybdenocene dimers via reaction with sodium cyclopentadienide.' During a study of this reaction we added triphenylphosphine to a mixture of (1) and Na(C5H5) to try to trap out intermediates in the reaction. This work led to the isolation of a new black molybdenocene dimer.2 In order further to understand the role of the PPh3 in the formation of this dimer we wished to synthesise the compounds [{ MoCl(PPh3)(p-OCOMe)),1 and [ { M O C I ~( P P ~~) ~} ~' J . We noticed a report by McCarley et aL3 of the synthesis of [Mo4CI,(PEt3),] from the reaction between [{ Mo(p-OCO-Me)*}?] and chlorotrimethylsilane in the presence of triethylphosphine. This led us to investigate the synthesis of related binuclear compounds using the same general principles.We have found that SiMe3C1 can act as a mild and selective reagent for the replacement of the acetato-groups in (1) by chlorine and thereby can provide a simple and very convenient route to the synthesis of a variety of new and known dimolybdenum compounds, as described below. 7 rrans-Di-~-acetato-OO'-bis[chloro(tri-n-butylphosphine)molybdenum(ri)](4Mo-M0).