Synthesis, characterisation, and molecular and electronic structure of CpMoCl2(R1C≡CR2) (R1, R2 = Ph, Et, Me): a new class of half-sandwich 17-electron molybdenum(III) organometallics

International audienceAddition of alkyne to [CpMoCl2]2 affords compounds CpMoCl2(η2-alkyne) (alkyne = EtC[triple bond, length as m-dash]CMe, 1; EtC[triple bond, length as m-dash]CEt, 2; PhC[triple bond, length as m-dash]CMe, 3; PhC[triple bond, length as m-dash]CPh, 4) in good yields. The compounds have been characterised by C,H analyses, IR, EPR and mass spectroscopies, magnetic susceptibility, and cyclic voltammetry. In addition, a single crystal X-ray diffraction analysis has been carried out for compound 4. The alkyne ligand adopts an almost parallel conformation relative to the Cp ring, essentially identical with that of previously reported Nb, Ta, and W analogues. Geometry optimisations on the CpMCl2(HC[triple bond, length as m-dash]CH) (M = Nb, Mo) model compounds show that the total energy is nearly independent of the alkyne orientation. The SOMO for M = Mo is an essentially metal-based orbital with a slight Mo-Cl π* component, in agreement with the observed trends in M–Cl bond lengths on going from Group 5 to Group 6. The cyclic voltammetric behaviour of 1–4 is similar to the analogous diene complexes CpMoCl2(η4-diene). A reaction between 1–4 and excess alkyne takes place only for the dialkylsubstituted alkyne complexes under forcing conditions. Compound CpMoCl2(η4-C4Et4H), 5, has been isolated from the reaction between 2 and excess EtC[triple bond, length as m-dash]CEt and crystallographically characterized