Preparation and characterisation of some dimeric η²-diyne complexes of cobalt

The organometallic dimers [{Co₂(CO)₆}₂(diyne)] (diyne = HC₂C₆H₄C₂H 1, HC₂C₆H₄C₆H₄C₂H 2, HC₂C₆H₄CH₂C₆H ₄C₂H 3, Me₃SiC₂C₆H₄C₆H ₄C₂SiMe₃ 4, Me₃SiC₂C₆H₄CH₂C ₆H₄C₂SiMe₃ 5 or Me₃SnC₂C₆H₄C₆H ₄C₂SnMe₃ 6) have been synthesised from the reaction of octacarbonyldicobalt(0), [Co₂(CO)₈], with the appropriate diyne. The products have been characterised by infrared spectroscopy, electron impact (El) mass spectrometry, microanalysis and ¹H NMR spectroscopy. The complexes 1, 3 and 4 have been characterised by single-crystal X-ray crystallography. In all cases both 'yne' fragments of the diyne bond to a Co₂(CO)₆ fragment with the C≡C vector essentially perpendicular to the Co-Co vector. A bonding analysis by a Fenske-Hall calculation using the crystallographically determined coordinates for dimer 1 is consistent with the observed distortion of the Co₂C₂ bonding pattern away from quasi-tetrahedral geometry.