Carbene complexes. Part 12. Electron-rich olefin-derived neutral mono- and bis-(carbene) complexes of low-oxidation-state manganese, iron, cobalt, nickel, and ruthenium

Abstract

The electron-rich olefin [:[graphic omitted]R]₂(L^R₂; R = Me or Et) provrdes a ready source of various thermally stable neutral mono- and bis-(carbene) complexes of Fe, Ru, Co, and Ni and, with greater difficulty, of Mn^I. Noteworthy is the range of co-ordination numbers, formal metal oxidation states, and geometries (tetrahedral, square planar, trigonal bipyramidal, or octahedral) of the complexes. Amongst the 29 new complexes are the novel neutral bis(carbene) specres [Fe(L^Me)₂(NO)₂], [Co(CO)(L^Et)₂(NO)], trans-[Fe(CO)₃(L^Me)₂], [Ni(CO)₂(L^Et)₂], [Fe(CO)₂l₂(L^Me)₂], and [Ni(L^Me)₂(NO₃)₂]; the last is the only diamagnetic nickel(II) dinitrate. Infrared spectra show that ν(CO) is low compared with analogous (e.g. phosphine) complexes: ν(CN₂) is in the range 1 480–1 540 cm^–1, the upper end being characteristic of the higher-oxidation state metal complexes. This trend is also observed in ¹³C n.m.r. chemical shifts when C_carb. moves to higher field (C_carb. is generally 200–230 p.p.m. downfield from SiMe₄); [Ru₃(CO)₁₁ L^Et] shows only one CO resonance, an indication of fluxionality, whereas [Fe₂(CO)(µ-CO)₂(η-C₅H₅)L^Et] exists at 25 °C in solution as a mixture of cis and trans isomers.