Reaction chemistry, NMR spectroscopy, and X-ray crystallography of [Fe2(μ-SiMe2)2(CO)4] and [Fe2(μ-SiMeCl)2(CO)4]. Electronic structure and bonding in Fe2E2 rings of [Fe2(μ-ER2)2(CO)4] binuclear complexes (E=C, Si, Ge, Sn, Pb)

The rings [Fe2(μ-SiRR′)2(CO)8] (R=Me, R′=Me or Cl) react with HMPA to give the base-stabilized silylenes [Fe(SiRR′(HMPA))(CO)4]. The reactions of [Fe2(μ-SiMe2)2(CO)8] with DABCO, THF and PMe3 have also been examined. The crystal structures of both [Fe2(μ-SiMe2)2(CO)8] and [Fe2(μ-SiMeCl)2(CO)8] show planar rings, CH⋯OC short contacts and relatively short but non-bonding SiSi distances. The possibility of through-ring MM or EE bonding in complexes of the type [M2(μ-ER2)2(CO)8] (M is a transition metal and E is a Group 14 element) is analyzed with the help of theoretical calculations based on density functional theory. For compounds with 20 ring electrons (or a framework electron count, FEC, of eight), regular M2E2 rings are expected, with no short through-ring distances. The framework electron counting rules, geometry optimization of several model complexes and a structural database analysis consistently indicate that through-ring bonding exists only when the FEC is less than eight. In that case, the isomer with a short metal–metal bond is found to be significantly more stable than that with a short EE distance.