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

Abstract

The rings [Fe 2(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 [Fe 2(μ-SiMe 2) 2(CO) 8] with DABCO, THF and PMe 3 have also been examined. The crystal structures of both [Fe 2(μ-SiMe 2) 2(CO) 8] and [Fe 2(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 [M 2(μ-ER 2) 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 M 2E 2 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.