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 [Fe-2(mu -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(mu -SiMe2)(2)(CO)(8)] with DABCO, THF and PMe3 have also been examined. The crystal structures of both [Fe-2(mu -SiMe2)(2)(CO)(8)] and [Fe-2(mu -SiMeCl)(2)(CO)(8)] show planar rings, C-H . . .O dropC 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,(I-l-ER,),(CO),I (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,E, 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. (C) 2001 Elsevier Science B.V. All rights reserved.