Reaction chemistry, NMR spectroscopy, and X-ray crystallography of [Fe-2(mu-SiMe2)(2)(CO)(4)] and [Fe-2(mu-SiMeCl)(2)(CO)(4)]. Electronic structure and bonding in Fe2E2 rings of [Fe-2(mu-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 t o give the base-stabilized silylenes [Fe(SiRR'(HMPA))(CO)(4)]. The reaction s of [Fe-2(mu -SiMe2)(2)(CO)(8)] with DABCO, THF and PMe3 have also been ex amined. 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 t hrough-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 c ompounds with 20 ring electrons (or a framework electron count, FEC, of eig ht), regular M,E, rings are expected, with no short through-ring distances. The framework electron counting rules, geometry optimization of several mo del 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.