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)