The Methylene Bridge

Abstract

Publisher Summary Organometallic chemistry encountered rapid expansion experienced by the synthesis, spectroscopy, structural chemistry, theory, and reactivity of compounds characterized by terminal carbene (methylene, A) and carbyne (methylidyne, B) functionalities. The chemistry of μ-methylene complexes did not develop as extensively as that of its mononuclear counterparts, the latter being characterized by terminal carbene ligands. As in the bonding of carbenes with single atoms metal, the methylene group has a filled orbital (a 1 ) that can act as a sigma donor to the system. Symmetrical methylene (alkylidene) bridges represent the predominant geometry. Stability against thermolysis and photolysis is one of the striking properties of dimetallacyclopropanes, especially of those involving carbonyl and cyclopentadienyl ligands on the metals. There is so far not a single exception to the rule that a μ-methylene complex is at least as stable as its μ-carbonyl counterpart. Substitution reactions at the methylene bridge have been observed, albeit the yields were quite low suggesting that major side reactions had occurred. The molecular regimes in discrete di- and polynuclear complexes are mostly coordinatively saturated, whereas metal surfaces, especially if they are relatively flat and close-packed, have no coordinatively saturated surface atoms, even in the presence of chemisorbed species. There is an array of well-established analogues of the μ-methylene complexes in which the heavier congeners of carbon adopt bridging positions.