DENSITY-FUNCTIONAL THEORY CALCULATIONS FOR SIMPLE OXAMETALLACYCLES - TRENDS ACROSS THE PERIODIC-TABLE

Density functional theory (DFT) was used to calculate the structures o f simple oxametallacycles, generated by inserting one or two metal ato ms in the C-O bond of the ethylene oxide molecule, for 14 different tr ansition metals (groups VIIIa-c, Ib, IIb). The calculated energetic st ability of these complexes generally decreases from left to right of t he periodic table and from top to bottom of each group of metals. Silv er, which is unique among these metals in catalyzing selective ethylen e epoxidation, gives an oxametallacycle intermediate that is more stab le than ethylene oxide by ca. 10 kcal/mol. The modest stability of thi s intermediate may explain the formation of the less energetically fav orable C2H4O isomer, ethylene oxide, on silver catalysts. The calculat ed relative stabilities of oxametallacycles versus acetaldehyde on var ious metal surfaces are consistent with previous experimental observat ions that an oxametallacycle is the preferred intermediate in ethanol decarbonylation on the Rh(111) surface, whereas eta(2)-CH3CHO is prefe rred on the Pd(111) surface.