Unsaturated trinuclear ruthenium carbonyls: large structural differences between analogous carbonyl derivatives of the first, second, and third row transition metals

For the saturated carbonyl trimer Ru-3(CO)(12) theoretical methods predict a doubly bridged Ru-3(CO)(10)(mu-CO)(2) structure to lie only 0.3 kcal mol(-1) above the unbridged global minimum in accord with the fluxional properties observed experimentally by carbon-13 NMR on the very stable unbridged Ru-3(CO)(12). For M-3(CO)(11) and M-3(CO)(10) the global minima for the Fe, Ru, and Os derivatives each have totally different arrangements of the carbonyl groups. Thus the global minimum of Ru-3(CO)(11) is singly edge-semibridged Ru-3(CO)(10)(mu-CO) in contrast to the doubly face-bridged Fe-3(CO)(9)(mu(3)-CO)(2) and triply edge-bridged Os-3(CO)(8)(mu-CO)(3) found as global minima for the iron and osmium analogues, respectively. Furthermore, comparison of our predicted nu(CO) frequencies for low-lying unbridged, singly bridged, and doubly bridged isomers of Ru-3(CO)(11) with the 1987 experiments of Bentsen and Wrighton indicates that three different Ru-3(CO)(11) isomers are generated in these low-temperature inert matrix Ru-3(CO)(12) photolysis experiments depending on the conditions. For Ru-3(CO)(10) the global minimum is a triply bridged structure Ru-3(CO)(7)(mu-CO)(3), which is very different from the Fe-3(CO)(9)(mu(3)-CO) and Os-3(CO)(8)(mu(3)-CO)(mu-CO) global minima found for the iron and osmium analogues, respectively. For Ru-3(CO)(9) the global minimum is a singly bridged structure Ru-3(CO)(8)(mu-CO) analogous to the global minimum for the osmium analogue but totally different from the triply bridged global minimum of Fe-3(CO)(9).