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

Abstract

For the saturated carbonyl trimer Ru₃(CO)₁₂ theoretical methods predict a doubly bridged Ru₃(CO)₁₀(μ-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 Ru₃(CO)₁₂. For M₃(CO)₁₁ and M₃(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 Ru₃(CO)₁₁ is singly edge-semibridged Ru₃(CO)₁₀(μ-CO) in contrast to the doubly face-bridged Fe₃(CO)₉(μ₃-CO)₂ and triply edge-bridged Os₃(CO)₈(μ-CO)₃ found as global minima for the iron and osmium analogues, respectively. Furthermore, comparison of our predicted ν(CO) frequencies for low-lying unbridged, singly bridged, and doubly bridged isomers of Ru₃(CO)₁₁ with the 1987 experiments of Bentsen and Wrighton indicates that three different Ru₃(CO)₁₁ isomers are generated in these low-temperature inert matrix Ru₃(CO)₁₂ photolysis experiments depending on the conditions. For Ru₃(CO)₁₀ the global minimum is a triply bridged structure Ru₃(CO)₇(μ-CO)₃, which is very different from the Fe₃(CO)₉(μ₃-CO) and Os₃(CO)₈(μ₃-CO)(μ-CO) global minima found for the iron and osmium analogues, respectively. For Ru₃(CO)₉ the global minimum is a singly bridged structure Ru₃(CO)₈(μ-CO) analogous to the global minimum for the osmium analogue but totally different from the triply bridged global minimum of Fe₃(CO)₉.