UV-PES, ₁₃C NMR, and Theoretical Studies on the Alkyne-Cluster Interaction in Fe₃(CO)₉(μ₃-η²-EtC₂Et)

13C NMR, UV-PES, and CNDO theoretical results of Fe3(CO)9(μ3-η2-EtC2Et) are reported. 13C alkyne resonances have very different chemical shifts according to the different coordination environments of the two acetylenic carbon atoms. Variable-temperature NMR experiments indicate localized exchange processes of the carbonyls at each Fe(CO)3 unit. The gas-phase UV-PE spectrum is assigned by comparison arguments with data on simpler related systems. The CNDO results contribute to the discussion of the PE data and provide interesting insights into the alkyne-cluster bonding mechanism. The relevant role of the cluster-↠alkyne back-donation is stressed by both the experimental and theoretical data. The qualitative picture of the electronic structure proposed by Blount et al. receives strong support by the present theoretical data.