430
Organometallics 1983,2, 430-434
parameters suggests that Charton is correct in his assessment that steric effects are more sensitive to reaction type than to the identity of the atom under attack and that a reaction involving expansion of coordination from four to five will show a different sensitivity to a and P branching than a reaction involving change of coordination from three
to four.
Acknowledgment. The statistical analyses were performed on PROPHET, a biomedical computer system sponsored by the National Institute of Health, Division of Research Resources.
UV-PES, 13C NMR, and Theoretical Studies on the Alkyne-Cluster Interaction in Fe,( CO),( pa-q2-EtC2Et) Gaetano Granozzi, * la Eugenio Tondello, la Maurizio Casarin,lb Silvio Aime,lc and Domenico Osella'c Istituto di Chimica Generaie ed Inorganica, University of Padova, 35100 Padova, Italy, Istituto di Chimica e Tecnologia dei Radioelementi del CNR, 35100 Padova, Italy, and Istituto di Chimica Generaie ed Inorganica, University of Torino, Torino, Italy Received June 29, 1982
13C NMR, UV-PES, and CNDO theoretical results of Fe3(CO)g(p3-vP-EtCZEt) are reported. 13Calkyne 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(C0I3unit. The gas-phaseUV-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 electronicstructure proposed by Blount et al. receives strong support by the present theoretical data.
Introduction The Dewar-Chatt-Duncanson model,2 originally developed to describe the bonding interaction in transitionmetal olefin complexes, has been successfully extended to include the description of the a-?T bond in mononuclear alkyne c~mplexes.~This bonding scheme, however, is unable to account for the interaction of alkynes chemisorbed on transition-metal surfaces; actually the strong alkyne-surface binding rises from multicentered interactions, which imply significant rehybridization of the acetylenic carbons and formation of strong a-carbon-metal bonds.4 The very rich chemistry of alkyne-cluster compounds5 provides a variety of structural arrangements, which can be envisaged as reasonable models for chemisorbed species on metallic surfaces.6 According to Muetterties et al,,6b three basic bonding modes (namely, p 4 y 2 ,p 3 q 2(I) and , p3q2(11) types) can account for the interaction between the (!) (a) University of Padova. (b) CNR of Padova. (c) University of Torino. Chem. Fr. 1951,18, C71. (b) Chatt, (2) (a) Dewar, M. J. S. Bull. SOC. 1953, 2939. J.; Duncanson, L. A. J . Chem. SOC. (3) For a recent detailed discussion see: Tataumi, K.; Hoffmann, R.; Templeton, J. L. Inorg. Chem. 1982, 21, 466. (4) For a general survey on this topic see: (a) Muetterties, E. L. Angew. Chem., Int. Ed. Engl. 1978,17, 545. (b) Ozin, G. A,; McIntosh, D. F.; Power, W. J.; Messmer, R. P. Inorg. Chem. 1981, 20, 1782. (c) Geurta, P.; Van Der Avoird, A. Surf. Sci. 1981,103, 431 and references cited therein. (5) Johnson, B. F. G.; Lewis, J. Adu. Inorg. Chem. Radiochem. 1981, 24. 225. (6).(a) Ugo, R. Catal. Reu. 1975, 11, 225. (b) Muetterties, E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.; Pretzer, W. R. Chem. Reo. 1979, 79, 91.
n
A A !J T, 2
Lc3-ri2
(1)
Lc3-riz
(11)
alkyne and a metallic cluster. It is generally accepted that in this class of molecules the interaction with three or four metal centers leads to an enhanced activation of the coordinated alkyne when compared with mono- and binuclear complexes. The bond order reduction can be compared with sufficient confidency to that occurring in chemisorption experiments on metallic surfaces. This cluster-surface analogy! then provides a novel methodological route to the study of surface phenomena by means of molecular spectroscopic techniques. In an attempt to get a more detailed picture of the multicentered ligand-metal cluster interactions, we have already investigated' compounds of type I, which, on the basis of the above considerations, represent good models for the dissociative chemisorption of monosubstituted alkynes (and acetylene itself) on metal surfaces by cleavage of the C-H bond. We report here the results of a 13CNMR, UV-PES, and theoretical (CNDO) study of the strictly related system Fe3(C0)9(p3-v2-RCzR) (11, R = C2H5),which can be envi(7) (a) Granozzi, G.; Tondello, E.; Aime, S.; Osella, D. "Euchem Conference the Challenge of Polynuclear Inorganic Compounds", Venice, 28-30 September 1981, Commun. C9. (b) Granozzi, G.; Tondello, E.; Bertoncello, R.; Aime, S.; Osella, D. Inorg. Chem., in press.
0276-7333/83/2302-0430$01.50/0 0 1983 American Chemical Society
Organometallics, Vol. 2, No. 3, 1983 431
Alkyne-Cluster Interaction in Fe,(CO)S(CL3-02-EtCZEt)
(hI=Ku, U = B u t l
1h
('
