The gas phase photofragmentation of transition metal cluster complexes is discussed. The information available for the gas phase dissociation of Mn 2 (CO) 10 , Co 3 (CO) 9 CCH 3 and Mo 2 (O 2 CCH 3 ) 4 is compared to the well established data base on cluster compounds in condensed phase. The gas phase results are discussed in light of the predictions of electronic structure theory concerning the photofragmentation of these cluster compounds.The study of metal containing compounds proved to be a difficult conceptual and technical task. To the experimentalist the difficulty comes from the fact that standard high resolution spectroscopic methods are often not applicable to the study of organometallic complexes.As experimental chemical physics added to its repertoire sensitive time and energy resolved spectroscopic probes, it opened the way for the exploration of metal containing molecules. At the same time, theoretical chemistry is developing methods able to handle many electron systems necessary for the study of transition metal complexes.The rich photochemistry of transition metal complexes has been used extensively to address questions concerning reactions of molecules in excited electronic states (1). These studies are motivated by expectations that such information will find practical application in the systematic design of organometallic reactions and catalytic processes (2^,4).To date, most of the photochemical data available for transition metal complexes comes from condensed phase studies (1). Recently, the primary photochemistry of a few model transition metal carbonyl complexes has been investigated in gas phase (5.). Studies to date indicate that there are many differences between the reactivity of organometallic species in gas phase (5,6) as compared with matrix (7-10) or solution (11-17) environments. In most cases studied, photoexcitation of isolated transition metal

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