Clusters, metal unsaturation

Photolytic and chemical methods of activation have also been employed in the reactions of clusters with unsaturated organic ligands. The photochemical activation of compounds containing metal-metal bonds has not received much attention until relatively recently (127). However, it is now being investigated in some detail. In some cases the photochemical products differ from those obtained by thermal activation alone (127). The reaction of Ru3(CO)12 with ethylene is an example of such behavior (128). 

Few other reactions between higher clusters and unsaturated organic ligands have been carried out (231, 232), but there are a number of studies which involve group IB organometallic derivatives (233-236). There are also a few high-nuclearity mixed-metal cluster alkynes which are the products of reactions between two heterometal complexes (237-242). 

Bridges between trinuclear metal clusters [of Ru, Os] Synthesis of ruthenium and osmium carbonyl clusters with unsaturated organic rings Oxyligand derivatives of triosmium dodecacarbonyl... 

The reactivity of metal-carbonyl clusters with unsaturated hydrocarbons gives an idea of what may happen on surfaces in heterogeneous catalysis, although the conditions and pathways are different, because surfaces of heterogeneous catalysts are not saturated with ligands whereas clusters are (see Chap. 20). Many bond activations have been shown (CO, PPhs, alkenes, alkynes, etc.), a simple one being the reaction of ethylene leading to double C-H activation. Such bond activations are common and rather facile because of the proximity of the metals in the clusters ... 

The main synthetic route to high nuclearity metal carbonyl clusters involves a condensation process (/) a reaction induced by coordinatively unsaturated species or (2) a reaction between coordinatively saturated species in different oxidation states. As an example of (/), Os2(CO)22 can be condensed to form a series of higher coordinated species (89). 

One of the m jor attractions in the metal-atom synthesis of dimer and cluster species is the ability to isolate highly unsaturated species, M Lm, that may then be considered to be models for chemisorption of the ligand, L, on either a bare, or a supported, metal surface (,100). It is quite informative to compare the spectral properties of these finite cluster-complexes to those of the corresponding, adsorbed surface-layers (100), in an effort to test localized-bonding aspects of chemisorption, and for deciphering UPS data and vibrational-energy-loss data for the chemisorbed state. At times, the similarities are quite striking. 

Preliminary indications are quite clear that coordin-atively unsaturated transition metal clusters are reactive, and, importantly, reactive in a controlled way. This chemistry is likely to include RC-H insertion, arom-atization etc., with the chemistry tending to produce... 

The reaction with [Mn(CO)5]2 or [Co(CO)4]2 also led to mixed-metal clusters.962,963 Osmium-palladium mixed-metal carbonyl clusters were made using the unsaturated cluster [Os3(CO)io(/r-H)2] and the carbido cluster [Os5(/U5-C)(CO)15].964 970 Treatment of [Os3(CO)10Gu-H)2] with [Pd2(/u-dppm)2Cl2] afforded the novel high-nuclearity osmium-palladium mixed-metal carbonyl clusters [Os5Pd6(CO)13( -CO)5(/u-H)2(/u-dppm)2], [Os5Pd6(CO)13(//-CO)6(/u-dppm)2], and... 

An alternative approach is to relate variations in reactivity with metal clusters structures. Unfortunately even less is known about their geometric arrangement than their electronic structure. Suggestions made in the literature will be summarized and simple ideas of the degree of coordi native unsaturation will be presented. 

In conclusion, the gas phase photofragmentation of Co (C0) CCH shows a wavelength dependence that, with the proper selection of wavelength, gives the experimentalist access to a range of interesting unsaturated metal and metal-carbon clusters. 

In our laboratory we have examined the reactivity pattern of [0s3(y-H)2(C0)10], an unsaturated cluster which can be represented as possessing an osmium-osmium double bond in its classical valence bond representation. We find (2,3) that this compound undergoes a number of reactions with metal carbonyls which in some cases can be formulated as proceeding through intermediates analogous to metal olefin complexes ... 

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