Metal Carbonyl Cluster Fluxionality

Only a few homo- mid heterometallic dodecacarbonyl compounds have been the subject of variable-pressure studies for various reasons insolubility (Ir4(CO)i2), intensity distortions in the NMR spectra (Co4(CO)i2 [39] and most other systems containing cobalt [40]), two parallel fluxional processes with similar rates (M3(CO)i2-x(NCCH3) r (M = Ru, jc = 1,2, 3 M = Os, x = 1, 2 [41], complex mixture of conformers (Fe3(CO)i2 [42]), or static behaviour (OS3(CO),2). 

The ground state geometry of Rli4(CO)i2 in solution has C3V symmetry [43]. A recent simulation of its variable-temperature and -pressure NMR spectra 

Carbonyl a remains unaffected, excluding site exchanges by a merry-go-round of six CO groups, as proposed for Rli4(CO)i2. Since restricted axial-basal site exchanges are observed, this fluxional process may be called a change of basal face (Rh(l)-Rh(2 -Ir(l) to Rh(l)-Rh(2)-Ir(2)). This new process, which has a positive volume of activation [49], is also encountered in the following systems. 

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Complex exchange networks, for example, CO fluxionality in a metal carbonyl cluster, can be readily mapped. 

The vast majority of transition metal clusters contain carbonyl ligands, which have been shown in many cases to be fluxional on the metal skeleton of the cluster (40,41). Therefore, the most obvious reactions to be catalyzed by such clusters should be those involving carbon monoxide. In fact, catalytic carbonylations are frequently encountered with transition metal carbonyl cluster catalysts, but very often the carbonylation step is followed by a consecutive step, e.g., a hydrogenation step, to give an overall hydroformylation. Simple carbonylation reactions have nevertheless been observed for various structures. 

MM modeling of the soft PES of metal carbonyl clusters has been made possible by a local connectivity approach which, freeing metal-to-ligand connectivity while tightening cluster-to-ligand connectivity, has enabled the fluxionality of ligands to be taken into account it has been shown theoretically that intramolecular steric interactions alone fail to account for the ligand stereochemistry of transition metal carbonyl clusters. At present, in our BMCCs model, and perhaps in the real... 

The dynamic behavior, or fluxionality, of metal carbonyl clusters (and indeed of organometallic molecules in general) was first recognized with the advent of nuclear magnetic resonance spectroscopy, especially C An early example of... 

G2.2 Dynamics and fluxionality in metal carbonyl clusters some old and new problems... 

The already voluminous review literature on clusters will be considered as a basis for this review. The topics treated so far are clusters in general (109, 241) and in connection with metal-metal bonding (30, 338, 380), special types of clusters like those with TT-acceptor ligands (231), hydrides (233), carbonyls (85, 86) or methinyl tricobalt enneacarbonyls (313, 317) properties of clusters like structures (56, 316), fluxionality (110), mass spectra (226), vibrational spectra (365), and redox behavior (292). Clusters have been treated in the context of metal carbonyls (3, 4), metal sulfur complexes (2, 381), and in relation to coordination polyhedra (297). Reviews... 

The metal clusters of the iron triad M3(CO)i2, 4 M = Fe, 5 M = Ru, and 6 M = Os, despite being some of the simplest carbonyl clusters known, are fascinating molecules, which still present interesting structural and dynamic problems. Because of their simplicity, they have been, and indeed remain, archetypal molecules for discussions on fluxional mechanisms. The two heavier congeners 5 and 6 have a clearer and less complicated history. In the solid state they are isostructural, and possess... 

Since 1970 or before, chemists have relied on classical, detailed temperature-dependent line-shape analyses.Indeed, fundamental contributions to our understanding of the dynamics of fluxional metal complexes with vr-hydrocarbon ligands,tertiary phosphorus donors, as well as TT-allyl anions,all stem from these types of measurement. Their contributions to metal carbonyl dynamics and rearrangements in cluster compounds is even more pronounced, and we cite selected studies in this very large area of organometallic chemistry.For slow exchange... 

Ligand fluxionality on metal clusters has been the subject of many studies, the majority of reports focusing on carbonyl migration on homometallic tri- and... 

Table XI summarizes reports of ligand fluxionality at "very mixed -metal clusters. A number of studies (c.g.. Refs. 28, 8I, 116, 213, 214) have reported that both metal-localized and global carbonyl fluxionality occur but give little mechanistic detail. The following discussion focuses on the examples for which detailed studies have been undertaken and/or those for which mechanistic speculation is available.

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