Interstitial carbido clusters

Some Structural Data for Interstitial Carbido Clusters... 

In clusters with octahedrally encapsulated carbon atoms, the observed values for the radius of the interstitial atom are much smaller. An illuminating comparison is provided by contrasting the optimum carbon radius in [Co8C(CO)l8]2- (0.74 A) with that for Co6C(CO)fr (0.56 A) 63), which has an octahedral core. Indeed all the octahedral carbido clusters have similarly compressed interstitial cavities (Table IV) and as has been pointed out previously, this fact militates against invoking minimum steric requirements of the carbon atom to explain distortions in octahedral carbido clusters. 

Fig. 16. Structures of the clusters (Cu2Ru6C(CO) 16(NCMe)2] and [Cu2Ru6(CO),8-(C6H5Me)2], with the CO ligands omitted for clarity, showing the change in skeletal geometry when an interstitial carbido ligand is formally replaced by two CO groups. [Reprinted with permission of Freund Publishing House Ltd. (11).]...

Os, and having the same essentially octahedral skeletons, nevertheless differ appreciably in the disposition of the attached carbonyl groups. The incorporation of interstitial (encapsulated) atoms such as C, H, S, N, P and, more recently, is a widespread and frequently stabilizing feature of these clusters. Carbido clusters are the most common the C contributing 4 electron.s... 

MoFe-Protein contains a second cluster, the M-cluster. [33] This is covalently bonded to cysteine and histidine of the a-suhunit. A homocitrate anion R)-2-Hydroxyhutane-l,2,4-tricarhoxylic acid) is complexed as a bidentate ligand with molybdenum (Fig. 4.5). On the basis of better resolved X-ray structural analysis and quantum mechanical calculations, initially it was thought, a nitrogen atom is located in the centre of the Fe/Mo complex, [34] but more recent results gave evidence for an interstitial carbido ligand [35,36], which is unprecedent in bioinorganic chemistry. 

There are as yet no thermochemical data to support the notion that the carbido carbon atom imparts additional stability to the cluster that surrounds it, but on the basis of observations made in the course of synthetic studies with these compounds, it is clear that the presence of the interstitial atom provides additional robustness to the molecule (86). Similar assertions have been made for clusters with other interstitial atoms (87). 

Paolo Chini began his work in the late 1950s with the characterization of cobalt carbonyl species involved in the hydroformylation of olefins with cobalt catalysts, and in the course of these studies developed improved synthetic methods for the known cobalt carbonyls Co2(CO)8 and Co4(CO)12 [132]. His next steps were the preparation of the heterometallic hydrido complex HFe-Co3(CO)i2 (isoelectronic to Co4(CO)12) and the corresponding anion [FeCo3(CO)12], both a novelty at that time, and of the new hexanuclear cobalt clusters [Co6(CO)15]2, [Co6(CO)14]4, and Co6(CO)16 [133-139]. This work was followed by the synthesis of carbido carbonyl cluster anions [Co6(CO)i4C], [Co6(CO)15C]2 and [Co8(CO)i8C]2, containing an interstitial... 

An interstitial H atom located in a tetrahedral environment was first reported in 1982, in the tetracapped octahedral cluster [HOsioC(CO)24] . Evidence came from X-ray studies which showed complete coverage of the metal core by carbonyl ligands very similar to that of [OsioC(CO)24] from which it was prepared. It was therefore assumed that the hydride was in an interstitial site, and because the octahedral cavity was already occupied by a carbido-atom it seemed reasonable to assume that the hydride was sited within a tetrahedral cap. The Os satellite pattern associated with the hydride signal in the H NMR spectrum was entirely as would be expected for a tetrahedrally coordinated hydride, which helped to confirm this proposal. 

Because the radius of an octahedral cavity is 0.414 times the metallic radius of the metal atoms on the surface of the cluster, only the first-row main-group atoms can be incorporated in this site. There are several octahedral carbido- and nitrido-clusters, but examples are also known that incorporate hydrogen and boron. A series of related clusters include [Ru6H(CO)i8] , [Ru6B(CO)i7] , [RueQCO)] ], and [Ru6N(CO)i6] , all of whieh contain a fully encapsulated interstitial heteroatom lodged in the center of the octahedral cavity. 

Core expansion of the /.ts-oxotriiron cluster 107 is reported (Scheme 8). Interaction with Mn and Re electrophiles leads to the formation of the heterotetranuclear /X4-0X0 clusters with the butterfly metal array 108 and the ji -oxo clusters with expanded metal frameworks 111-113, " which arise by Fe20 108 and Fes face capping 111-113, respectively. Interstitial clusters (e.g., 14- and 15-type complexes Scheme I) are rare for the 0x0 cluster compounds (see below) in contrast to the related carbido and nitrido clusters. 

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