Types of Cluster Compounds

Thomas A. Albright, Jeremy K. Burdett, and Myung-Hwan Whangbo. 

I + I ] = 72 electrons. For an 18 electron count at each metal one needs 18(5) = 90 electrons. Therefore, there need to be 18/2 = 9 Os-Os bonds. This is exactly the number of edges in 22.4. In 22.5 there are [12(2)+ 10(6)+ 2] =86 electrons. For each nickel atom to be 18 electrons we need 18(6) =108 electrons. The number of Ni-Ni bonds is then 22/2= 11. But there are 12 edges therefore, the molecule must posses two delocalized three-center two-electron bonds in the molecule, (c) A few metallacarboranes are given by 22.6 [14] and 22.7 [15], 

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To illustrate the distinction between the two types of cluster compounds the examples shown in Fig. 1 may be examined. 

The [Ms(/i3-E)(/t2-E)8Lg] (I), [ (/ - )(/ 2- 2) 8] (II), and [M3E5Lg] (III) types of cluster compounds (Fig. 29) are known, and because of the importance of triangular cluster units as building blocks of higher clusters, much effort has been put into the synthesis of such clusters. 

The [Bai4CaN6] cluster can be considered as a Ca-centered Bas cube with each face capped by a Ba atom, and the six N atoms are each located inside a Bas tetragonal pyramid, as shown in Fig. 12.6.6. The stmcture can be viewed alternatively as a cluster composed of the fusion of six N-centered BasCa octahedra sharing a common Ca vertex. In the synthesis of this type of cluster compounds, variation of the atomic ratio of the Na/K binary alloy system leads to a series of compounds of variable stoichiometry Bai4CaN6]Na,v withx = 7,8,14,17,21... 

These types of cluster compounds can also be obtained from reactions in solution. For example, [Na(12-C-4)2]2[Cu4(Se4)3] results from the reaction of CuO, NazSe, and 12-C-4 in DMF. [132, 133]... 

A third type of cluster, [FePt2(CO)s P(OPh)3 3], was isolated from the reaction between Fe2(CO)g and [Pt P(OPh)3 4], for which the structure (73a) was proposed.These clusters join a growing number of such compounds which do not obey the effective atomic number rule. 

Johnson et al. (132) have also discussed the validity or otherwise of such analogies. Spectra from the same type of surface species that give similar vibrational spectroscopic patterns can show readily measurable frequency variations that may be much more significant in reactivity terms. However, it seems clear that in many cases the overall spectral patterns of cluster-compound ligands do provide useful and reliable fingerprints for identifying the structures of surface species. 

In their original paper (2) on the structure of Fe5C(CO)l5, Dahl and co-workers assigned two bands in the infrared spectrum of hydrocarbon solutions of the cluster, at 790 and 770 cm-1, to vFeC modes. This assignment has been confirmed by a recent study of the infrared spectra of the series M5C(CO)15, (M = Fe, Ru, Os) (78). The room temperature spectra of the compounds (Table II) in the solid state are quite similar to each other, comprising three bands assigned as the a, and e modes (split in the solid state) expected for the C4 symmetry of the isostructural clusters. At low temperature the ruthenium and osmium clusters exhibit five absorptions associated with M-C stretches, whereas the iron cluster retains its room temperature spectrum. This is ascribed to the presence of two types of cluster molecule in the crystal lattices of the ruthenium and osmium clusters which are isostructural with, but not isomorphous with, the iron analog in which all the molecules are identical. 

The most unusual geometries among these types of clusters are clearly the multiple-decker sandwich structures observed for vanadium ion-benzene clusters Vn(C6H6)+ [142, 151-157]. One interesting finding is that early transition metals all appear to form sandwich compounds with benzene but for the late metals the benzene adducts coat a central metal cluster [158]. 

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