Metal polyhedra

Cluster complexes containing opened transition metal polyhedra. M. O. Albers, D. J. Robinson and N. J. Coville, Coord. Chem. Rev., 1986,69,127 (357). 

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

As indicated in Scheme 11, there are basically two classes of chemistry that have been observed for [Os CO) ]2-. One involves expansion of the ligand coordination sphere, without changing the stereochemistry of the metal cluster, and is electronically related to the parent carbonyl Os5(CO)18, while the second involves redox reactions, with addition of two electrons to the metal cluster and concomitant structural changes in the metal polyhedra (204). 

Bonding concepts for cubanes, as discussed in Chapter 2.3., predict that the metal polyhedra in the cubanes will contract upon oxidation because of increased metal-metal interaction and vice versa. This has been verified for [CpFeS] 381), [CpFe(CO)]4 167, 177), and [CpCoS]4 361). Furthermore, the ease of oxidation and reduction of several cubane-type clusters 166,167, 361, 381), and the delocalization of electrons in the charged species 48, 176,177, 401) is noticeable. This, together with the prefered formation of iron-sulfur clusters, 381), is borne out by the fact that Nature uses iron-sulfur proteins for redox reactions 207). 

Degens, E. T., Matheja, J. Molecularmechanisms on interactions between oxygen coordinated metal polyhedra and biochemical compounds. Techn. Report, Woods Hole Oceanogr, Inst., Ref. No. 57—67 (1967)... 

The metal/ligand ratios in the complexes listed in Table 2 are obviously related to the proportions of vertices, edges and faces of the various metal polyhedra. The charges of the metals and ligands need not balance in these compounds. There is, however, another set of complexes (MX)f or (RMXR )p with no net charge, where the thermodynamics of solvation rather than symmetry might appear to be the primary determinant of composition and structure. In these cages the structure must adapt to the fixed M/X ratio. 

Concentric metal polygons or concentric metal polyhedra occur inner and outer triangles in Section 4.6.1.3 an octahedron outside a cube in Section 4.8.2.4 and inner and outer tetrahedra in Section 4.8.3. 

All these results are summarized in Fig.l 1. However, in any cases, the stability of hydrides is well understood in terms of the nature of the chemical bond between atoms in small metal polyhedra and also of the crystal structural evolution in the course of hydrogenation (25, 28). 

The metal-carbon cluster systems we have considered so far in the present chapter, like the carboranes considered in the previous chapter, have contained one or more skeletal carbon atoms occupying vertex sites on the cluster deltahedron or deltahedral fragment. We now turn to some molecular cluster systems in which hypercoordinated carbon atoms occupy core sites in the middle of metal polyhedra. Most are metal carbonyl carbide clusters of typical formulae Mj (CO)yC. Their carbide carbon atoms are incorporated within polyhedra, which in turn are surrounded by y carbonyl ligands. Such compounds, for which few controlled syntheses are available, have been found primarily among the products of thermal decomposition of polynuclear metal carbonyls Mj (CO)j, their carbide carbon atoms result from disproportionation reactions of carbonyl ligands (2 CO CO2 + C). 

Figure 28 The metal polyhedra in (a) [Mri9Na2(0)7(02CPh)i5(MeCN)2] 27 and (b) [Niio(OH)4(mhp)io(02CCMe3)6(sol)2] 61 illustrating the resemblance to deltahedra. The additional circles required to complete the tetraicosahedron and icosahedron, respectively, are shown as small open circles.

Table 5. Characteristic electron (Ac) for shared units in fused transition metal polyhedra...

This was the first example of a facile, intra-molecular, metal polyhedral rearrangement in a transition metal-carbonyl cluster. Other work showed that related metal polyhedral rearrangements occur when the metal polyhedra are not well close-packed, as when they contain a hetero-interstitial atom, e.g. [Rh9E(0O)2 (E = P or As), [RhioE(00)22]" (n = 3, E = P or As n = 2,... 

The stacked platinum clusters, [Pt (CO)2 ] (n = 6, 9, 12, 15, and 18) consist of trigonal prismatic stacked triangular Pt3 units. The metal-metal bonds within the triangular units are significantly shorter than those between the layers. Variable temperature Pt NMR indicates that the metal polyhedra undergo interconversion between prismatic and antiprismatic structures via the rotation of the triangular units relative to each other. 

The structure of these rhodium clusters with the unique rhodium centre, posed a general question that has underpinned much of the research in this area, namely, when do compounds of this type begin to exhibit the properties of metals This of course involves the question of what criteria are required to define metal properties. No satisfactory solution has as yet been found to the initial question but work in this area has established that there is a gradual variation in properties as the size of the metal polyhedra increases. It would now appear that a spectrum of properties are possible, and that for the very high nuclearity clusters, of the types discussed below, new and distinctive behaviour may be anticipated. If this proves to be the case then this enhanees the possible application of these materials. 

P-07 - The structure of a copper molybdate and its relation to other natural and synthetic porous materials based on transition metal polyhedra... 

A new non-stoichiometric copper molybdate of ideal composition (NH40H)3/2(CuMo04)2 was synthesized hydro thermally and its crystal structure was solved from powder data. The material shows the hexagonal parameters a=6.0775(3) and c=21.601(1) A in the space group R-3m, The green powder exhibits the same building units observed in the mineral Volborthite and the layered structure of other materials based on transition metal polyhedra. 

The conducting skeletons of both the Chevrel phases MMogSg and the ternary lanthanide rhodium borides LnRh4B4 are thus seen to consist of edge-localized discrete metal polyhedra (Mog octahedra and Rh4 tetrahedra, respectively) linked Into a three-dimensional... 

Transition metal analogues of closo boranes are known. They are characterized by a number of valence electrons equal to 14n+2 as a consequence of the filling of the additional five d orbitals. Also, tranation metal polyhedra exhibiting nido and arachno structures (with electron counts of 14n+4 and 14n+6 respectively) have been identified. [227] Thus, the PSEP model represents a very ample, yet powerful method to rationalize or predict the structures of duster compounds... 

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