Main-group cluster ligands

Although the principles of cluster bonding developed for main-group clusters carry over to transition-metal clusters of the group 8/9 metals with carbonyl ligands, we fully expect transition metals to exhibit variations on this cluster bonding theme as well as novel behavior not seen in main-group systems. In this section we introduce those aspects of cluster chemistry characteristic of transition-metal clusters. 

To summarize this long section on metal effects, we can state metal clusters can mimic main-group clusters (late-metal clusters with acceptor ligands) metal clusters with four-connect or higher vertices can be described with localized bond models (early-metal clusters with donor ligands) and metal clusters can have reduced (Pt clusters) or no tangential bonding (Au clusters). The characteristically more... 

NMR studies of solutions of inter-metallic phases led to recognition of the potential use of polyatomic Zintl ions as reagents for the generation of new cluster compounds (Eichhorn et al., 1988). Examples of transition-metal derivatives have been discussed in Chapter 5 and the structures of the ions themselves were used as examples of ligand-free main-group clusters in Chapter 2 (Corbett, 1985 Fassler, 2001). The following examples illustrate recent developments in the chemistry of these clusters derived from solid-state syntheses. 

The analysis of these molecules is complicated by the presence of the d orbitals, which contribute one orbital of CT symmetry (d 2 in local axes), two of n symmetry (d and Ayg), and two of 5 symmetry (d y and dj2 j,2). There are far more examples of deviations from the usual patterns in transition metal clusters than in main group clusters, and many of these must be treated specifically, although TSH theory may again provide a useful framework in which to perform the analysis. However, in this section the objective will simply be to understand the most common patterns, as set out in Section 3, which are typically found in clusters with u-acceptor ligands such as CO. 

In transition metal-main group element clusters, there is the possibility of ligand substitution at either type of element center. Displacement of an exo-cluster ligand on one of the metal centers (equation 10) is to be expected (see Mechanisms of Reaction of Organometallic Complexes) However, displacement at a main group cluster site has also been observed (equation 11 ). Indeed, phosphine substitution takes place exclusively at the boron atom, and the osmium-substituted BCO complex can only be prepared by synthesizing it from the phosphine-substituted osmium carbonyl starting material. 

Fig. 2.13. Examples of structures of triangular metal clusters containing main-group bridging ligands...

Of the iron clusters with main group element ligands summarized in this chapter, those with group 16 ligands, in particular, the heavier S, Se, and Te ligands, have been studied most extensively. 

Halet )-F, Saillard )-Y (1997) Electron Count Versus Structural Arrangement in Clusters Based on a Cubic Transition Metal Core with Bridging Main Group Elements. 87 81-110 Hall DI, Ling JH, Nyholm RS (1973) Metal Complexes of Chelating Olefin-Group V Ligands. 15 3-51... 

The Wade rules can be applied to ligand-free cluster compounds of main-group elements. If we postulate one lone electron pair pointing outwards on each of the n atoms, then g — 2n electrons remain for the polyhedron skeleton (g = total number of valence elec-... 

This chapter will mainly use examples of coordination phenomena involving transition metals, but where necessary and useful examples may include the coordination behavior of main group metals. There may also be occasion to give examples involving nonmetal systems. The ions used as examples will be both positive and negative ions such as the simple bare metal ions M+ or M , cluster metal ions M and M , and other metal containing ions M E (where E can be another metal, element, or ligand). 

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