Delocalized cluster bonding

The versatility of transition-metal-ligand combinations in stabilizing localized, partially localized and delocalized cluster bonding is evident in the three examples discussed to this point. But we still have no analog of a late metal-carbonyl cluster that follows the main-group counting paradigm for an octahedral shape. Centered zirconium chloride clusters provide an example of an octahedral cluster with seven... 

The organometallic compound shown below is represented with a Cr=Cr triple bond in the literature. Describe it as a cluster compound. The measured Cr-Cr distance is 2.34 A consistent with a triple bond. Comment on factors that favor localized Cr-Cr multiple bonding vs. delocalized cluster bonding as a mechanism for accommodating the electronic unsaturation. 

Quantitative Approaches to Delocalized Cluster Bonding Models... 

Cluster Configuration Peripheral Bonds Delocalized (r-honds Delocalized ir-bonds Antiaromaticity... 

When it comes to metal-rich compounds of the alkaline earth and alkali metals with their pronounced valence electron deficiencies it is no surprise that both principles play a dominant role. In addition, there is no capability for bonding of a ligand shell around the cluster cores. The discrete and condensed clusters of group 1 and 2 metals therefore are bare, a fact which leads to extended inter-cluster bonding and results in electronic delocalization and metallic properties for all known compounds. 

Prior to any work on heteroatom clusters the notion was expressed (20) that heteroatom placement within the polyatomic clusters would lead to a decrease In delocalization and bonding and thence stability. Although this may lessen stability the substitution clearly does not preclude It. Furthermore, many of the likely polyhedra already have Inequlvalent atom positions, the 5, 7, 9 and 10 atom examples already considered here for example, and mixed species especially with elements from different groups may be quite stable within the discrimination provided by Inequlvalent positions. Even the nominally equivalent atom positions In a tetrahedron can obviously accommodate substantial differences. Additional examples of mixed element polycations are certainly to be expected. An Inadequate foresight was revealed In a review of polycations (20) written for a 1974 award symposium, about one year before the crypt discoveries, by the expectation that polycations should be more stable than polyanions for the metallic elements. In hindsight, metallic behavior Is a property of the dense solid state and has little to do with the stability of small clusters where electronic and geometric factors are far more important. 

Figure 8.12. The two pertinent empty orbitals are of b2g symmetry (cluster and ligand antibonding) and a2g symmetry (cluster antibonding) and cannot mix. Bending the Ge-H bonds to model the doubly connected Ge9 cluster units in the dimer and trimer lowers the symmetry to D2h and these two orbitals now are of big symmetry and can mix. One is stabilized and the other destabilized as shown. The former is now available at low energy to accommodate the extra lone pair. An important consequence of this exocluster orbital mixing is that the oligomers are not viewed as delocalized clusters connected by localized bonds but single delocalized entities.

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