Bonding tetrahedral clusters

The bonding situation of the tetrahedral clusters may be described by the qualitative MO scheme that is depicted in Figure 2.3-5 and in which the orbitals involved in the bonding of the terminal ligands are ignored [33, 35, 53], The monomeric fragments M-X have a lone electron pair in a cr-orbital as the HOMO and a degenerate set of two orbitals of re-symmetry as the lowest unoccupied state. Linear... 

The crystal structure of the complex between silver iodide and piperidine has been determined.57 The colourless crystals were prepared by warming silver iodide with sufficient piperidine to allow the silver iodide to dissolve and then allowing the resulting solution to cool. The structure consisted of tetrahedral clusters of iodide ions with the silver atoms embedded into the faces of the tetrahedron. The (Agl)4 clusters were separated by the piperidine molecules which were bound to the silver via the N atom. The Ag—N bond lengths were 232.9 pm, while the Ag—I distances were 285.3, 293.6 and 294.2 pm. 

Let us compare the MO descriptions of six and four sep R4E4 tetrahedral clusters. In Figure 2.5 the cluster MO energies for E = C and E = Ga are shown alongside the localized descriptions discussed above. The number of filled and unfilled MOs is equal for E = C a consequence of the two-center-two-electron bond model. Conversely, the number of filled MOs is less than the number of empty MOs for E = Ga a consequence of the three-center-two-electron bond model. Note also that an e symmetry pair of orbitals lies in between lower-energy filled orbitals and... 

For example, the thermal reactions of Os6(CO),8 with CO (56, 425) and P(OMe)3 (56, 98, 392) lead to the open raft species Osg(CO)2i and [Os6(CO)2i ) P(OMe)3 J (n = 1-6), respectively, probably by sequential bond hssion in the bicapped tetrahedral cluster. As indicated in Scheme 21, under slightly different reaction conditions, elimination of an osmium vertex occurs, resulting in the formation of clusters containing a trigonal-bipyrami-dal metal framework. These react further with nucleophiles to give open Osj clusters and finally substituted triosmium species. Cluster unfolding has also beeen observed in the reactions of Os5(CO),jH2 with a variety of nucleophiles (Scheme 22). 

The tetrahedron constitutes a special case in that the number of skeletal bonding MOs is usually 6, accommodating 6 electron pairs since this is also the number of edges, most tetrahedral clusters can also be viewed as classical species having one electron pair for each bonding interaction. 

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