Fragments, isolobal

In this section we will briefly discuss some open problems. The metala-logy principle leads to new chemical relationships of preparative importance. The concept of isolobal fragments (the metal with all ligands except the combined ir-systems), developed by R. Hoffmann and coworkers will be extremely helpful from the theoretical standpoint. On the other hand this empirical principle raises some questions, the answering of which will acquire preparative relevance. 

Parallels with Nonmetal Chemistry Isolobal Fragments... 

We shall encounter further examples of isolobal fragments later in this and subsequent chapters, but for now we can sum up their essential features as follows Two fragments are isolobal if the number, symmetry properties, approximate energies, and shapes of their frontier orbitals and the number of electrons in them are similar. 

Exercise 4.2. What hydrocarbon cluster is analogous to Co4(CO)i2 Write the main-group and metal isolobal fragments. Write out the compositions and draw the structures of all the mixed Co/C clusters possible between these two homonuclear Co and C clusters. 

Scheme 1 illustrates how isolobal fragments can be generated and specifies what the frontier orbitals are that were... 

Using the second row of isolobal fragments (40)-(43), one can replace one, two, or all three CH2 units in cyclopropane (51) by isolobal partners to form the molecules listed in (52). Notice that the top row of structures have been drawn as metallacyclopropanes and that these are equivalent (see Section 5.3) to metal-aUcene complexes. The essential details of the bonding and their conformational preferences are identical in each molecule. 

As a final example of the isolobal analogy consider the bottom row of isolobal fragments in Scheme 1. In particular. 

The existence of mixed metal-boron clusters constitutes one example of the validity of the isolobal principles when applied to a class of compounds. The stoichiometric and structural diversity of metallaboranes is truly striking, and the idea of isolobal fragments combined with the electron counting rules is sufficient to rationalize this diversity—this fact is impressive (67,68,69, 70, 71, 72, 73, 74). We have explored the mixing of boron and metal characters in cluster MOs in the Bs nHg-nlFeCCOla) ] nido cluster series (Fig. 5). Examples of the first four members of this series (a, b, c, and d) have been well characterized (75, 76, 77), and suggestive evidence has been published for a B2H6RU3-... 

Similarly, 6-electron CH2 and 16-electron ML4 are isolobal. Both CH2 andML4 are 2 electrons short of a filled shell octet or 18-electron configuration, so they are electronically equivalent each has 2 single electrons occupying hybrid orbitals at otherwise vacant sites. Absence of a third ligand similarly gives a pair of isolobal fragments, CH and ML3. 

The concept of isolobal fragments can be extended beyond the examples given so far to include charged species, a variety of ligands other than CO, and organometallic fragments based on structures other than octahedral. Some of the ways of extending the isolobal parallels can be summarized as follows ... 

Gain or loss of electrons from two isolobal fragments yields isolobal fragments. For example,... 

Examples of isolobal fragments containing CO and ligands are given in... 

Other octahedral isolobal fragments can be found by changing the metal and the charge on the complex. A positive charge compensates for a metal with one more electron and a negative charge compensates for a metal with one less electron ... 

Examples of electron configurations of isolobal fragments of polyhedra having five through nine vertices are given in Table 15-4. 

Hoffmann has described the following molecules to be composed of isolobal fragments. 

Verify that the following compounds are composed of isolobal fragments ... 

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