Isolobal replacement

Consider, for example, the species shown in Fig. 1-24. The practical importance of these isolobalities is that, in general we may expect the existence of stable, isostruct-ural molecules in which one component is replaced by another that is isolobal to it. As an illustration of this, all of the following clusters may be regarded as related by isolobal replacements in each case, there is a tetrahedral core ... 

Summary Isolobal replacement of the NRj-unit in silatranes 1 by a d ML4-unit leads to metallosilatranes 2. The cage compounds 2 show weak, attractive donor-acceptor interactions. As observed by X-ray difihaction there exist sub-van-der-Waals distances between silicon and nickel. The proposed 3-center 4-electron interaction is in good agreement with some unusual NMR data of 2. 

Silatranes 1 are meanwhile classical cage compounds with donor-acceptor interactions and represent examples of hypercoordinated silicon [2], The donor-acceptor contact in 1 is formed by an interaction of the Lewis-basic amino group with the Lewis-acidic silicon center favored by the chelate effect. Numerous examples show that electron-rich transition metal complexes also possess Lewis-basic properties [3, 4]. Isolobal replacement of the NR3-unit in 1 by a d ML4-unit [5] leads to compounds of type 2 [1,6, 7]. These Si/Ni-cages 2 can be regarded as metallosilatranes. Here we report on the s5mthesis, structure and bonding of 2. 

Isolobal replacement of the exocyclic methylene group of fulvene with a cobalt fragment generates cobaltafiilvenes ... 

FIGURE 25 Structure of B4HsFe(CO)3. The metalloborane derived from the isolobal replacement of B—H by Fe(CO)3. 

FIGURE 13,5 Stepwise isolobal replacement of CH by CofCO) in tetrahedrane. Co4(CO)i2 has the CO bridged structure shown. 

The isolobal replacement of the BH group can also be tried with the saturated ring B3H9 (24). The replacement of the terminal B-H bonds by Si leads to 25 (A = Si). This is indeed very high in energy compared with the classical structure (26). However, the energy difference between 25 and 26 decreases and eventually reverses with A = Pb when the terminal B-H bonds are replaced (Table 3). It is not easy to arrive at these structures in any other way. 

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