Isolobal approach

A.W. Adamson, University of Southern California You demonstrated the usefulness of the isolobal approach as a correlating one. Can you, however, give some examples of how the approach has led to experimental chemistry that would not have been suggested by other rationales ... 

F.G.A. Stone I had thought that the answer to your question had been embodied in the lecture. However, perhaps I failed to stress that the isolobal approach in our work has been predictive rather than a mere correlation of apparently unrelated results. I would highlight two experiments which emphasize the predictive character of the research. 

The isolobal approach was used in the previous section to describe the formation of metal-metal bonds. These bonds differ from others only in the use of d orbitals on both atoms. In addition to the usual ct and n bonds, quadruple bonds are possible in transition metal compounds. Furthermore, bridging by ligands and the ability to form cluster compounds make for great variety in structures containing metal-metal bonds. 

A theoretical approach addresses the question of alkynes bonded to PtL2 fragments in both parallel and perpendicular geometries. With each mode of alkyne coordination there is required a different coordination geometry at platinum. The authors use the isolobal analogy to calculate the electronic structures of complexes, and propose several unknown complexes to be stable.842... 

An isolobal relationship exists between 5 and ethene. The logic goes that it should therefore be possible to construct molecules containing the Cp2-Rh2(CO)2 unit that are formally analogous to ethene complexes. In practice, 5 is used because of the essential nonavailability of the Cp species. Many compounds have been prepared by using this approach (195,236). 

A third approach notes the relationship between the anion [(q -CsHs) Fe(q5-C2B9Hii)]2 and the parent carborane from which it is derived, C2B10H12. An anionic [(q5-C5H5)Fe] unit in the former replaces a neutral BH unit in the latter. The capacities of these two units, [(CsHsjFe] and BH, to participate in cluster bonding are similar. Each can function as a source of two electrons, and three AOs, for use in skeletal bonding. Not only are the numbers of their frontier orbitals the same, but so are their lobal characteristics they are isolobal (Fig. 3.21).The metal unit can effectively supply a radially oriented Pzdz. hybrid orbital and two pd p.xL i,P-,d., ) hybrid orbitals to bond to the nido... 

Theoretical Aspects. - Relatively few articles of an original nature have been published. Instead, most concerned the extension of previous work such as Wade s approach to cluster bonding, or the suitability of BH or CH units to cap four-, five-, or slx-membered rings. A hypothetical "B H CHICH" model was used to analyse the observed C-H bond weakening in the Isolobal analogue [Fe CH] CCD)... 

---