Generation of Isolobal Fragments

20 are just metallacyclopropane formulations of metal-olefin complexes and 

21 is the Dew ar-Chatt-Duncanson model of bonding as described more fully in Section 19.2 and 19.4. 

We will cover several more complexes that can be traced back to cyclopropane in the next section. For now, we note that the isolobal analogy nicely solves conformational issues. Let us rotate the ethylene unit in 21.19 and 21.20 by 90°. The ethylene tt orbital is now of Z i symmetry and the / 2 orbital in CIU or ML is left occupied and nonbonding. Thus, 21.22 and 21.23 arc high-energy structures. 21.22 

In fact it is isolobal to both. If wc use Ni(CO)3 to reconstruct a trigonal bipyramid, then it is isolobal to CH2 however, ifit is used to form a tetrahedral complex then it is isolobal to CH3. Suppose we remove an equatorial CO ligand from Fc(CO)5, 21.24. That creates a C21, Fc(C0)4 ligand with one empty hybrid orbital and the Fc(C0)4 group is isolobal to CH3. Ethylcne-Fc(CO)4,21.29, is then equivalent to protonated cyclopropane, 21.30. But wc had said that ethylene-Fe(CO)4, drawn in the mctallacyclopropanc form in 21.20, was equivalent to cyclopropane The metallacyclopropane drawing emphasizes octahedral coordination at iron. The OC—Fe—CO angle in the equatorial plane should be 90. In molecular orbital 

Contour plots of the frontier orbitals in four isolobal fragments at the HF 3-21G level. 

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