Isolobal Principle - Applications and Examples

Applications and Examples

Uses of the isolobal analogy include providing a short-cut to understanding electronic structure, predicting reactivity and reaction mechanisms, and a method of classifying molecules. Applications are typically utilized to make connections between well-known systems and less familiar systems. For example, the possibility of unsynthesized compounds can be imagined from those of known molecular conformations. The isolobal analogy does not guarantee these products are capable of being produced, but only proposes a possibility. Consider the molecule Fe(CO)₃ complexed with cyclobutadiene. Fe(CO)₃ is isolobal with CH+. Therefore one can predict that CH+ will coordinate with cyclobutadiene in a similar fashion that Fe(CO)₃ will. Thus the molecule C₅H₅+ can be envisioned regardless of its actuality.

Predicting the reactivity of complexes can also be accomplished using the isolobal analogy. From the simple expectation of two CH₃ radicals reacting to form ethane one can use the analogy to predict M–C or M–M bonding such as (CH₃)M(CO)₅ and M₂(CO)₁₀, where M is d7.

Another application of the isolobal analogy is assisting in predicting reaction mechanisms. As in the other applications the mechanisms of well-known reactions can be used to help predict mechanistic pathways of lesser-known reactions. There is no limit on the potential comparisons between organic and inorganic complexes. The analogy can flow in either direction (Organic to Inorganic) or within each division (Organic to Organic).

Arteaga-Müller et al. utilize the isolobal analogy to relate imido half-sandwich complexes with isoelectronic dicyclopentadienyl complexes. The isolobal relationship of the imido and the cyclopentadienyl ligands is the key to this comparison. The study found the reactivity of these two types of complexes to be similar although their catalytic abilities differed in some respects. This study shows that the isolobal analogy does not make perfect predictions between two isolobal fragments, as Hoffman warned in his Nobel Lecture.

Wu et al. apply the isolobal analogy to explore relationships involving structures, energies and magnetic properties between polyhedral boron carbonyls and their hydrocarbon relatives. As determined in this study, although isolobal, these two sets of molecules have significant differences in their strain energy.

Goldman and Tyler used the isolobal analogy to determine the most likely mechanism for a deletion reaction. One of the products of the irradiation of CpW(CO)₃Me in the presence of PPh₃ is CpW(CO)₃−. The mechanism of said reaction was studied and theorized to be isolobal to the disproportionation of metal-metal bonded dimers involving 19-valence electron intermediates. The reactions are composed of isolobal fragments and the key intermediates of both reactions are isolobal. Thus, the reaction pathways are mechanistically isolobal.