Moving Beyond the Single Center-Ways To Reinforce Molecular Orbital Theory in an Inorganic Course

Abstract

Rather than ending the discussion of molecular orbital (MO) theory in an inorganic chemistry course with molecules such as octahedral ML6 or square planar ML4, we suggest moving beyond the single-atom center to include the MO diagram of ethene (C2H4). Embracing the use of group theory and symmetry adapted linear combinations (SALCs) at the level already taught, we extend the coverage to include this simple organic dimer as a means of teaching skills that enable students to tackle significantly more complicated molecules. Ethene provides a bridge to the fragment approach pioneered by Roald Hoffmann and used in some high-level treatises. Larger molecules are split into component fragments poised to form bonds. The MO diagram is constructed for each fragment and then combined to obtain the MO diagram of the full molecule. In this way, more sophisticated examples can be addressed if the process is reduced to smaller, more chemically intuitive steps. To bring the fragment approach to the undergraduate curriculum, we present the MO diagram of C2H4 first using SALCs of atomic orbitals and then contrasting this with a two-step fragment approach that first creates the MO diagram of the CH2 diradical.