After several days of lecturing on the topic of polyatomic molecular orbital diagrams, students break into small groups of 3-4 and form LGO’s that can be used to interact with a central atom to form a Molecular Orbital (MO) diagram. This assignment is part of a larger 4-5 week unit on MO theory.
The first third of my inorganic course is devoted to polyatomic MO theory as it is the basis of modern understanding of bonding, reactivity and spectroscopy. Generally, even students at the advanced level have not ever formed an MO diagram for a polyatomic compound (MXn; e.g. PF5, CCl4, CoF63-) whether main group or coordination compound. After lecturing on symmetry and symmetry operations, and a brief review of Lewis theory, Valence Bond theory, and diatomic MO theory, students are shown that interaction of three or more atoms to form a compound is significantly more difficult unless all of the ligand orbitals are taken together as a group. The LGO’s are derived using an intuitive, symmetry-based approach that does not require linear algebraic techniques (projection operators). This allows the students to quickly and easily derive LGO’s and thus MO diagrams for complicated molecules and coordination compounds at a “back-of-the-envelope” level of theory suitable for drawing during a seminar or on a cocktail napkin.
Doing the problems in this way allows me to cover much more complex material that I would not want to ask on a homework assignment. I circulate through the room answering questions and providing guidance in real time. The last 15-20 minutes of class are reserved for each group to describe their solution to the problem.
after doing this exercise, a student should be able to:
i)From a predicted molecular geometry, determine the central atom's hybrid orbitals and use them as generator orbitals
ii) Generate the LGOs by taking linear combinations of the ligand bonding orbitals
iii) assign proper symmetry labels to the LGOs using a character table
iv) predict the symmetry of lone pairs (if applicable)
I spend a good deal of time explaining the technique in class (one to two lectures) and do simple examples in class. This in-class practice session really cements it home for them. One of these days I will post a five-slides-about LO in order to more fully explain the technique. Please email me if you want additional details on implementation.