The colors of transition metal compounds are highly variable. Aqueous solutions of nickel are green, of copper are blue, and of vanadium can range from yellow to blue to green to violet. What is the origin of these colors? A simple geometrical model known as crystal field theory can be used to differentiate the 5 d orbitals in energy. When an electron in a low-lying orbital interacts with visible light, the electron can be promoted to a higher-lying orbital with the absorption of a photon. Our brains perceive this as color.

Students are often presented with the finished MO correlation diagrams of molecules like bis benzene chromium or ferrocene in classes and in organometallic chemistry text books. This activity helps them match the ligand group orbitals of the two benzene rings with the metal valence orbitals. Their understanding and appreciation of such diagrams is significantly enhanced when they find out how only some matches have the appropriate symmetry requirements.

This is a truly hands-on activity in which students manipulate paper cutouts of carbon atomic orbitals and oxygen group orbitals to identify combinations with identical symmetry and build the carbon dioxide molecular orbital diagram. The activity pairs well with the treatment of MO theory in Miessler, Fischer, and Tarr, Chapter 5. An optional computational modeling component can be added at the end.

In this activity, a pair of students are show an object or molecule and are asked to determine the point group before their competitor.

A set of questions to be used in General or Introductory Inorganic Chemistry as a review or “quiz” of shapes and polarities.