This paper describes the synthesis and characterization of a Cr(I) dimer with a very short Cr-Cr distance. Computational studies support fivefold bonding between the chromium atoms. I have used this paper to introduce metal-metal multiple bonds and discuss the molecular orbital interactions of homonuclear diatomics including d-orbitals. More generally, it is a nice example to stimulate the discussion of what constitutes a bond and the various interpretations of bond order.
Students are asked to read the paper and answer the discussion questions before coming to class. I used this paper in a second-year inorganic course after we had talked about MO theory of diatomics but fairly early in our discussion of transition metal chemistry. There is a Perspectives article in Science that goes along with this paper, but I decided not to give it to the students as I wanted them to work out the MO diagram for themselves.
Student performance on the discussion questions is assessed.