Multiple bonds between metal atoms! My favorite! I am in love with them since I've seen the first structure and molecular orbital diagram of [Re2Cl8]2-. I think there is nothing more amazing than having multiple bonds between two metals.
This is a "Viewpoint" and it's written for the "50th anniversary of the first paper describing a species with a quadruple bond by a team led by F. A. Cotton..."
It's not only a good summary of great work by him and his students, but also a great text full of hints how and with which instrumentation you should approach problems in your research.