The concept of the chemical bond has been around for quite some time and there are many models that try to explain what is going on in that hazy world of electron density that glues atoms together. But molecules that challenge our notion of just what a chemical bond is continue to be reported, often presenting us with more questions than answers.
 
H.S. Rzepa, Nature Chemistry 1, 510 - 512 (2009)doi:10.1038/nchem.373