Direct 1-3 Metal-Carbon Bonding and Planar Tetracoordinated Carbon in Group 6 Metallacyclobutadienes

A comprehensive analysis of the nature of metal carbon bonding group 6 metallacyclobutadienes has been carried out by means of energy decomposition analysis and topological properties of electron density. The results suggest strong sigma- and pi-bonding interactions between the metal and the beta-carbon (direct 1,3-metal-carbon bonding) as well as the existence of the beta-carbon in a new type of planar tetracoordinate state characterized by a catastrophe bonding interaction with the metal. The importance of this bonding for alkyne metathesis is indicated.