Data have been presented which show that dispersed metals can be considered as viable catalysts for synthetically useful organometallic reactions, In addition, the material used as the catalyst support has been shown to influence the regiochemistry of the reaction. In the Heck arylation an acidic support, such as silica, withdraws electrons from the palladium and this leads to predominant β enol ether formation. Palladium supported on a basic support, such as magnesia, which is electron donating, produces primarily the α product. Electron withdrawal also increases the rate of the reaction. As with a number of other synthetically useful reactions the heterogeneously catalyzed Heck arylation takes place on the coordinately unsaturated corner atoms and adatoms on the metal surface. These single atom active sites are the only ones having a sufficient number of orbitals available for interaction with both the aryl species and the enol ether at the same time, a procedure needed for the reaction to take place. A mechanism for the reaction on a tetrahedral corner atom is proposed. This is based on the Frontier Molecular Orbitals of this site and the corresponding mechanism established for the homogenously catalyzed reaction.