When reaction of an unsaturated hydrocarbon with a transition metal centre occurs, two isomeric products may result - a π complex in which the organic substrate remains essentially intact or oxidative addition of the metal to the substrate with fission of a C-H bond. The mechanisms by which these products result, and the relative stability of the isomers, have been examined in detail, particularly for complexes of rhodium, iridium, iron and ruthenium. The π complex is not an intermediate on the pathway to formation of vinyl hydride or aryl hydride complexes. Reaction to give the isomeric products occurs by independent routes, at least in the cases studied in depth. The observation that alkene π complexes tend to be more stable than the isomeric vinyl hydride complexes while the relative  stability of the isomers is reversed for aromatic systems, while true in most cases, is an over simplification and the nature of the metal and ligand set must be considered. Insertion products are generally formed when terminal alkynes react to form complexes and may be either alkynyl hydride or vinylidene complexes, the latter dominating for d ⁶ and d ⁸ metal centres.