- During the past decade, soluble transition-metal catalysts have become increasingly more important in a wide range of reactions; included in these are the oxo process or hydroformylation, olefin metathesis or isomerization, hydrogenation, polymerization, and oligomerization. Langford and Gray have classified the three possible intimate mechanisms for ligand substitution processes as dissociative, associative, and interchange. The dissociative mechanism that affords an intermediate of reduced coordination number is by far the most common pathway for substitution reactions involving 18-electron complexes. The associative mechanism involves addition of incoming ligand to the complex in the first step affording an intermediate of increased coordination number. Many ligand substitution processes involving 18-electron transition-metal organometallic compounds proceed through concurrent ligand-independent and ligand-dependent pathways. The use of highly enriched 13CO and C18O (>90%) has contributed significantly to an understanding of the intimate mechanistic details of the ligand substitution processes of metal carbonyl derivatives. An illustration of the sorts of information accessible from labeling studies is seen in photochemical and thermal substitution reactions of the group VIB metal tetracarbonylnorbornadiene derivatives. 1982 Academic Press Inc.