Book excerpt: The section devoted to iron in this volume reflects the tremendous progress in the area. Specifically cluster chemistry, ligand transformations and detailed structural results are more prominent in COMC II. The organic chemistry of ruthenium and osmium is an area which has burgeoned during the period since the publication of COMC. This is especially true for the cluster chemistry of these elements, which have provided most of the advances in this important field. Consequently, this volume will include an update (1981-1993) of the chemistry of mono- and bi-nuclear complexes of ruthenium and osmium, with a rather more extensive treatment of tri- and tetra-nuclear complexes. This is because many of the early results in ruthenium and osmium cluster chemistry described in COMC are now much better understood and can thus be placed in a more general context. In the case of complexes containing clusters with five or more metal atoms, the coverage is essentially complete, again because this chemistry has developed during the 1980s.