This review deals with rhodium and iridium complexes of the hydridotris(pyrazolyl)borate (Tp') ligands. In addition to outlining the synthesis of precursor compounds, an overview of the coordination modes of the Tp' ligands is given. Recent developments in the chemistry of some important families of compounds (carbonyls, isonitriles, classical and non-classical polyhydrides) are discussed. Particular attention is given to C-H activation reactions with these compounds. Over 100 references are covered, of which approximately half stem from the last 3 years. (C) 2001 Elsevier Science B.V. All rights reserved.