Low oxidation state transition metal clusters which contain one or two boron atoms in fully or semi-interstitial sites are reviewed. In such environments, the boron atom does not bear a terminal hydrogen atom. This area of research has been developed primarily over the last decade, and now includes a range of homo- and heterometallic systems. Metal frameworks that are represented include the M 4 butterfly, the M 5 square-based pyramid, the M 6 octahedron and the M 6 trigonal prism, together with a variety of more unusual geometries. Synthetic and structural studies form the basis of the review. Chemical reactivity patterns of the butterfly clusters (which support semi-interstitial boron atoms) are discussed in detail; reactions include B--C and B--N couplings at the semi-interstitial atom as well as metal framework expansion which transforms the boron atom from semi- to fully interstitial.