In this review a detailed overview is given of the (electronic) structure and reactivity of open-shell (paramagnetic) alkene rhodium and iridium complexes M-x(alkene) (M = Rh, Ir; x = 0, +II). In most cases these species are highly reactive, but some notable exceptions of stable species for which not much reactivity has been described are also included (section 2). Understanding the electronic structure of the reactive nitrogen-ligand-based [M-II(por)(alkene)] and [M-II(alkene)(N-ligand)]21 species (section 3) is complicated by a substantial spin distribution over the metal and the alkene ligand. This dual-mode metallo-radical and ligand radical behavior is reflected by their high and diverse reactivity. Metallo-radical character is demonstrated by a variety of metal- to -radical coupling reactions with a series of other radicals, as well as metal-based allylic hydrogen atom abstractions from M-II(alpha-alkene) species. On the other hand, (donor induced) ligand radical character is crucial in a variety of M-C and C-C radical coupling reactions, C-O bond formation by oxygenation of M-II(alkene) species and alkene "insertions" into (por)M-H bonds. The review concludes with some speculations about possible roles of paramagnetic M(olefin) (M = Rh, Ir) species in catalytic olefin oxygenation and carbene transfer reactions. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007.