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Square-planar cobalt diimine pyridine complexes LCoR (L = 2,6-[RN CMe]2C5H3N; R = n-C6H13 for L hex , 2,6-(i-Pr)2C6H3 for L dip ) are active in the hydrogenation of monosubstituted and disubstituted olefins; sterically more hindered trisubstituted olefins do not react. For the L dip Co system, a diamagnetic hydride intermediate was observed, although a small amount of paramagnetic product is also formed upon reaction of L dip CoR with H2. DFT studies suggest a traditional hydrogenation cycle starting with LCoH, except that intermediate LCo(R)(H2) transfers a hydrogen atom directly from H2 to the alkyl group in a -bond metathesis step, without going through a discrete Co III intermediate. Autoclave experiments show that conversion is not linear in catalyst intake. Diffusion limitation was ruled out as an explanation, and we propose a concentration-dependent catalyst decay. At low catalyst intake conversion rates up to 2 × 10 4 (mol octene/mol Co/bar/h) can be reached. Reducing the steric bulk at the imine positions (L dip → L hex ), or changing the metal from cobalt to rhodium, do not alter the activity or specificity of the hydrogenation much. For the L hex Co and L dip Rh systems, no diamagnetic products corresponding to L dip CoH were observed.