Square-planar cobalt diimine pyridine complexes LCoR (L=2,6-[RN=CMe](2)C5H3N; R=n-C6H13 for L h,,, 2,6-(i-Pr)(2)C6H3 for L dip) are active in the hydrogenation of monosubstituted and disubstituted olefins; sterically more hindered trisubstituted olefins do not react. For the (LCo)-Co-dip system, a diamagnetic hydride intermediate was observed, although a small amount of paramagnetic product is also formed upon reaction of (LCoR)-Co-dip with H-2. DFT studies suggest a traditional hydrogenation cycle starting with LCoH, except that intermediate LCo(R)(H-2) transfers a hydrogen atom directly from H-2 to the alkyl group in a sigma-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 x 104 (mol octene/mol Colbar/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 (LCo)-Co-hex and (LRh)-Rh-dip systems, no diamagnetic products corresponding to (LCoH)-Co-dip were observed. (c) 2005 Elsevier B.V. All rights reserved.