Planar, low-spin cobalt(II) dialkyl complexes bearing bidentate phosphine ligands, (P-P)Co-(CH2SiM3)(2), are active for the hydrogenation of geminal and 1,2-disubstituted alkenes. Hydrogenation of more hindered internal and endocyclic trisubstituted alkenes was achieved through hydroxyl group activation, an approach that also enables directed hydrogenations to yield contrasteric isomers of cyclic alkanes.