The mechanisms of hydrogenation and dehydrogenation reactions catalyzed by a series of aliphatic PNP cobalt pincer complexes, [(PNRPiPr)CoH]+ (R = H and CH2X; X = H, Me, NH2, OMe, OH, F, and Cl) and [(PNPiPr)CoH], are studied by density functional theory calculations. In the hydrogenation of propylene catalyzed by [(PNRPiPr)CoH]+, a propylene molecule first inserts into the Co-H bond to form a Co-C bond. Then a H2 molecule is inserted into the Co-C bond for the formation and release of propane. The influence of different substituents on the N atom of the pincer ligand for the hydrogenation process is investigated. The relations between the field/inductive effect (σF) and total free energy barriers and the properties of the lowest energy intermediates, including the Co-N bond lengths, the lowest unoccupied molecular orbital energies, and the Wiberg bond indices of Co-N bonds, are analyzed. The results show that σF plays a crucial role in the substituent effect. The mechanism of acceptorless dehydrogenation of alcohols is also elucidated with a detailed free energy profile for the whole catalytic cycle. We found that the very low catalytic activity of [(PNPiPr)CoH] is caused by the easily transfer of H from cobalt to nitrogen to form stable intermediates.