Selective reactions that combine H 2 , CO and organic electrophiles (aldehyde, ketones, isocyanide) to form hydrogenated C 3 and C 4 carbon chains are reported. These reactions proceed by CO homologation mediated by [W(CO) 6 ] and an aluminum(I) reductant, followed by functionalization and hydrogenation of the chain ends. A combination of kinetics (rates, KIEs) and DFT calculations has been used to gain insight into a key step which involves hydrogenation of a metallocarbene intermediate. These findings expand the extremely small scope of systems that combine H 2 and CO to make well-defined products with complete control over chain length and functionality.