The reversible activation of H-2 via a pathway involving metal-ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Nevertheless, there is a paucity of experimental information probing the mechanism of this transformation. Here, we present an in-depth kinetic study of the 1,2-addition of H-2 via an MLC pathway to the widely used iron catalyst [((PNP)-P-iPr)FeH(CO)] (1) ((PNP)-P-iPr = N((CH2CH2PPr2)-Pr-i)(2)(-)). We report one of the first experimental demonstrations of an enhancement in rate for the activation of H-2 using protic additives, which operate as "proton shuttles". Our results indicate that proton shuttles need to be able to both simultaneously donate and accept a proton, and the best shuttles are molecules that are strong hydrogen bond donors but sufficiently weak acids to avoid deleterious protonation of the transition metal complex. Additionally, comparison of the rate of H-2 activation via an MLC pathway between 1 and two widely used ruthenium catalysts enables more general conclusions about the role of the metal, ancillary ligand, and proton shuttles in H-2 activation. The results of this study provide guidance about the design of catalysts and additives to promote H-2 activation via an MLC pathway.