The preparation and reactivity with H-2 of two Ru complexes of the novel ZnPhos ligand (ZnPhos = Zn(o-C6H4PPh2)(2)) are described. Ru(ZnPhos)(CO)(3) (2) and Ru(ZnPhos)(IMe4) 2 (4; IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) are formed directly from the reaction of Ru(PPh3)(C6H4PPh2)(2)(ZnMe)(2) (1) or Ru(PPh3)(3)HCl/LiCH2TMS/ZnMe2 with CO and IMe4, respectively. Structural and electronic structure analyses characterize both 2 and 4 as Ru(0) species in which Ru donates to the Z-type Zn center of the ZnPhos ligand; in 2, Ru adopts an octahedral coordination, while 4 displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, 2 loses CO to give Ru(ZnPhos)(CO)(2) that then adds H-2 over the Ru-Zn bond to form Ru(ZnPhos)(CO)(2)(mu-H) 2 (3). In contrast, 4 reacts directly with H-2 to set up an equilibrium with Ru(ZnPhos)(IMe4)(2)H-2 (5), the product of oxidative addition at the Ru center. DFT calculations rationalize these different outcomes in terms of the energies of the square-pyramidal Ru(ZnPhos)L-2 intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H-2 to add across the Ru-Zn bond, but for L = IMe4, this species is kinetically inaccessible and reaction can only occur at the Ru center. This difference is related to the strong pi-acceptor ability of CO compared to IMe4. Steric effects associated with the larger IMe4 ligands are not significant. Species 4 can be considered as a Ru(0)L-4 species that is stabilized by the Ru. Zn interaction. As such, it is a rare example of a stable Ru(0)L4 species devoid of strong pi-acceptor ligands.