Two-reagent reactions of Ni-19 and Ni-23 with hydrogen/deuterium and ammonia are studied to probe adsorbate-induced cluster structural changes. H/D and NH3 bind to the clusters noncompetitively, i.e., to different sites, so simultaneous saturation by the two reagents can be achieved. It is found that HID adsorption causes a change in Ni-19 from the double icosahedral structure to a face-centered cubic (fee) or hexagonal close-packed (hcp) one, but that subsequent saturation with NH3 Converts the cluster back to the double icosahedron. No such structural changes are seen for the triple icosahedral Ni-23. The results for Ni-19 are interpreted in terms of the electron withdrawing or donating character of the ligands and the consequent effect on the extent of d-orbital bonding in the cluster. Possible configurations of the H/D atom binding sites on the icosahedral Ni-19 and Ni-23 clusters and on the fcc/hcp Ni-19 cluster are presented.