We study the electronic structure of a Pt overlayer and Pt7 adatom cluster, respectively, on the Ni(111) surface by using a self-consistent tight-binding model. Together with epitaxial geometries, a mean-field 'non-epitaxial' model is also employed. For these structures, calculations for hydrogen adsorption are performed. Correlation between the geometry, surface reactivity, core-level shifts and local surface electronic structure has been recently discussed for Pd and Pt ultrathin films on BCC substrates. Similar correlation is also found in the present study. Nevertheless, its origin is now traced to the change of the local d-band width due to overlayer compression rather than to a specific hybridization between orbitals on different atoms. An analogy with PtNi alloy surfaces and their catalytic properties is pointed out.