NiCo nanoparticles are active catalysts for a number of reactions, as an interesting alternative to noble metals. In particular, phosphine-covered NiCo nanoparticles with a 1:1 metal ratio were found to be active at room temperature for Si-H bond activation. However, it was unclear which of the two metals was the active site and if the neighboring atoms influenced the reaction efficiency. We designed nanoparticles with a nickel core and a limited amount of cobalt surface sites, just below or just above a monolayer amount, to investigate this question. We showed that the trend in catalytic activity is consistent with cobalt being the active site, and it shows a higher activity when its immediate neighbors are cobalt atoms. This was consistently observed with two phosphine ligands, PPh 3 and PnBu 3 , while the first allowed a higher conversion rate than the second.