"Weakly ligated/labile ligand" nanoparticles, that is nanoparticles where only weakly coordinated ligands plus the desired catalytic reactants are present, are of fundamental interest. Described herein is a catalyst system for benzene hydrogenation to cyclohexane consisting of "weakly ligated/labile ligand" Ir(0) nanoparticles and aggregates plus dry-HCl formed formed in situ from commercially available [(1,5-COD)IrCl](2) plus 40 +/- 1 psig (similar to 2.7 atm) H-2 at 22 +/- 0.1 degrees C. Multiple control and other experiments reveal the following points: (i) that this catalyst system is quite active with a TOF (turnover frequency) of 25 h(-1) and TTO (total turnovers) of 5250; (ii) that the BE4- and PF6- iridium salt precursors, [(1,5-COD)Ir(CH3CN)(2)]BF4 and [(1,5-COD)Ir(CH3CN)(2)]PF6, yield inferior catalysts; (iii) that iridium black with or without added, preformed HCl cannot achieve the Tor, of 25 h(-1) of the in situ formed Ir(0)/dry-HCl catalyst. However and importantly, CS2 poisoning experiments yield the same activity per active iridium atom for both the Ir(0)/dry-HCl and Ir black/no-HCl catalysts (12.5 h(-1) Ir-1), but reveal that the Ir(0)/dry-HCl system is 10-fold more dispersed compared to the Ir(0) black catalyst. The simple but important and key result is that "weakly ligated/labile ligand" Ir(0) nanoparticles and aggregates have been made in situ as demonstrated by the fact that they have identical, per exposed Ir(0) activity within experimental error to (40) black and that they have no possible ligands other than those desired for the catalysis (benzene and H-2) plus the at best poor ligand HCl. As expected, the in situ catalyst is poorly stabilized, exhibiting only 60% of its initial activity in a second run of benzene hydrogenation and resulting in bulk metal precipitation. However, stabilization of the Ir(0) nanoparticles with a ca. 2-fold higher catalytic activity and somewhat longer lifetime for the complete hydrogenation of benzene was accomplished by supporting the Ir(0) nanoparticles onto zeolite-Y (TOE of 47 h(-1) and 8600 TTO under otherwise identical conditions). Also reported is the interesting result that Cl (present as Proton Sponge center dot H+Cl-) completely poisons benzene hydrogenation catalysis, but not the easier cyclohexene hydrogenation catalysis under otherwise the some conditions, results that suggest different active sites for these ostensibly related hydrogenation reaction. The results suggest that synthetic routes to "weakly ligated/labile ligand" nanoparticles exhibiting improved catalytic performance nee is an important goal worthy of additional effort.