Tripalladium clusters of the type [Pd 3 (PPh 3 ) 4 ] 2+ , wherein three linearly connected Pd atoms are stabilized by phosphine and arsine ligands, have been detected and isolated as intermediates during the reduction of well-defined mononuclear [Pd(OTf) 2 (XPh 3 ) 2 ] (X = P and X = As, respectively) to Pd nanoparticles (PdNPs). The isolated [Pd 3 (PPh 3 ) 4 ] 2+ cluster isomerizes on broad-band UV irradiation to form an unexpected photoisomer, produced by a remarkable change in conformation at one of the bridging PPh 3 ligands. A catalytic role for these [Pd 3 (XPh 3 ) 4 ] 2+ species is exemplified in Suzuki−Miyaura crosscoupling (SMCC) reactions, with high activity seen in the arylation of a brominated heterocyclic 2-pyrone. Use of the [Pd 3 (PPh 3 ) 4 ] 2+ cluster enables a switch in site selectivity for SMCC reactions involving 2,4-dibromopyridine from the typical C2bromide site (seen previously for mononuclear Pd catalysts) to the atypical C4-bromide site, thereby mirroring recently reported cyclic Pd 3 clusters and PdNPs. We have further determined that the thermal isomer and photoisomer of [Pd 3 (PPh 3 ) 4 ] 2+ are similarly catalytically active in the Pd-catalyzed hydrogenation of phenylacetylene to give styrene. Our findings link the evolution of mononuclear Pd(II) salts to PdNPs via the intermediacy of linear [Pd 3 (XPh 3 ) 4 ] 2+ clusters.