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Four new palladium(II) complexes of the type [Pd(NHC)2X2] with N‐heterocyclic carbene (NHC) ligands of relatively small steric hindrance were prepared and characterized by using spectroscopic and X‐ray methods. For [Pd(bmim‐y)2Br2] (bmim‐y=1‐butyl‐3‐methylimidazol‐2‐ylidene), crystals of both cis and trans isomers were obtained. All the studied complexes demonstrated very high activity in Suzuki–Miyaura cross‐coupling in ethylene glycol, which yielded turnover numbers of up to 760 000. High activity was also observed if NaBPh4 was used instead of PhB(OH)2, and the best results (turnover number=580 000) were obtained with [Pd(emim‐y)2Cl2] (emim‐y=1‐ethyl‐3‐methylimidazol‐2‐ylidene). In the reaction mixture, different forms containing [Pdx(NHC)y]+ fragments (x=1–4, y=2–5) were identified by using ESI‐MS. In the presence of Suzuki–Miyaura reaction substrates, catalytic palladium intermediates with aryl groups—[Pd(NHC)2Ph]+ and [Pd3(NHC)4Ph]+—were detected. Additional mechanistic investigations, such as TEM observations and mercury poisoning experiments, substantiated the formation of nanoparticles as a catalyst resting state. These heterogeneous particles serve as a reservoir for soluble palladium species—atoms or clusters that function as homogeneous catalysts for the Suzuki–Miyaura reaction.

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Rhodium(I) carbene complexes of the type [Rh(NHC)(cod)X] (where NHC ) N-heterocyclic carbene obtained from dialkylimidazolium cation; cod ) 1,5-cyclooctadiene; X ) Cl, Br, I, SCN) with P(OPh) 3 as a modifying ligand have been found to be very active catalysts for 1-hexene hydroformylation under mild conditions (80 °C, 10 atm of H 2 /CO, 4 h). Even at a low rhodium concentration ([1-hexene]/[Rh] ) 2000), aldehydes have been obtained in yields of up to 90% with a high n/iso ratio of ca. 7. During the catalytic process, rhodium(I) carbene complexes, [Rh(NHC)(P(OPh) 3 ) 2 X], reacted with H 2 /CO, giving a catalytically active rhodium(I) hydrido complex containing an N-heterocyclic carbene ligand. The presence of [HRh(CO)(P(OPh) 3 ) 3 ] and [Rh(NHC)(P(OPh) 3 )(CO)X] complexes in the catalytic system has been confirmed. When P(OCH 2 CF 3 ) 3 was used as modifying ligand with [Rh(NHC)(cod)Br] as catalyst precursor, formation of [HRh(CO)(NHC)(P(OCH 2 CF 3 ) 3 ) 2 ] with a square-pyramidal structure was evidenced by 1 H and 31 P NMR.

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