**1. Introduction**

Recent studies involving the chemistry of the heterocyclic nitrogen carbene complexes of transition metals have demonstrated that they can act as precatalysts for a variety of reactions [1–11]. These new species offer many opportunities to advance this field of study [12–30]. The use of palladium carbene complexes for the Heck reaction [31–34] and platinum carbene compounds for the C─H activation reactions [11] has created new opportunities in catalytic chemistry.

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Over the last two decades, imidazolium-based ionic liquids have also found many applications in catalysis [35–39], or as nonaqueous alternatives for biphasic catalysis [4, 40–42]. The studies by Cavell and co-workers [43] showed that the reaction of imidazolium-based ionic liquids with low-valence Ni0 and *σ*-donor ligands that bear Pd0 is an easy procedure for the production of unusually stable carbene-metal-hydride complexes (see **Scheme 1**). The major feature of the study was the direct formation of a carbene-metal-hydride, which offers an atom-efficient direct route to an active catalytic species. Besides these experimental facts, it is not surprising that N-heterocyclic carbenes (NHCs) [44–46] have found applications in a series of catalytic reactions, such as amination reactions, the Suzuki-Miyaura and Sonogashira coupling reactions, hydroformylation, hydrosilylation, and polymerization and olefin metathesis [47–50].

**Scheme 1.** 

The crucial experimental works that are presented in **Scheme 1** inspire this study of the potential energy surfaces of these oxidative addition reactions, using density functional theory (DFT). There have been a number of reports concerning the conventional oxidative additions-reductive eliminations of alkanes to low-valence metals, which has led to an understanding of the factors that affect these reactions [51–56]. These studies have mostly focused on the catalytic reactions of saturated hydrocarbons to zerovalent group 10 elements (i.e., Ni, Pd, and Pt). To the authors' best knowledge, there has been neither experimental nor theoretical study of the catalytic oxidative addition reactions for the group 9 atoms (i.e., Co, Rh, and Ir) or the imidazolium cation. This study gives a thorough understanding of the catalytic reactions for potential transition metal complexes with imidazolium cations (ICs). Accordingly, a study of the important C─H activation reactions, Eqs. (1) and (2), is undertaken:

(M = Ni, Pd, Pt; L = 1, 3 - aryl-NHC, aryl = 2, 4, 6 - trimethylphenyl) (1)

( M ′ = Co, Rh, Ir; L = 1, 3 - aryl - NHC, aryl = 2, 4, 6 - trimethylphenyl ) (2)

Since oxidative addition involves charge transfer from the metallic center of both L2 M and CpM′L to the approaching IC, an electron-donating L that increases the electron density on the central metal stabilizes its transition state and lowers the barrier height. That is to say, increasing the electron density on the central metal atom of both L2 M and CpM′L increases the chance of its triplet participating in the oxidative addition reaction (vide infra). Therefore, the reactivity of both substituted 14-electron L2 M and 16-electron CpM′L is verified by the singlet-triplet splitting (∆*E*st = *E*triplet − *E*singlet), which can result from several factors, such as the effect of the geometrical structure (i.e., linear or bent for the L2 M system) [55], the nature of electron-withdrawal or electron-acceptance for the ancillary ligand, L, and the character of the central transition metal atom. In the organometallic field, the NHC groups are stronger *σ*-donors and weaker *π*-acceptor ligands than the traditional PR3 ligands [47–50]. Therefore, the model systems (both L2 M and CpM′L complexes) that are studied in this work use the NHC as the ancillary ligand L.

Since the transition-metal-catalyzed reactions that contain imidazolium salt are both helpful and novel, a comprehensive understanding of the factors that control the magnitude of the activation barriers and the reaction enthalpies allows a greater understanding of their reactivity. Full realization of the factors that influence the reactivities of transition metal complexes with ICs benefits basic science and a continued expansion of their applications.
