Russian Chemical Bulletin, International Edition, Vol. 69, No. 11, pp. 2073—2081, November, 2020 2073 Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2073—2081, November, 2020. 1066-5285/20/6911-2073 © 2020 Springer Science+Business Media LLC Metal—ligand bond dissociation energies in the Ni, Pd, and Pt complexes with N-heterocyclic carbenes: efect of the oxidation state of the metal (0, +2) A. V. Astakhov, S. B. Soliev, and V. M. Chernyshev M. I. Platov South-Russian State Polytechnic University, 132 ul. Prosveshcheniya, 346428 Novocherkassk, Russian Federation. E-mail: chern13@yandex.ru A DFT study was carried out of how the nature of metal, the oxidation state of the metal (0 and +2), as well as the structures of N-heterocyclic carbene (NHC) and other ligands influ- ence the heterolytic dissociation energies of the metal—ligand bond in the complexes M-NHC (M = Ni, Pd, Pt). It was shown that a change in the oxidation state of the metal can be followed by a considerable change in the M—NHC bond dissociation energy (up to nearly 21 kcal mol –1 ), which is also strongly influenced by the ligand in the trans-position to NHC. Key words: N-heterocyclic carbenes, coordination compounds, nickel, palladium, platinum, bond dissociation energies, catalysis. Nickel, palladium, and platinum complexes with N-heterocyclic carbenes (NHC) are widely used as homo- geneous catalysts. 1—6 They are also intensively studied as luminescent materials 7—9 and cancer drugs. 10—12 Wide use of metal complexes with NHC (M-NHC) is underlain by higher stability of such systems compared to other types of homogeneous catalysts since the M—NHC bond is very strong. 1,2,13 However, M-NHC complexes can decompose in solutions due to dissociation of the М—NHC bond, which often occurs in rather mild conditions. 14—20 Dis- sociation of the metal—ligand bond is of great importance for catalysis and other practical applications because it usually causes deactivation of catalysts 6,21—23 or changes in the nature of active centers and in the mechanism of catalysis. 24—27 Contrary to this, elimination of auxili- ary ligands is often necessary to activate the catalyst, M-NHC. 16 Thus, the catalytic properties of complexes M-NHC depend strongly on their ability to break down with hetero- lytic dissociation of the metal—ligand bond. Therefore, the heterolytic bond dissociation energy (BDE), 28 which can be calculated by quantum chemistry methods, is often used to predict the catalytic properties of complexes. 29—31 To date, both experimental and theoretical BDE values have been determined for various complexes M-NHC. 32—40 For instance, the influence of (i) the structure of the NHC ligand on the M—NHC bond dissociation energies (BDE M—NHC ) in complexes with Ni 32,34,37 and Pd 36,37,39,40 and (ii) halide ligands on the BDE in complexes Pd II -NHC 39 were studied. Steric factors, especially, bulky substituents at nitrogen atoms of the NHC ligands afect strongly the BDE M—NHC values. 29,30 However, it should be noted that the oxidation state of the metal usually changes in the course of catalysis and these changes can influence the stability of the metal—ligand bonds. For instance, the catalytic cycles of the cross-coupling, hydro- genation, and СН-activation reactions very often involve interconversions of the intermediates M 0 -NHC and M II -NHC. 1—4 Therefore, prediction of catalytic activity and stability of complexes requires taking account of the mutual influence of the oxidation state of the metal and the nature of the NHC ligand and other ligands on the BDE values. It is commonly accepted that reduction of the complexes M II L (L is ligand) to M 0 L considerably reduces their thermodynamic stability. 41 However, it was shown that BDE Ni—NHC in the Ni 0 -NHC complexes can be higher than in the Ni II -NHC complexes with the same NHC ligands. 35 Note that systematic studies of the efect of the oxidation state of the metal on the metal—ligand BDE have not been reported as yet. In this work we used the density functional quantum chemical calculations to study the joint efect of the oxida- tion state of the metal (0 and +2) and the nature of the NHC ligand and other ligands on the metal—ligand BDE in the Ni, Pd, and Pt complexes belonging to the abundant structure types (NHC)M 0 L and (NHC)M II (X) 2 L, where NHC = 1,3-dimethylimidazol-2-ylidene, 1,3-dimethyl- benzimidazol-2-ylidene, 1,4-dimethyl-1,2,4-triazol-5- ylidene; X = Cl – , Br – , I – ; L = Py, NHC, I – . Calculation Methods Calculations were carried out within the framework of the density functional theory (DFT) using the Gaussian-09 pro- gram. 42 To choose the optimum computational method, the most often used DFT methods including PBE1PBE, B3LYP,