The reactions of a mother solution of RuCl₃ with benzimidazole derivatives 2-(2′-pyridyl)benzimidazole (2,2′-PyBIm, L₁) and 2-hydroxymethylbenzimidazole (2-CH₂OHBIm, L₂) yielded three novel ruthenium complexes: (H₂L₁)₂[Ru^IIICl₄(CH₃CN)₂]₂[Ru^IVCl₄(CH₃CN)₂]·2Cl·6H₂O (1), mer-[Ru^IIICl₃L₁(CH₃CN)]·L₁·3H₂O (2), and (HL₂)₄[Ru^IVCl₆]·2Cl·4H₂O (3). The isolated compounds were characterised by elemental analyses, UV-Vis and IR spectroscopy, and magnetic measurements. The nature of the ligands bound to the metal ions of these compounds and the experimental conditions significantly influenced the ruthenium complexes in different oxidation states. The N,N-donor ligand bound to the metal centre is a recognised stabiliser of the +III state of ruthenium, whereas the lack of ligand coordination promotes the formation of a mixed (Ru^III/Ru^IV) complex. In the case of complex 3, the absence of a N,O-donor ligand in the coordinate sphere facilitates the formation of the compound in a higher oxidation state. X-ray single crystal analyses revealed an octahedral geometry in each of the complexes. The crystal structure of ruthenium complexes is formed by a network of intermolecular classical and unconventional (C–H⋯π) hydrogen bonds. The most interesting feature of the supramolecular architecture of complexes is the existence of a very rare Cl⁻⋯π interaction and π⋯π stacking, which also contribute to structural stabilisation. Ruthenium compounds 2 and 3 behave as paramagnets with an octahedral geometry, corresponding to the presence of one or two unpaired electrons, respectively. The cyclic voltammetric data of complex 2 show three one-electron redox processes. The first redox couple is reversible, whereas the two other couples have a quasi-reversible nature. In the case of complex 3, two redox couples are reversible and the electrode processes are connected with exchange of one electron.

Accurate prediction of spin–state energetics for transition metal (TM) complexes is a compelling problem in applied quantum chemistry, with enormous implications for modeling catalytic reaction mechanisms and computational discovery of materials. Computed spin–state energetics are strongly method-dependent and credible reference data are scarce, making it difficult to conduct conclusive computational studies of open-shell TM systems. Here, we present a novel benchmark set of first-row TM spin–state energetics, which is derived from experimental data of 17 complexes containing Fe^II, Fe^III, Co^II, Co^III, Mn^II, and Ni^II with chemically diverse ligands. The estimates of adiabatic or vertical spin–state splittings, which are obtained from spin crossover enthalpies or energies of spin-forbidden absorption bands, suitably back-corrected for the vibrational and environmental effects, are employed as reference values for benchmarking density functional theory (DFT) and wave function methods. The results demonstrate a high accuracy of the coupled-cluster CCSD(T) method, which features the mean absolute error (MAE) of 1.5 kcal mol⁻¹ and maximum error of −3.5 kcal mol⁻¹, and outperforms all the tested multireference methods: CASPT2, MRCI+Q, CASPT2/CC and CASPT2+δ MRCI. Switching from Hartree–Fock to Kohn–Sham orbitals is not found to consistently improve the CCSD(T) accuracy. The best performing DFT methods are double-hybrids (PWPB95-D3(BJ), B2PLYP-D3(BJ)) with the MAEs below 3 kcal mol⁻¹ and maximum errors within 6 kcal mol⁻¹, whereas the DFT methods so far recommended for spin states (e.g., B3LYP*-D3(BJ) and TPSSh-D3(BJ)) are found to perform much worse with the MAEs of 5–7 kcal mol⁻¹ and maximum errors beyond 10 kcal mol⁻¹. This work is the first such extensive benchmark study of quantum chemistry methods for TM spin–state energetics making use of experimental reference data. The results are relevant for the proper choice of methods to characterize TM systems in computational catalysis and (bio)inorganic chemistry, and may also stimulate new developments in quantum-chemical or machine learning approaches.