DFT computational insight into the mechanism of the monomer–trimer isomerism of Ni(II) bis-acetylacetonate

[Display omitted] •Dissociation of trimeric Ni(II) bis-acetylacetonate into the monomers was studied.•Four possible reaction paths for this spin-forbidden rearrangement were found.•The energy preferred mechanism involves three consecutive changes of spin state.•Energy barriers necessary to overcome along preferred route do not exceed 33 kcal/mol.•This reaction is an example of a thermally induced multi-step CISSS phenomenon. The possible reaction paths for the spin-forbidden dissociation of paramagnetic nickel(II) bis-acetylacetonate trimer (Ni(acac)2) have been studied based on the DFT UB3LYP/6-311++G(d,p) calculations. Two energy preferred reaction routes involve three consecutive steps resulting in spin state changes of the system: (S = 3) → (S = 2) → (S = 1) → (S = 0). The highest theoretically-estimated limits for energy barriers necessary to overcome along these routes, as estimated by the relative energies of the minimum energy crossing points (MECPs) on the seams of the intersecting potential energy surfaces of different multiplicity, do not exceed the value of 33 kcal/mol. The results obtained are in a good agreement with available experimental data. The considered dissociation – association rearrangement represents an example of a thermally induced multi-step Coordination-Induced Spin-State Switching phenomenon.