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Using fully internally contracted (FIC)-CASPT2 analytical gradients, geometry optimizations of spin-crossover complexes are reported. This approach is tested on a series of Fe(II) complexes with different sizes, ranging from 13 to 61 atoms. A combination of active space and basis set choices are employed to investigate their role in determining reliable molecular geometries. The reported strategy demonstrates that a wave function-based level of theory can be used to optimize the geometries of metal complexes in reasonable times and enables one to treat the molecular geometry and electronic structure of the complexes using the same level of theory. For a series of smaller Fe(II) SCO complexes, strong field ligands in the LS state result in geometries with the largest differences between DFT and CASPT2; however, good agreement overall is observed between DFT and CASPT2. For the larger complexes, moderate sized basis sets yield geometries that compare well with DFT and available experimental data. We recommend using the (10e,12o) active space since convergence to a minimum structure was more efficient than with truncated active spaces (e.g., (6e,5o)) despite having similar Fe–ligand bond distances.

Accurate treatment of electron correlation and spin-orbit coupling is essential for understanding the electronic structure and spectroscopy of high-spin, open-shell transition metal dihalides. The experimental photoelectron spectra of MnX2, X = Cl, Br, I, have been interpreted on the basis of multi-configurational ab initio electronic structure calculations. The intensity of the photoelectron bands have been calculated from the norms of the Dyson orbitals, which represent the transition probability to a particular ionic state from a given starting state. The present study provides a detailed account of the contribution of the one- and two-electron transitions in the experimental photoelectron spectra of the title molecules and outlines the nature of the photoelectron bands in terms of the closely spaced quintet and septet states of the cationic species.

Linear or near linear bicoordinated mononuclear Co(II) complexes are studied as potential single molecule magnets due to the strong spin-orbit constant of Co(II) and its low coordination number that retains the metal orbital angular momentum unquenched. The spin-orbit coupled unquenched orbital angular momentum of the metal center upon interaction with ligand field produces strong magnetic anisotropy. The role of ligand environment, structural distortion and conformational change on the magnetic anisotropy of the Co(II) complexes are investigated employing ab initio electronic structure calculations. The zero-field splitting parameters, g-tensors, and the transition magnetic moment matrix elements among the Kramer's pairs are evaluated to obtain the effective anisotropy barriers (Ueff) and mechanism of relaxation of magnetization in a series of Co(II) complexes, where the estimated values of Ueff range between 394 cm-1 to 974 cm-1, representing the largest effective anisotropy barrier reported for transition metal complexes. The calculations reveal that the ligand field strength, structural distortion and conformational changes not only affect the magnetic anisotropy barrier but also significantly alter the mechanism of relaxation of magnetic moments.

(PMe3)2FeCl3 is an Fe(III) complex that exists in the intermediate-spin ground state in a distorted trigonal bipyramidal geometry. An electronic state with high-spin configuration lies in close vic...

Mononuclear Fe(iii) complexes commonly exist in high-spin or low-spin states, whereas their occurrence in the intermediate-spin state (S = 3/2) is scarce. The magnetic anisotropy in two trigonal-bipyramidal mononuclear Fe(iii) complexes, (PMe3)2FeCl3 (1) and (PMe2Ph)2FeCl3 (2), in their intermediate-spin ground state has been examined by ab initio electronic structure calculations. The calculations successfully reproduce the experimental magnetic anisotropic barrier, Ueff in 1 (81 cm−1) and 2 (42 cm−1), which is shown to arise due to thermally assisted quantum tunneling of magnetization from the second Kramer’s doublets. The magnetic anisotropy in both the complexes is found to be significantly influenced by the axial ligands, while the equatorial ligands have negligible contribution. The large reduction in Ueff of 2 has been shown to arise due to the phenyl groups, which results in the lifting of orbital degeneracy of e″ and e′ frontier orbitals and leads to a net quenching of the orbital angular momentum of the metal center causing a diminished spin-orbit splitting in 2. While the crystal structure of 2 shows two phenyl rings out of plane to each other, the present study discovered another stable conformation of 2, where the two phenyl rings are in the same plane (2a). Unlike 2, the planarity of the two phenyl rings in 2a restores the degeneracy of the frontier orbitals, thereby increasing the spin-orbit splitting and a consequent rise in Ueff from 42 to 80 cm−1 in 2a.