Limitations on the D -Parameter in Ni(II) Complexes.

A number of hexacoordinate, pentacoordinate, and tetracoordinate Ni(II) complexes have been investigated by applying ab initio CASSCF + NEVPT2 + SOC calculations and Generalized Crystal Field Theory. The geometry of the coordination polyhedron covers D 4 h , D 3 h , D 2 h , D 2 d , C 4 v , C 3 v , and C 2 v symmetry. The calculated spin-Hamiltonian parameters D and E were compared to the available experimental data. The limiting values of the D -parameter in the class of Ni(II) complexes are identified. Magnetic anisotropy in Ni(II) complexes, expressed by the axial zero-field splitting parameter D , seriously depends upon the ground and first excited electronic states. In hexacoordinate complexes, the ground electronic term is nondegenerate 3 B 1g for the D 4 h symmetry; D is slightly positive or negative. In tetracoordinate systems, D is only positive when the electronic ground state is nondegenerate 3 A or 3 B; this diverges on the τ 4 path when oblate bisphenoid approaches the prolate geometry and a level crossing with 3 E occurs. In pentacoordinate systems, D could be extremely negative when approaching a trigonal bipyramid (Addison index τ 5 ∼ 1, ground state 3 E″). In pentacoordinate Ni(II) complexes with the D 3 h and C 3 v symmetry of the coordination polyhedron, the ground electronic term is orbitally doubly degenerate which causes the D -parameter stays undefined. It is emphasized that one has to inspect compositions of the spin-orbit multiplets from the spin states | M S ⟩ and check whether the weights confirm the expected spin-Hamiltonian picture: with D > 0, the ground state contains a dominant part of |0⟩ (close to 100%) whereas with D < 0 the spin-orbit doublet is formed of |±1⟩ with high weights (approaching 50 + 50%). The calculations show that the situations are not black and white, and the mixing of the states might be more complex especially when the rhombic zero-field splitting parameter E is in the play. In the case of the 3 E ground term, six spin-orbit multiplets are formed by mixing six | M S ⟩ states from the ground and quasi-degenerate excited states.