Density functional studies on dinuclear {(NiGdIII)-Gd-II} and trinuclear {(NiGdNiII)-Gd-II-Ni-III} complexes: magnetic exchange and magneto-structural maps

Theoretical calculations using density functional methods have been performed on two dinuclear {Ni-II-Gd-III} and two trinuclear {Ni-II-Gd-III-Ni-II} complexes having two and three mu-OR (R = alkyl or aromatic groups) bridging groups. The different magnetic behaviour, having moderately strong ferromagnetic coupling for complexes having two m-OR groups and weak ferromagnetic coupling for complexes having three mu-OR groups, observed experimentally is very well reproduced by the calculations. Additionally, computation of overlap integrals MO and NBO analysis reveals a clear increase in antiferromagnetic contribution to the net exchange for three mu-OR bridged {Ni-Gd} dimers and also provides several important clues regarding the mechanism of magnetic coupling. Besides, MO and NBO analysis discloses the role of the empty 5d orbitals of the Gd-III ion on the mechanism of magnetic coupling. Magneto-structural correlations for Ni-O-Gd bond angles, Ni-O and Gd-O bond distances, and the Ni-O-Gd-O dihedral angle have been developed and compared with the published experimental {Ni-Gd} structures and their J values indicate that the Ni-O-Gd bond angles play a prominent role in these types of complexes. The computation has then been extended to two trinuclear {Ni-II-Gd-III-Ni-II} complexes and here both the {Ni-Gd} and the {Ni-Ni} interactions have been computed. Our calculations reveal that, for both structures studied, the two {NiGd} interactions are ferromagnetic and are similar in strength. The {Ni-Ni} interaction is antiferromagnetic in nature and our study reveals that its inclusion in fitting the magnetic data is necessary to obtain a reliable set of spin Hamiltonian parameters. Extensive magneto-structural correlations have been developed for the trinuclear complexes and the observed J trend for the trinuclear complex is similar to that of the dinuclear {Ni-Gd} complex. In addition to the structural parameters discussed above, for trinuclear complexes the twist angle between the two Ni-O-Gd planes is also an important parameter which influences the J values.