title: Unraveling σ and π Effects on Magnetic Anisotropy in cis-NiA4B2Complexes: Magnetization, HF-HFEPR Studies, First-Principles Calculations, and Orbital Modeling.
abstract: By using complementary experimental techniques and first-principles theoretical calculations, magnetic anisotropy in a series of five hexacoordinated nickel(II) complexes possessing a symmetry close to C2v, has been investigated. Four complexes have the general formula [Ni(bpy)X2]n+(bpy=2,2'-bipyridine; X2=bpy (1), (NCS-)2(2), C2O42-(3), NO3-(4)). In the fifth complex, [Ni(HIM2-py)2(NO3)]+(5; HIM2-py=2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-hydroxy), which was reported previously, the two bpy bidentate ligands were replaced by HIM2-py. Analysis of the high-field, high-frequency electronic paramagnetic resonance (HF-HFEPR) spectra and magnetization data leads to the determination of the spin Hamiltonian parameters. The D parameter, corresponding to the axial magnetic anisotropy, was negative (Ising type) for the five compounds and ranged from -1 to -10 cm-1. First-principles SO-CASPT2 calculations have been performed to estimate these parameters and rationalize the experimental values. From calculations, the easy axis of magnetization is in two different directions for complexes 2 and 3, on one hand, and 4 and 5, on the other hand. A new method is proposed to calculate the g tensor for systems with S=1. The spin Hamiltonian parameters (D (axial), E (rhombic), and gi) are rationalized in terms of ordering of the 3 d orbitals. According to this orbital model, it can be shown that 1) the large magnetic anisotropy of 4 and 5 arises from splitting of the eg-like orbitals and is due to the difference in the σ-donor strength of NO3-and bpy or HIM2-py, whereas the difference in anisotropy between the two compounds is due to splitting of the t2g-like orbitals; and 2) the anisotropy of complexes 1-3 arises from the small splitting of the t2g-like orbitals. The direction of the anisotropy axis can be rationalized by the proposed orbital model.
authors.affiliation: Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS, Université de Paris-Sud 11, 91405, Orsay Cedex, France.|Laboratoire MSC, Université Paris 7 Diderot, Paris, France.
authors.forename: Gaëlle
authors.initials: G
authors.lastname: Charron
authors.affiliation: Laboratoire de Chimie et Physique Quantiques, CNRS, Université Toulouse III, 118 route de Narbonne, 31062, Toulouse, France.|CTCC, Department of Chemistry, University of Tromsø, 9037, Tromsø, Norway.
authors.forename: Elena
authors.initials: E
authors.lastname: Malkin
authors.affiliation: Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, 23 rue du Loess, BP 43, 67034, Strasbourg Cedex 2, France.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Rogez
authors.affiliation: Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS, Université de Paris-Sud 11, 91405, Orsay Cedex, France.
authors.forename: Luke J
authors.initials: LJ
authors.lastname: Batchelor
authors.affiliation: Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS, Université de Paris-Sud 11, 91405, Orsay Cedex, France.
authors.forename: Sandra
authors.initials: S
authors.lastname: Mazerat
authors.affiliation: Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS, Université de Paris-Sud 11, 91405, Orsay Cedex, France.
authors.forename: Régis
authors.initials: R
authors.lastname: Guillot
authors.affiliation: Laboratoire de Chimie et Physique Quantiques, CNRS, Université Toulouse III, 118 route de Narbonne, 31062, Toulouse, France.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Guihéry
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses, CNRS, 25 rue des Martyrs, B.P. 166, 38042, Grenoble Cedex 9, France.
authors.forename: Anne-Laure
authors.initials: AL
authors.lastname: Barra
authors.affiliation: Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS, Université de Paris-Sud 11, 91405, Orsay Cedex, France.
authors.forename: Talal
authors.identifier: http://orcid.org/0000-0002-9311-3463
authors.initials: T
authors.lastname: Mallah
authors.affiliation: Laboratoire de Chimie et Physique Quantiques, CNRS, Université Toulouse III, 118 route de Narbonne, 31062, Toulouse, France.
authors.forename: Hélène
authors.identifier: http://orcid.org/0000-0002-6302-7820
authors.initials: H
authors.lastname: Bolvin
keywords: EPR spectroscopy; ab initio calculations; magnetic properties; nickel; quantum chemistry