Analyzing Anisotropic Exchange in a Pentanuclear Os(2)Ni(3) Complex

Spin Hamiltonian parameters of a pentanuclear Os [Formula: see text] Ni [Formula: see text] cyanometallate complex are derived from ab initio wave function based calculations, namely valence‐type configuration interaction calculations with a complete active space including spin‐orbit interaction (CASOCI) in a single‐step procedure. While fits of experimental data performed so far could reproduce the data but the resulting parameters were not satisfactory, the parameters derived in the present work reproduce experimental data and at the same time have a reasonable size. The one‐centre parameters (local [Formula: see text] matrices and single‐ion zero field splitting tensors) are within an expected range, the anisotropic exchange parameters obtained in this work for an Os−Ni pair are not exceedingly large but determine the low‐T part of the experimental χT curve. Exchange interactions (both isotropic and anisotropic) obtained from CASOCI have to be scaled by a factor of 2.5 to obtain agreement with experiment, a known deficiency of such types of calculation. After scaling the parameters, the isotropic Os−Ni exchange coupling constant is [Formula: see text] cm(−1) and the D parameter of the (nearly axial) anisotropic Os−Ni exchange is [Formula: see text] (−1), so anisotropic exchange is larger in absolute size than isotropic exchange. The negative value of the isotropic J (indicating antiferromagnetic coupling) seemingly contradicts the large‐temperature behaviour of the temperature dependent susceptibility curve, but this is caused by the negative g value of the Os centres. This negative g value is a universal feature of a pseudo‐octahedral coordination with [Formula: see text] configuration and strong spin‐orbit interaction. Knowing the size of these exchange interactions is important because Os(CN) [Formula: see text] is a versatile building block for the synthesis of [Formula: see text] / [Formula: see text] magnetic materials.