Highly Anisotropic Exchange Interactions in a Trigonal Bipyramidal Cyanide-Bridged NiII 3OsIII 2 Cluster

This article is a part of our efforts to control the magnetic anisotropy in cyanide-based exchange-coupled systems with the eventual goal to obtain single-molecule magnets with higher blocking temperatures. We give the theoretical interpretation of the magnetic properties of the new pentanuclear complex {[NiII(tmphen)2]3[OsIII(CN)6]2}·6CH3CN (NiII 3OsIII 2 cluster). Because the system contains the heavy OsIII ions, spin−orbit coupling considerably exceeds the contributions from the low-symmetry crystal field and exchange coupling. The magnetic properties of the NiII 3OsIII 2 cluster are described in the framework of a highly anisotropic pseudo-spin Hamiltonian that corresponds to the limit of strong spin−orbital coupling and takes into account the complex molecular structure. The model provides a good fit to the experimental data and allows the conclusion that the trigonal axis of the bipyramidal NiII 3OsIII 2 cluster is a hard axis of magnetization. This explains the fact that in contrast with the isostructural trigonal bipyramidal MnIII 2MnII 3 cluster, the NiII 3OsIII 2 system does not exhibit the single-molecule magnetic behavior.