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We report the first single‐molecule magnet (SMM) to incorporate the [Os(CN)6]3−moiety. The compound (1) has a trimeric, cyanide‐bridged MnIII‐OsIII‐MnIIIskeleton in which MnIIIdesignates a [Mn(5‐Brsalen)(MeOH)]+unit (5‐Brsalen=N,N′‐ethylenebis(5‐bromosalicylideneiminato)). X‐ray crystallographic experiments reveal that 1is isostructural with the MnIII‐FeIII‐MnIIIanalogue (2). Both compounds exhibit a frequency‐dependent out‐of‐phase χ′′(T) alternating current (ac) susceptibility signal that is suggestive of SMM behaviour. From the Arrhenius expression, the effective barrier for 1is found to be Δeff/kB=19 K (τ0=5.0×10−7s; kB=Boltzmann constant), whereas only the onset (1.5 kHz, 1.8 K) of χ′′(T) is observed for 2, thus indicating a higher blocking temperature for 1. The strong spin–orbit coupling present in OsIIIisolates the E′1g(1/2)(Oh*) Kramers doublet that exhibits orbital contributions to the single‐ion anisotropy. Magnetic susceptibility and inelastic neutron‐scattering measurements reveal that substitution of [Fe(CN)6]3−by the [Os(CN)6]3−anion results in larger ferromagnetic, anisotropic exchange interactions going from quasi‐Ising exchange interactions in 2to pure Ising exchange for 1with ${J{{{\rm MnOs}\hfill \atop \parallel \hfill}}}$=−30.6 cm−1. The combination of diffuse magnetic orbitals and the Ising‐type exchange interaction effectively contributes to a higher blocking temperature. This result is in accordance with theoretical predictions and paves the way for the design of a new generation of SMMs with enhanced SMM properties.

In the present article, we proffer an explanation for the origin of single molecule magnetism in [CuIILTbIII(hfac)2]2, a member of the novel class of mixed transition−lanthanide metal clusters. The theoretical model takes into account the crystal field acting on the TbIIIion as well as the exchange interaction between the TbIIIand CuIIions and provides a good account of the low-lying energy levels, rigorously measured by inelastic neutron scattering and magnetic data. We demonstate that the single molecule magnet (SMM) behavior is not a sole consequence of the low-lying levels of the lanthanide metal ion but a property of the tetranuclear cluster itself. The energy levels are shown to increase with the decrease of the mean value of the Z-projection of the total angular momentum of the cluster, thus forming a barrier for magnetization reversal that is wholly consistent with the observed SMM behavior. On the basis of this study, recommendations are formulated on how the SMM properties for nd−4f clusters may be further improved.

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