Single-molecule magnets (SMMs) are a class of coordination compounds that attract attention because of their magnetic bistability.[1] These finite-size molecules show a slow relaxation of the magnetization at low temperatures owing to an energy barrier for spin reversal resulting in a hysteresis of the magnetization which is of purely molecular origin. [2, 3] SMMs promise access to dynamic random access memory (RAM) devices for quantum computing and to ultimate high-density memory storage devices in which each bit of digital information might be stored on a single molecule.[4] The energy barrier (D S t ·S t 2 ) for spin reversal arises from the combination of a high spin ground state S t and a magnetic anisotropy of the easy-axis type (negative zero-field splitting parameter D S t ). [2,5] Since the discovery of the fascinating SMM behavior of Mn 12 , a lot of synthetic efforts have been devoted to the preparation of new molecules with an increased anisotropy barrier and impressive new structural motives have been reported. [6] The necessary requirements for SMMs are a high spin ground state S t and a strong magnetic anisotropy D S t . To rationally design polynuclear complexes with high spin ground states S t , a control of the exchange couplings is highly desirable. The main component of the magnetic anisotropy of the ground state (D S t ) usually comes from the projection of the single-site anisotropies (D i ) onto the spin ground state S t , while dipolar and anisotropic interactions yield only minor contributions. [7,8] Since zero-field splittings are tensor quantities, the projection of the single-site zerofield splittings onto the spin ground state may vanish when the metal-ion arrangement approaches a cubic symmetry. Thus, a rational design of SMMs requires a control of the molecular topology which can not be achieved by simply increasing the nuclearity of complexes and using small bridging ligands. Another prerequisite for a SMM to function as a data storage is the minimization of the quantum-mechanical magnetization tunneling which provides an alternative pathway for spinreversal and thus the loss of information. This tunneling mechanism is directly related to the rhombic component of the magnetic anisotropy expressed by E S t which is exactly zero for complexes with at least a threefold axis. Thus, there are several requirements for a targeted synthesis of SMMs which must be considered when designing a polynucleating ligand of low flexibility.We have designed the C 3 -symmetric triplesalen ligand C (Scheme 1) which combines the phloroglucinol bridging unit (A) with the coordination environment of a salen ligand (B). [9] In trinuclear Cu II [10] and Mo V [11] complexes the phloroglucinol bridging unit A acts as a ferromagnetic coupler by the spin-polarization mechanism. To introduce magnetic anisotropy we have been choosing a salen-like coordination environment B which causes a pronounced magnetic anisotropy through its strong ligand field in the basal plane. [8,12] A well documented example is t...

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YbPtSn and Yb

The nature of chemical bonding in the complex carbides Sc3[Fe(C2)2] (1) and Sc3[Co(C2)2] (2) has been explored by combined experimental and theoretical charge density studies. The structures of these organometallic carbides contain one-dimensional infinite TC4 (T = Fe, Co) ribbons embedded in a scandium matrix. Bonding in 1 and 2 was studied experimentally by multipolar refinements based on high-resolution X-ray data and compared to scalar-relativistic electronic structure calculations using the augmented spherical wave method. Besides substantial covalent T-C bonding within the TC4 ribbons, one also observes discrete Sc-C bonds of noticeable covalent character. Furthermore, our study highlights that even tiny differences in the electronic band structure of solids might be faithfully recovered in the properties of the Laplacian of the experimental electron density. In our case, the increase of the Fermi level in the organometallic Co(d9) carbide 2 relative to its isotypic Fe(d8) species 1 is reflected in the charge density picture by a significant change in the polarization pattern displayed by valence shell charge concentrations of the transition metal centers in the TC4 units. Hence, precise high-resolution X-ray diffraction data provide a reliable tool to discriminate and analyze the local electronic structures of isotypic solids, even in the presence of a severe coloring problem (Z(Fe)/Z(Co) = 26/27).