Metal cluster-centered transition

The CD signals in the near-UV to visible region are typically LMCT transitions. The authors also suggest that the higher wavelength signal at 354 nm may be assigned to a spin-forbidden "id — 4s metal cluster-centered transition that is favored by interactions of adjacent Cu+ ions. 

The metal-containing centers in MOFs can be either a mononuclear metal ion or a cluster unit, which are assembled by two or more metal ions. The metal cluster center commonly refers to as SBUs, which offer a prospective avenue toward the design and construction of MOFs [22], The formation of the SUBs can be controlled through the selection of suitable synthetic conditions, to some extent. Until now, more than hundred kinds of geometries of SBUs have been reported, among them, most are transition-metal carboxylate assemblies produced during the formation of MOFs under given conditions (Fig. 3.3). 

For mixed lanthanide-transition metal clusters, Yukawa et al. have synthesized an octahedral [SmNi6] cluster by the reaction of Sm3+ and [Ni(pro)2] in nonaque-ous medium [66-68]. The six [Ni(pro)2] ligands use 12 carboxylate oxygen atoms to coordinate to the Sm3+ ion, which is located at the center of an octahedral cage formed by six nickel atoms. The coordination polyhedron of the central Sm3+ ion may be best described as an icosahedron. The [SmNir, core is stable in solution but the crystal is unstable in air. The cyclic voltammogram shows one reduction step from Sm3+ to Sm2+ and six oxidation steps due to the Ni2+ ions. Later, similar [LaNis] and CjdNif> clusters were also prepared. 

The chemistry of octahedral metal clusters culminates in the center of the Periodic Table with the heavy transition metals Nb, Ta, Mo, W, and Re. There is a plethora of clusters where the M-M bonded core is surrounded (and shielded) by non-metal ligands. When moving to the left of the Periodic Table the decrease in valence electron concentration calls for a stabilization through incorporation of interstitial atoms into the cluster core. Actually, the stabilization of the cluster occurs... 

In molecules, the interaction of surrogate spins localized at the atomic centers is calculated describing a picture of spin-spin interaction of atoms. This picture became prominent for the description of the magnetic behavior of transition-metal clusters, where the coupling type (parallel or antiparallel) of surrogate spins localized at the metal centers is of interest. Once such a description is available it is possible to analyze any wave function with respect to the coupling type between the metal centers. Then, local spin operators can be employed in the Heisenberg Spin Hamiltonian. An overview over wave-function analyses for open-shell molecules with respect to local spins can be found in Ref. (118). 

In practice, however, already with a comparatively small number of metal atoms it is no longer feasible to investigate all possible spin states with all potential realizations by various local spin distributions. Assumptions on the interaction of the metal centers on the basis of their structural arrangement and experimental susceptibility measurements have to be made. For example, for the BS state of a tetranuclear transition-metal cluster, one has to decide which of the four metal atoms couple in an antiferromagnetic fashion with each other. Prominent coupling schemes are, e.g., the dimer-of-dimers 2-plus-2-type or the 3-plus-... 

It has been long known that for a given transition-metal cluster the open-shell and the closed-shell species may differ. Typically, metal-ligand bond lengths are elongated for the open-shell structures, where the metal centers carry local spins and electrons occupy antibonding orbitals, in comparison to their closed-shell... 

The method described above is of general validity and can be applied to transition metal clusters of arbitrary shape, size, and nucleanty. It should be noted that in the specific case of a system comprising only two interacting exchanged coupled centers, our general treatment yields the same result as that of Bencini and Gatteschi (121), which was specifically formulated for dimers. In this case, the relation between the spin-projection coefficient and the on-site spin expectation value is simply given by... 

The first carbidocarbonyl transition metal cluster to be recognized was Fe5C(CO),5 (1), which was isolated in very low yield from the reaction of triiron dodecacarbonyl with methylphenylacetylene and characterized by X-ray diffraction by Dahl and co-workers (2). The molecule (Fig. 1) comprises a square pyramidal Fe5 core with the carbide situated. 08 A below the center of a square face. Each iron atom bears three terminal carbonyls. Improved syntheses of 1 by protonation (5) or oxidation of [Fe6C(CO)l6]2-... 

The recent discovery of many metal complexes having t/2-H2 ligands or agostic C—H bonds has focused attention on a type of bonding that had hitherto seemed to be a domain of early main group elements and metal clusters three-center, two-electron bonds. As more and more examples are found, it has become obvious that such bonds are probably more widespread among transition metal compounds than had been commonly assumed and that it is about time to recognize the features the various species have in common. 

The use of transition metals or transition metal clusters to act as nodes for the modular self-assembly of diamondoid networks that are sustained by coordinate covalent bonds is also well established. Such architectures are of more than aesthetic appeal. Indeed, such structures have resulted in a class of compound with very interesting bulk and functional properties. Metal-organic diamondoid structures in which the spacer moiety has no center of inversion are predisposed to generate polar networks since there would not be any inherent center of inversion. Pyridine-4-carboxylic acid is such a ligand and bis(isonicotinato)zinc exists as a three-fold diamondoid structure that is thermally stable and inherently polar.33... 

---