Main-group metal ions

These reports sparked off an extensive study of metalloporphyrin-catalyzed asymmetric epoxidation, and various optically active porphyrin ligands have been synthesized. Although porphyrin ligands can make complexes with many metal ions, mainly iron, manganese, and ruthenium complexes have been examined as the epoxidation catalysts. These chiral metallopor-phyrins are classified into four groups, on the basis of the shape and the location of the chiral auxiliary. Class 1 are C2-symmetric metalloporphyrins bearing the chiral auxiliary at the... 

These hydrogen-bonded salts show distinct potential for the controlled design of new crystalline structures, and are applicable to incorporation of a wide variety of transition metal and main group metal ions. The work of Mitzi on perovskite structures in which perhalometallate layers are linked via organic alkyl ammonium cations that interact via N-H X-M hydrogen bonds shows another area of potential application [43] (see Sections 4.4.2 and 7). 

More than 400 compounds of ABjX, stoichiometry adopt the ThCr Si, type struc-ture.27 In these A is typically an alkali, alkaline earth, or rare earth metal. B may be a transition metal ora main-group metal. X is a group VA (15), IVA(14), or occasionally IIIA (13) nonmeial. The compounds in which we shall be most interested are composed of an alkaline earth metal (A = Ca, Sr, Ba), a transition metal (B = Mn, Fe. Co, Ni, Cu), and phosphorus (see Table 7.2). These compounds are isostructural and crystallize in the ThCr2Si2 structure with space group /4/mwm. The unit cell (Fig. 7.28) consists of eight A" ions at the corners of a rectangular parallelepiped plus one... 

Neither gold(i) nor gold(lll) forms a stable aquated ion ([Au(OH2)2 4 ] or [Au(OH2)4 ], respectively) analogous to those found for many transition metal and main group cations. Both are thermodynamically unstable with respect to elemental gold and can be readily reduced ... 

The number of ligands bound to the metal ion is designated as the coordination number of the metal. Some main group elements form complexes that are similar to transition metal complexes. These include aluminum, tin, and lead, for example, but for the most part organometallic chemistry employs transition metals. This is due to the partially-filled d-orbitals. 

Metal ions with higher (>6) coordination numbers, such as lanthanides and actinides, have less predictable coordination geometries than lower-coordination-number transition metal and main group ions, while it is difficult to incorporate these ions into rationally designed self-assembled supermolecules, a number of structures have been reported. The unique electronic properties of the /-elements have been exploited in supramolecular complexes to generate functional molecular devices. ... 

Hagihara and coworkers in the 1970 s and early 1980 s have reported a successftil condensation polymerization strategy to incorporate late second- and third-row transition metal ions (mainly Pt(II) and Pd(II)) as part of a polymeric linear chain [65-67]. Since these metal ions prefer square-planar geometric structures, they designed compounds that contained two reactive chlorine groups in a trans orientation. Condensation of such di-fimctional monomers with traw -diacetylides afforded linear polymers which are calledpolyynes (Fig. 8.32). 

Several literature sources contain information on the synthesis of complexes with metal-metal bonds between transition metals or between transition metals and main group metals. o The use of ion exchange reactions in the rational synthesis of heterobimetallic compounds is widespread. Such reactions are of considerable utility because the starting materials are readily accessible and typically the reactions proceed cleanly and in high yield. i A number of ion exchange reactions that lead to the isolation of complexes containing metal-metal bonds have been described in Inorganic Syntheses... 

The volatile, air-sensitive Hquid species (CH3)2AlB3Hg and (CH3)2GaB3Hg are prepared by the direct reaction of the corresponding main group metal hahde and salts of the [B3Hg] ion, in the absence of solvent (178). The reaction of (CH3)2AlB3Hg and A1(BH 3 results in the species (BH 2AlB3Hg. These small metallaboranes are fluxional in solution and have limited thermal stability at room temperature. 

Two classes of aldolase enzymes are found in nature. Animal tissues produce a Class I aldolase, characterized by the formation of a covalent Schiff base intermediate between an active-site lysine and the carbonyl group of the substrate. Class I aldolases do not require a divalent metal ion (and thus are not inhibited by EDTA) but are inhibited by sodium borohydride, NaBH4, in the presence of substrate (see A Deeper Look, page 622). Class II aldolases are produced mainly in bacteria and fungi and are not inhibited by borohydride, but do contain an active-site metal (normally zinc, Zn ) and are inhibited by EDTA. Cyanobacteria and some other simple organisms possess both classes of aldolase. 

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