Asymmetrical metal atoms

The model we have used for the description of the transition states implies that also the corresponding n-olefin complexes contain an asymmetric metal atom and that changes of configuration at the metal occur more slowly than the intramolecular transformation of the 7i-complex into the metal-alkyl complex. 

Brunner, H., Angeto. Chem. Internal. Edn., 1971, 10, 249 (optical activity due to asymmetric metal atoms). 

Asymmetrical metal atoms, 293 Atomic compounds, August Kekule s definition, 5... 

In the vast majority of reactions involving asymmetric complexes, the chirality has resided either in the molecule as a whole or in chiral ligands attached to the metal atom. There are a few examples, however, of complexes containing an asymmetric metal atom. The preparation of these complexes is illustrated by Eqs. (199) (Brunner, 1971) and (200) (Simmonneauxc/n/., 1975). 

Optical activity from asymmetric transition metal atoms. H. Brunner, Angew. Chem., Int. Ed. Engl., 1971,10, 249-260 (88). 

As in organic chemistry, there are several sources of chirality at a metal center. As for an asymmetric carbon atom in an organic molecule, the coordination of the metal ion by four different monodentate hgands in a tetrahedral con-... 

Abstract Sonoluminescence from alkali-metal salt solutions reveals excited state alkali - metal atom emission which exhibits asymmetrically-broadened lines. The location of the emission site is of interest as well as how nonvolatile ions are reduced and electronically excited. This chapter reviews sonoluminescence studies on alkali-metal atom emission in various environments. We focus on the emission mechanism does the emission occur in the gas phase within bubbles or in heated fluid at the bubble/liquid interface Many studies support the gas phase origin. The transfer of nonvolatile ions into bubbles is suggested to occur by means of liquid droplets, which are injected into bubbles during nonspherical bubble oscillation, bubble coalescence and/or bubble fragmentation. The line width of the alkali-metal atom emission may provide the relative density of gas at bubble collapse under the assumption of the gas phase origin. 

Structural details in CpNa(15-crown-5) 8499 show the Cp and the crown ether to be arranged in an almost coplanar manner with the oxygen atoms coordinated in a slightly asymmetric manner to sodium (Na-O 2.42-2.55 A). The Cp ring displays an asymmetric metal coordination mode with two close (2.67 and 2.71 A), two medium (2.85 and 2.91 A), and one longer Na-C distance (2.99 A). The asymmetric metal coordination may be accredited to the small sodium... 

How then, can one recover some quantity that scales with the local charge on the metal atoms if their valence electrons are inherently delocalized Beyond the asymmetric lineshape of the metal 2p3/2 peak, there is also a distinct satellite structure seen in the spectra for CoP and elemental Co. From reflection electron energy loss spectroscopy (REELS), we have determined that this satellite structure originates from plasmon loss events (instead of a two-core-hole final state effect as previously thought [67,68]) in which exiting photoelectrons lose some of their energy to valence electrons of atoms near the surface of the solid [58]. The intensity of these satellite peaks (relative to the main peak) is weaker in CoP than in elemental Co. This implies that the Co atoms have fewer valence electrons in CoP than in elemental Co, that is, they are definitely cationic, notwithstanding the lack of a BE shift. For the other compounds in the MP (M = Cr, Mn, Fe) series, the satellite structure is probably too weak to be observed, but solid solutions Coi -xMxl> and CoAs i yPv do show this feature (vide infra) [60,61]. 

X-Ray photoelectric ionization is believed to take place in a time interval of about 10-18 s. Therefore separate XPS peaks are possible for atoms if the lifetime of the asymmetric electronic state is greater than about 10 18 s, whether or not the atoms are structurally equivalent. We may represent the ground state of a localized mixed valence compound (involving two metal atoms differing in oxidation state by one unit) by the following formula, where the dot represents the extra valence electron M—M. The two possible XPS transitions can then be represented as follows, where the asterisk indicates core ionization,... 

Even metals like Cu, Pt, or Pd which form tetrahedral coordination compounds also from asymmetric compounds. In all these cases, therefore, the centre of asymmetry has a tetrahedral configuration just like an asymmetric carbon atom. 

In the case of [Fe3(CO)n]2-, a triply bridging carbonyl group is asymmetrically positioned (as deduced by the bond distances) above the Fe3 triangle, which is not found for [Ru3(CO)i i]2-,18 and [Os3(CO)i i]2-.15 In all the three compounds there is a carbonyl group bridging two metal atoms. 

Later on, Kagan [7] reported an important result with DIOP bisdentate ligand, the first ligand with a C2-symmetry axis, in which the stereoselectivity is improved by restricting the mobility around the metal atom and Morrison [8] reported the interesting neomenthyl diphenylphosphine ligand, which is devoid of any chiral phosphorus atom. As stated by Kotha [lb] "Early lessons learned in asynunetric hydrogenation paved the way to some of the new asymmetric catalytic processes". 

Coordination of ammonia or a substituted ammonia to a metal ion alters markedly the N — H dissociation rate (see See. 6.4.2). Since also proton dissoeiation of complexed ammines is base-catalyzed, then exchange can be made quite slow in an aeid medium. Thus, in a eoordinated system of the type 12, containing an asymmetric nitrogen atom (and this is the only potential souree of optical activity), there is every chance for a successful resolution in acid conditions, since inversion is expected only after deprotonation. It was not until 1966 that this was suc-eessfully performed, however, using the complex ion 12. A number of Co(III), Pt(II) and Pt(IV) complexes containing sarcosine or secondary amines have been resolved and their raeemizations studied.Asymmetrie nitrogen centers appear eonfined to d and d ... 

The structure of the cluster compound [H2FeRu3(CO)j3] is based on a tetrahedron of metal atoms. There are three terminal CO groups bonded to each Ru, two terminal CO groups bonded to Fe, two asymmetric bridging CO groups bonded between Fe and Ru atoms, and it is suggested that the two... 

Section 5.3 The vibrations of CO adsorbed on the Cu(100) surface were examined using an asymmetric slab model of 4 layers with the atoms in the two bottommost layers fixed at bulk positions and all remaining atoms allowed to relax previous to the calculation of the normal modes. The supercell had c(2 x 2) surface symmetry, containing 2 metal atoms... 

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