Chirality octahedral structures

The most common structures of arsenic compounds are tetrahedral and pyramidal, which are similar when the sterically active lone pair is counted. Tetrahedral symmetry holds the potential for chirality and indeed many chiral organoarsenic compounds have been prepared. Arsenic may also use d orbitals for (d-d)n bonding and for hybridization with s2 and p3 orbitals, resulting in trigonal bipyramidal or octahedral structures. In the former the more electronegative substituents occupy the apical position. 

Bridged binuclear, trinuclear and tetranuclear chelated octahedral structures were examined by Schaffer,17 who used the skew line helical definition for the chirality symbols A and A. The configurational isomers for a tetrakisbidentate, edge-fused-bis-octahedral structure are AA, AA, AA, and AA. 

Because ansa metallocenes had been found to be effective polymerization catalysts, exploration of chelating bis(arylimido) complexes has been undertaken, typical examples being (48-51). In general, these have distorted octahedral structures with mo-n in the range 1.73-1.75A and ZMo=N-R in the range 155-162°. Complex (51), which is chiral, catalyzed the kinetic resolution of styrene oxide with ane.e of 30%. 

Bis 8-quinolmato metal complexes are representative examples of distorted octahedral structures of the type (87). It was noted that these systems bear a topological resemblance to the chiral, octahedral, active metal sites that have been proposed for supported Ziegler-Natta catalysts, and are interesting for modeling catalytic properties. Similar comportment has been observed with diamine bis (phenolate) ligands. ... 

Examples 1 and 2 represent the delta (A) and lambda (A) forms of a complex such as [Co(NH2CH2CH2NH2)3]3+. The rules define the chiralities of two additional families of structures. These are the cTv-bisfbidentate) octahedral structures and the conformations of certain chelate rings. It is possible to use the system described below for complexes of higher poly dentate ligands, but additional rules are required.15... 

The efficacy of ansa metallocenes as polymerization catalysts has stimulated research into chelating bis(arylimido) complexes, including those with chiral ligands. The first of these, (62), and derivatives (63) and (64) (n= 1, 2) were reported by Gibson et al. in 1996.121 The complexes display distorted octahedral structures with d(Mo=N), Z(Mo=N—C) and Z(N=Mo=N) in the ranges 1.725-1.754 A, 155-162° and 100-103°, respectively. Complexes featuring strained, seven-membered, unsymmetrical ansa bis(imido ligands), e.g., (65), have also been reported.113 The first chiral bis(imido)-MoVI complex, C2-symmetric (66), catalyzes the kinetic resolution of styrene oxide and enantioselective trimethylsilylcyanation of benzaldehyde with 30% and 20% e.e., respectively.151... 

E7.17 Below is a picture of both isomers. The best way to do this problem is to draw both isomers as mirror images of each other, and look along one of the four C3 symmetry axes found of ideal octahedral structure. If the ligand backbone is rotating clockwise then the structure is the A isomer, if it rotates counter clockwise, it is the A isomer. When you do this, it is obvious which isomer you have, in the case of this problem, the complex drawn in the exercise is the A isomer. For more help with this concept read Section 7.10(b) Chirality and optical isomers. 

Consider, for example, Werner s ingenious determination of the structures of cis- and trans-[Pt(NH3)2Cl2] (2), and his demonstration of the octahedral structure of six-coordinate complexes through the optical resolution of [Co(en)2(NH3)X]2+ (X = Cl, Br) (3). We are now able, by modern techniques, not only to perform these demonstrations quickly, but, in the case of chiral complexes, to show the actual configurations of the isomers (A,5). ... 

It is fitting to begin this brief historical overview of chirality in organometallic and coordination chemistry with the name of Alfred Werner (1866-1919) who, as far back as 1893, applied van t Hoff and Le Bel s stereochemical ideas of the tetrahedral nature of the carbon atom to the structure of hexacoordinated metal complexes. He established their octahedral structure and predicted that some could exist in an enantiomeric form with the power of optical rotation. This prediction was followed in 1911 by the resolution of the two enantiomers of the complexes [Co (en)2(NH3)X]X2 (X = Cl, Br) (2.1)-X2 (en = ethylene diamine) (Figure 2.1). This overall work won him the Nobel Prize for Chemistry in 1913, following which he then went on to resolve the inorganic complex Co (OH)6[Co (NH3)4]3 Br6 (2.2)-Br6 (Figure 2.2). ... 

In 1897 Werner began the search for a definitive proof of his hypothesis of the octahedral structure of chiral cationic Co complexes, and to do so he required an efficient anionic resolving agent. Whether or not he himself was familiar with the work of Pope on the resolving power of the (+)-3-bromo-camphor-9-sulphonate anion (2.3) (Figure 2.3), it was his student L. King who chose to use this anionic agent to resolve the complexes [Co° en)2(NH3)X]X2 (X = Cl, Br) (2.1)-X2. ... 

With multidentate ligands such as terpyridine where the octahedral structure has restricted rotation due to the bonds between nitrogen heterocycles, atropisomerism involves axial chirality. Thus, Constable, Lacour et al. prepared an iron terp)uidine cation (Figure 38) that exists as two enantiomers (09NJC376). In the presence of a chiral anion and in low... 

Six is the most common coordination number. The most common structure is octahedral, but trigonal prismatic structures are also known. Octahedral compounds exist for to d transition metals. Many compounds with octahedral structures have already been displayed as examples in this chapter. Others include chiral tris(ethylenediamine) cobalt(III), [Co(en)3], and hexanitritocobaltate(III), [Co(N02)6], shown in Figure 9.30. 

Racheli Yes, Usha, Sason is right. In fact, if you look at Figure 9.3d, you will see that when the ligand is ethylene diamine (NH2CH2CH2NH2), Werner s octahedral coordination would predict a chiral compound that has a propeller shape. The octahedral structure is critical for such a prediction. 

Results from such a calculation for 1,4,7,10,13,16-hexaazacyclooctadecane (18-azacrown-6) [40] in a range of conformations are shown in Figure 3. Similar calculations has been carried out on 18-crown-6 [44]. Four possible conformations are considered for 18-azacrown-6 which encapsulate the metal in different geometric environments, i.e. meso octahedral, chiral octahedral, trigonal prismatic and hexagonal planar. Crystal structures are available for all but the last geometry and molecular mechanics calculations show an excellent fit to these experimental structures. 

Although tetrahedral carbon is probably the most important and certainly the most common chiral structure, many other chiral molecular structures can be classified as asymmetric or dissymmetric. In the very late nineteenth century, the "father of coordination chemistry" Professor Alfred Werner of the University of Zurich developed a theory for the structure of compounds where a central atom other than carbon could form bonds to different numbers of atoms. This "coordination theory" was most successful and applicable to structures in which the number of bonds (the coordination number) was 6. Werner was very familiar with the work of van t Hoff and Le Bel on tetrahedral carbon, and he spent many years trying to synthesize and separate compounds that were based on octahedral geometry. 

It was easy to see that in an octahedral structure four different compounds could be made with a central metal atom with two each of three different substituents. This compound would have the general formula MA2B2C2. Different compounds that could be formed with identical formula are called isomers. It was also easy to see that two of the six possible isomers were chiral. All of the possible isomers of octahedral MA2B2C2 are drawn in Figure 1.15. The solid lines in this figure represent the bonds from the central metal atom to the substituents, and the dashed lines have been added to show the outline of the octahedron, which contains eight triangular faces. A real proof of the coordination theory of Werner was the successful measurement of optical rotation in octahedral complexes in 1911 [3]. 

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