Chirality symbol

Symbols R and 5, There are two established and well-u<>ed systems for chirality symbols and these differ in fundamental ways. The first, the convention for chiral carbon atoms is equally appropriate to metal complexes and is most often used in conjunction with ligand chirality. However, it can be applied to metal centers Hnd has been useful for pseudotetrahedral organometallic complexes when, for example, cyclopcnhidienyl ligands are treated as if they were monodentate ligands of high priority. 

Attempts to produce descriptors similar to cis and trims for stereochemicidly more complicated coordination entities have tailed to achieve generality, and labels such as foe and mer are no longer recommended. Nevertheless, a diastereoisomeric structure may be indicated for any polyhedron using a configuration index as an affix to the name or formula. Finally, the chiralities of enantiomeric structures can be indicated using chirality symbols. 

Chirality symbols based on the priority sequence. The procedure is applied as for tetrahedra, but modified because there is a unique principal axis. The structure is oriented so that the viewer looks down the principal axis, with the ligand having the higher priority closer to the viewer. Using this orientation, the priority sequence of ligating atoms in the [horizontal] plane is examined. If the sequence proceeds in a clockwise fashion, the chirality symbol C is assigned. Conversely, if the sequence is anticlockwise, the symbol A is assigned. 

An example of a real compound to which this system may be applied is found on page 488. c/.v-dicarbonylchloro(cyclopentadienyl)(methyldiphcnylphosphine)mo]ybdenum(II). The enantiomer shown there has the chirality symbol C.]... 

Applying the pseudo-atom convention to the iron complex in Figure 8, the iodine atom is priority 1, the t1S-C5H3R2 ligand is priority number 2, the phosphorus atom is priority number 3, and carbon is priority number 4.15 When the iron is viewed from the side opposite the priority number 4, the sequence is 1,2,3 in the anticlockwise or 5 direction. The highest priority carbon in the cyclopentadienyl ligand, indicated by an asterisk, is designated with the R chirality symbol by application of the extended CIP sequence rule. 

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. 

Use italic type for certain chirality symbols and symmetry site terms. 

Beginning with the highest priority atom in the plane perpendicular to the reference axis, the clockwise and anticlockwise sequences of priority numbers are compared, and that with the lower number at the first point of difference is chosen. If the chosen sequence results from a clockwise reading of the priority numbers, then the structure is given the chirality symbol C, otherwise it is given the symbol A. 

Chirality symbol = C Chirality symbol = A Chirality symbol = C... 

Example 4 shows the compound [CoBr2(en)(NH3)2]+ which has the polyhedral symbol OC-6 and the configuration index 32. The chirality symbol is C. 

The configurations of centers or other stereoelements in a ligand such as the configuration of an amino acid ligand are noted after the chirality symbol of the central atoms. 

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