Priorities of ligands

Since the priority of ligands in stereochemical descriptors is indicated by numbers each ligand has also been given... 

Primary groups, 54 Priorities of ligands, I iff Probability factor, 39, 59 Propagation, 34 Pyran, 285 Pyrazine, 458 Pyridine, 454... 

Re and Si Labels used in stereochemical descriptions of heterotopic faces. If the CIP priority of the three ligands a, b, and c is assigned as a > b > c, the face that is oriented clockwise toward the viewer is called Re, while the face with a counterclockwise orientation of a — b — c is called Si, as shown in Figure 1-34 ... 

The CIP ranking of ligands is termed the CIP priority and is determined according to the subrules 0) Nearer end of axis or side of plane precedes further. 1) Higher atomic number precedes lower. 2) Higher atomic mass precedes lower. 3) z (seqcis) precides E (seqtrans) ( ). 4) Like pair (R,R or S,5)... 

The tin compound in Figure 5 illustrates the CIP priorities for ligands of different atomic number. Chlorine is atomic number 17, silicon 14, nitrogen 7, and carbon 6. Note, the higher the priority, the lower the CIP priority number. It is not uncommon to have ligating atoms of the same atomic number being compared as shown for the cobalt compound in Figure 6. 

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