The names of some common ligands are listed alphabetically below, the numbers preceding them indicating the relative priorities assigned to the ligands under the sequence rule. A higher number denotes greater preference; for example, a hydroxy group (numbered 57) takes precedence over an amino group (numbered 43). It should be noted that any change to the structure of a listed ligand may alter the order of preference. Where a ligand consists of or contains a heavy or radioactive isotope of hydrogen, the relative priorities are tritium > deuterium > protium. Racemates with a single chiral centre are labelled (RS); extensions of the sequence rule are used in the E/Z, pro-R/pro-S, pro-E/pro-Z, and Re/Si nomenclature systems.
From Oxford Dictionary of Biochemistry and Molecular Biology in Oxford Reference.