Highest seniority

Find the shortest path in atoms, irrespective of their nature, from the subunit of the highest seniority to the subunit of the same seniority (Rules 15,16), if present, or of the second highest seniority (Rules 13,14). Where paths of equal length are identified as shortest, the choice depends on the seniority of the remaining subunits and the number and positions of substituents ... 

Orient the structure so that the direction from the most senior subunit to the subunit of the next highest seniority reads left to right ... 

Identify the preferred CRU starting from the highest seniority subunit and moving in the direction determined above. 

When two paths from the starting subunit (A) to both the next subunits of seeond highest seniority (B) are equally long, the shorter path from the starting subunit A to the subunit of third highest seniority (C) is to be followed. 

When three or more identical subunits of the highest seniority (A) are present in a CRU in the main chain (backbone), the starting point and direction are chosen in such a way that the shortest path through all the subunits A results. If there is a choice, the CRU with the shorter path to the subunits of second or third highest seniority (B or C) is selected. 

The CRU is written from left to right beginning with the subunit of highest seniority and proceeding in the direction involving the shortest route to the subunit next in seniority. 

Organic ligands are cited under C, and NO and CO are regarded as neutral. Because square brackets are always of highest seniority (or priority), a hierarchical sequence of enclosing marks is adopted to ensure that this seniority is preserved [ ], [()], [ () ], [ [()] ], [ [()] ], etc. 

The constituent subunit of highest seniority must contain one or more central atoms bridging groups between central atoms in the backbone of the polymer cannot be senior subunits. This is consistent with the principle of coordination nomenclature, in which the emphasis is laid on the coordination centre. 

The name of the polymer is poly(CRU). The preferred CRU is one beginning with the subunit of highest seniority. The order of seniority is heterocyclic rings, chains containing heteroatoms (in the descending order O, S, Se, Te, N, P), carbocyclic rings, and chains containing only carbon. The seniority is expressed by brackets and internal parentheses (see examples in Table 1.4). 

Within each of the structures above, the seniority is governed by the seniority of the individual constituent, see rules of Organic Nomenclature, e.g., the Blue Book mentioned in Section 1.18.5. The preferred CRU then is the one starting with the subunit of highest seniority and the direction is the one to the subunit with the same or next highest seniority. In the above example, the preferred CRU is... 

Begin with subunit of highest seniority and continue along the chain in the direction of... 

A single CRU must be selected following the rules given in Table 5.2.1. Here, the subunit of highest seniority is the oxygen atom. The lowest locant for substitution is the 1-bromo substitution. 

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