Ligands priority numbers

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. 

The structural formula of this new cytostatic (picoplatin) is shown below. In determining the stereo descriptor, the situation arises here that the highest ranking ligand occurs twice. In such cases the priority number of the ligand coordinated trans to it with the lowest priority (i.e. with the highest priority number) is used as the configuration index (principle of trans maximum difference). 

Priming convention. The configuration index is especially useful for bis(tridentate) complexes and for more complicated cases. Bis(tridentate) complexes exist in three di-astereoisomeric forms which serve to illustrate the utility of a priming convention. These isomers are represented below, along with their site symmetry symbols and configuration indexes. For Examples I, 2, and 3, the two ligands are identical and the ligating-atom priority numbers are indicated. 

Priority is assigned to the central atoms as follows. For cases of type (i) the central atom carrying the greater number of alphabetically preferred ligands is numbered 1. For cases of type (ii) the number 1 is assigned to the higher priority central element of Table VI, whatever the ligand distribution. 

The presence of polydentate ligands may require the use of primes on some of the numbers in the configuration index. The primes are used to indicate either that donor atoms are not part of the same polydentate ligand as those that have unprimed priority numbers, or that the donor atoms belong to different parts of a polydentate ligand that are related by symmetry. A primed priority number means that that donor atom has lower priority than the same kind of donor atom without a prime on the priority number. More detail on the priming convention can be found in Section IR-9.3.5.3. 

The chirality of an SPY-4 system is defined in a similar way. In this case, the viewer looks along the formal C4 axis in such a way that the ligands are further away than the central atom. The priority numbers are then used to assign the symbol C or A, as for the SPY-5 system. 

The C/A assignment for polydentate ligands is illustrated by Example 6 which uses the priming convention developed in Section IR-9.3.5. Note that priority number 2 has higher priority than 2/. 

The methods for differentiating between stereoisomers outlined earlier in this Chapter require the assignment of priorities for the ligand atoms attached to the central atom (i.e. the donor atoms). These priority numbers are then used in the configuration index, which describes the relative positions of the ligands, and in the assignment of the absolute configuration of the compound. 

The following sections outline the method used to arrive at the priority numbers for a given set of donor atoms, and the ways that the basic mles have to be modified in order to describe adequately systems that include polydentate ligands. These modifications, which are collectively referred to as the priming convention, make use of primes on the priority numbers to indicate which donor atoms are grouped together within a particular polydentate ligand. 

Once the ligands have been compared, the priority numbers are assigned as follows ... 

The priority numbers on one of the ligands are arbitrarily primed. The primed number is assigned lower priority than the corresponding unprimed number, but a higher priority than the next higher unprimed number. Thus 1 has lower priority than 1, but higher than 2. 

The technique also distinguishes between stereoisomers for complexes of higher polydentate ligands as indicated in Examples 4, 5 and 6 for linear tetradentate ligands such as N,A,-bis(2-aminoethyl)ethane- 1,2-diamine. In this case, the donor atom priority numbers in half of the tetradentate ligand have been primed. 

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. 

Figure 6. Assignment of CIP priority numbers for ligands with donor atoms of the same atomic number...

Figure 7. CIP relative priority numbers by ligand exploration...

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