notes on nomenclature adequately describe the difference hetween the two isomers as indicated above. The isomerism encountered with three substituents presents a somewhat different problem.
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Coordination Nomenclature. II. Names for Isomers A previous Note' was devoted to a general description of the pattern of coordination nomenclature and the considerations involved in naming ligands. This note deals with the designation of structure and the principles covering the nomenclature of various configurations. The incorporation of structural details into nomenclature is desirable at all times (if i t can be done without greatly lengthening names) hut, for meaningfully distinguishing isomers, some indication of the differences in structure is essential. Isomers of the type M-SCN
M-NCS
are readily distinguished by specific names for the ligands, thiocyanato versus isothiocyanato, or by designating the atom through which the ligand is coordinated. Isomerism of the type
is indicated in the normal names pentaamminehromocobalt(2+) sulfate pentaamminesulfatocobalt(l+) bromide
When A. Werner first discovered geometrical isomerism among coordination compounds and conducted a sufficient number of reactions to establish that in one isomer two ligands were near to each other and in the other farther apart
NH3
0s-
flMS-
cis-tetraamminedichlor(lcobalt(m) ion tma5-tetraamminedichlorocohalt(III)ion
he could borrow two prefixes from organic chemistry to
Originally the prefix cis- was applied to the first to indicate that each substituent was near to the other like substituents and trans- was applied to the other to indicate that one of the ligands was across the molecule from an identical ligand.2 Later the prefixes fac- for facial and mer- for meridional came into common use. Further, cis- and trans- were applied to isomers of square planar complexes
Nevertheless, such descriptors are of limited usefulness. The isomers of the general type Mazbzcd include one in which each like pair is cis, one in which each like pair is trans, and three in which two pairs are cis and the third trans, and for Mazbzcd the number of possible isomers increases to eight. For compounds of the type Mahcdef there are 30 possible isomers. Even for square planar complexes there are three isomers for compounds of the type Mabcd. Hence, a general system is needed which will treat all possible isomers. Locants: General Considerations To decide upon a scheme for assigning locants to the spatial positions around an atom, i t is necessary first to decide what kinds of locants to use for this purpose. Numbers are widely used as locants for atoms in a chain, ring or polycycle. Hence, if lower case letters are used for positions around an atom, there should he no confusion. Then one must decide upon an order lor assigning locants. If one assigns locants to the corners of a square continuously, it makes no difference which direction one proceeds hecause on turning one of the squares over, the two directions produce the same numbering.
'Fernelius, W. C., Loening, K. L., and Adams, R. M., J. CHEM. EDUC., 51,000 (1974). It seems wise today to limit the use of cis to those cases where t w o identical groups occupy adjacent positions and trans to those
cases where t c o identical groups occupy positions acrurr a molecule at the greatest distance from each other.
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This statement cannot be made for a three-dimensional object. If one affixes the first locant to the apex of the tetrahedron then, depending on the order of procedure for the points in the base, one obtains two different designations.
We recognize these two situations as the familiar enantiamorphic pair of optical antipodes. A similar situation is encountered for the octahedral configuration 0
the axis is not symmetrical, choose that end with a single atom (or smallest number of atoms) in the first plane to be numbered; third, locate the first plane of atoms (atom) to receive locants; fourth, orient the molecule so that the first position to receive a locant in the first plane with more than one atom is in the 12 o'clock position; fifth, assign locant designators to the axial position or to each coordinating position in the first plane, beginning at the 12 o'clock position and moving in whichever direction (clockwise or anti-clockwise) will give the lowest locants to the first (or earliest) named Iigands; sixth, from the first plane, move to the next lower and continue assignments moving in the same direction as was done with the previously assigned plane, always returning to the 12 o'clock position or in the position just beyond i t before assiming any locants in that plane; seventh, continue this operation h i l all positions are assigned. The application of this procedure in typical configurations of cwrd~nation number eight are shown helou,. (The direction of rotation in each case shows the right handed isomer.)
n
5 q ~ ~m r et l p r ~ s m dlgonol dodecahedron
(Other configurations possess the same property.) So then for the octahedral (and other configurations) one has the choice of affixing locants in a constant direction, say clockwise, and getting a different set of locants for optical antipodes or of proceeding in alternate directions so that enantiomers have the same locants. The 1971 IUPAC Rules3 follow the first procedure. However, objection has been strong and nearly universal for the procedure which gives both enantiomers the same set of locants and uses an additional descriptor to define the sign of rotation. The sequence-rule method for specifying the absolute configuration of enantiomers on the self-contained basis of a few general rules4 will be presented in a later Note. Gaining a consensus in the matter of assigning locants was no easy matter. A few felt that a shift from a square planar to an octahedral configuration should not involve any alteration in the locants already assigned to the former. Such a procedure would assign the last locants to the polar positions and the assignment of locants would not be continuous. During the time that number locants, instead of lower case letters, were being considered there was a desire to have the sum of the locants trans to each other add to a common sum: 5 for the square planar and 7 for the octahedral
cube (regular
hexahedron)
A convenient device for visualizing the assignment of locant designators is the representation of the successive planes and the atoms contained in the planes. Thus, the dodecahedron is seen to involve the following unit planes of atoms h-g f
Indication of Configuration In the systems so far described the actual configuration is not included in the name. Instead, it has been assumed that the user of the name will have independent information about the configuration. This assumption produced no great difficulty when coordination compounds were largely square planar, tetrahedral, and octahedral. However, today an abundance of compounds in other configurations is known. Chemical Abstracts is using a simple scheme to indicate the configuration of a complex. A symmetry code and the coordination number are included in the name. Tetrahedral T-4 Square planar SP-4 Trigonal hipyramid TB-5 Square pyramid SP-5 OC-6 Octahedral Trigonal prism TP-6 In the future the symmetry code probably should be extended to include -8 Square antiprism Digonal dodecahedron -8 and possibly others
Once again this is not a continuous assignment; further, it is not readily applicable to all configurations. General Assignment of Locants to Include All Configurations The assignment of locant designators is based on locating planes of atoms (often called zones) perpendicular to a major axis in each configuration and assigning locants in a fixed manner in each successive plane. The actual procedure is: first, locate the highest (and longest in case of a choice) order axis of rotational symmetry; second, where
Listing Points of Attachment Not only must there be a specific order of assigning locants to the positions around a coordination center but also for the listing of the points of attachment of a chelate group. 3"Nornenclature of Inorganic Chemistry, Definitive Rules 1970," (2nd Ed.) Butterworths. London, 1971, Sec. 7. Also pubIishedinPure. Appl. Chem., 28(11,39 (1971). 4Cahn, R. S., Ingold, C., and Prelog, V., Angew. Chem. Inter-
"at. Ed. 5.385 (19661.
Symmetrical linear multidentate groups present no problem because starting a t one, end to specify points of attachment is the same as starting a t the other. HINCH,CH,NH, H,NCH,CH,NHCH,CH,NHI -O?CCH,0CH,CH90CH,C0,-
For unsymmetrical groups in general, then, the needed agreement involves the choice of one end of an unsymmetrical chelating group to list first. Since a variety, of factors make for dissymmetry, there are many facets to the agreement and what might appear to be a simple matter becomes rather complicated. For details, consult the Red
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