notes on nomenclature
W. C. FERNELIUS
University of Soulh Florida Tampa 33620
KURT LOENING
Chemical Abslrmb Service Columbus, Ohio 4 3 2 1 0
ROY M . ADAMS
Bridging groups can be divided into (1) those in which the linkages t o the nuclear atoms are through the same coordinating atom and (2) those in which the linkages to the nuclear atoms are through different atoms. Although in many cases the very nature of the bridging groups themselves makes obvious to which of these two classes the bridging group belongs, there are other cases in which i t is desirable to indicate the points of attachment. This is especially true where a given group can he present in two or more different ways in different compounds. Examples
Geneva Cdlege Bearer Falls, ?enn%ylvonio 1 SO1 0
Coordination Nomenclature. Ill1 Polynuclear Compounds wlth Bridging Groups
Note that the order of listing element symbols is that in which the elements are encountered on the line running through the periodic table as shown below.
The pattern of chemical compound formation2 ( l a ) known as coordination is not limited to single centers of coordination. There are many varieties of compounds in which one or more ligands are attached simultaneously to two or more central atoms. Such bridging groups are indicated in the name by p-L (L = name of ligand), di-p-L [or his(@-L)],etc. and separating the name from the rest of the complex hy hyphens. Bridging groups are listed with other groups in alphabetical order and, where the same ligand is present both as a hridging and a non-bridging ligand, it is cited first as a hridging ligand. Examples
There are a numher of cases in which a hridging group is hound to more than two central atoms. For these a subscript after p indicates the number of centers hound. Examples
[(H,N)5Cr-OH-Cr(NH,,ijlClj
[Cr ,O~CH.,COOi,]Cl
decaammine-p-hydroxa-diehromium(5+) chloride
h e x a - b - a c e t a t a - 0 , 0 ' ) - p 3 - ~ ~ ~ - t r i ~ h r ~ mchloride im(1+
[Br,Pt(SMe,),PtBr,]
[(CH.,Hg),Sl'+
tetrabromo-bisb-dimethyl sulfide)-diplatinum(I1)
or
/OH\ [CH,,N),CO~H-CO(NH.,,,~:'+
\NO/
heraammine-di-~-hydroxo)-~.(nitrito-OON).dicoblt(3+) ion
[(OCi,FetCO),,FetCOi.,] tri-01-carbony1)-hexacarbonyldiiron
Inasmuch as many hinuclear compounds are symmetrical, like the examples, many feel that names clearly indicative of this symmetry are more suitable. Such names for the first two examples above would be p -hydroxo-bis(pentaamminechromi~m)( 5 + ) chloride bisb-dimethyl sulfide)-bis[dibromoplatinum(II)] However, the use of this system might he considered to be inconsistent with the alphabetical order of listing ligands. No rules covering priorities in such cases have been formulated. Actually, this treatment is similar in many respects to the multiplicative nomenclature used for many organic compound^.^
tetramethyl-p,-thio-tetramercury(2+) ion pa-thio-tetrakis(rnethylmercury)(Z+) ion
[Be,O(CH,COOi,,] hexa-01-aeetata~O,O')-p,-oxo-tetraberyllium [( PtIMe.,),l
tetra-p,-ioda-dadecamethyltetraplatinum
or tetra-p,-iodo-tetrakis(trimethylplatinum)
In the first compound the oxygen is at the center of a trianele of three chromium atoms each of which is linked to the urher two chromiwn atoms hg two bridging acetate ligands. The distribut~mo! metal atoms ahout rhe central bridvine u atom in the second and third examples is tetrahedral. How-
-
'
For previous notes on coordination nomenclature see footnotes 2 and 3. Fernelius. W. C.. Loenine. K. L.. and Adams. R. M.. J. CHEM. Vernelius, W. C., Loening, K. L., and Adams, R. M., J. CHEM. EDUC., 51,603 (1974). See subsection C-0.7. Nomenclature of Assemblies of Identical Units. IUPAC, "Nomenclature of Organic Chemistry Sections A, B, and C," Buttenuorths, London, 1971, p. 127. Volume 52, Number
12, December 1975 / 793
ever, in the third example each beryllium atom is bound to the other three beryllium atoms by a bridging acetate group (i.e., each acetate group occupies an edge of a tettahedron). In the last example the platinum atoms occupy alternate positions a t the corners of a distorted cube. Hence, each iodine is linked t o three platinum atoms and each platinum atom to three iodine atoms. There is a wide variety of types of polynuclear complexes and the possibilities of isomer formation legion. Such structures may be named by the use of locant designators which are assigned by the use of the same principles3 as for the various configurations around a single center of coordination. The "axis" of operation must be chosen so as to pass continuously through the largest number of the nuclear atoms even if this results in a bent axis. The following examples indicate the application of these principles in simple cases. For more complicated examples, see the Red Book.S
different states of oxidation. Two possible names may be given catena-diammine-M-bromo-dibromoplatinum (11, IV) ca tena-~bromo[diammine-dibmmo-rbromo-platinum
(1V)I-diamminedibromo platinum (11) The first is short but does not indicate unequivocahly the alternation of platinum in two oxidation states. T o some the second seems undesirably long. Extended structures are not always neutral entities but may carry electrical charges. Thus, the compound represented by the formula Cs[CuCIz] (cesium copper(I1) chloride) has an ion with the structure
The structure may he expressed by the formula (Cs+),[(CuCIz),]"- which leads to the name cesium catena-ll-chloro-dichlorocuprate(I1). In recent years there has been a good deal of effort toward'the preparation of coordination polymers. They can be named by simple extension of the above pattern. The nomenclature committee of the ACS Division of Polymer Chemistry is working on an extended report covering such compounds. Dired Linking between Centers of Coordinalion
Nomenclature proposals for polynuclear complexes did not anticipate the direct linkage between two centers of coordination rather than linkage through a bridging group. However, many examples of such compounds are known. When symmetrical, they are readily named by using a multiplicative prefix with the structural segment. Examples
Extended Structures
Bridging does not always lead to the formation of structures of fixed (and comparatively low) molecular weight. Some crystals consist of anions of infinite extension containing repeating units. The structure of such compounds can be indicated by prefixing catena- (meaning chain) to the name of the repeating unit. Thus, the crystal of palladium(I1) chloride does not consist of discrete Pd2+ and CIions but of chains of the structure
When unsymmetrical, the group consisting of one central atom and its attached ligands is treated as a ligand on the other atom. The metal to be considered as the primary central atom is the last encountered in the line running through the periodic table shown above. The names for the substituted metallo ligands are constructed by prefixing the names of the organic and/or inorganic ligands to a modified name of the metal.? Examples
The compound Pt(NH3)2Br3
presents some difficulty because platinum is present in two S"Nomenelature of Inorganic Chemistry-Definitive Rules 1970," 2nd ed., Buttemortbs, London, 1971, pp. 67-71; also, Pure Appl. Chem., 27(1),67 (1971). The structure of this compound may he indicated more definitely by a structural prefix such as triangulo- or eyelo-. For a complete list of such modified names of metals see Table V of the Red Book (footnote5), p. 105. 794 / Journal of Chemical Education
There are several radicals of elements usually regarded as metals which act as ligands in forming coordination compounds: Ge(CH&, Sn(CH&, SnCI3, etc., for example [Fe(CO)z(C5H~)iGe(CH3)911, and [Co(CO)&iFsl, [Mn(C0)5Sn(CH3)3]. Although some may think of these compounds as containing metal-metal bonds for nomenclature purposes, the radical containing the fourth group metal is probably best treated as an ordinary ligand. [(Cl,Sn),RhCl,Rh(SnCI.,,?P~ or di-~-chloro-his[his(trichlor0~tannyl)rh](4-) ion di-p-chlora-tetrakis(trichlomstannyl)di~(4-)ion
Bridges as Well as Direct Llnklng between Centers of Coordination
In some polynuclear compounds with bridging groups, the distance between the two metal coordination centers is so close as to strongly indicate direct linking as well as bridging. Such compounds are named as bridged compounds and the direct linking indicated by adding the italicized symbols of the centers in parentheses a t the end of the name. Examples
Occastonally, a bridging group is linked to more than two directly linked metal atoms
(OC),cQ(CO),~(CO), di-p-carbanyl-hexacarbonyldicobalt(Co-Co) di-p-carbanyl-bis(tricarbonylcobaIt)(Co-Co)
or
Volume 52 Number 12, December 1975 / 795
