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Their Derivation and Nomenclature OLUS J. STEWART University of Kentucky, Lexington, Kentucky
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H E PURPOSE of this paper is to show that in studymg complex compounds i t is convenient to thmk of these bodies as substitution derivatives of hydroxides, or their corresponding hydroxylated acids. An example will make the matter clear. Consider the complex salt, K3[Fe(CN)a]. Since this compound contains tervalent iron, the starting material required for deriving the formula is iron (111) hydroxide, Fe(OH)3, or H3Fe03,which through ionization yields the orthoferrite ion, FeOa'. This ion may be said to contain three oxide ions, 30-, or the equivalent thereof, because if three oxygen atoms are extracted from the composite ion, six units of negative charge must also be removed in order that the iron atom may retain its original charge of plus three. This hypothetical pairing-off of atoms and charges within the orthoferrite ion serves to emphasize the importance placed on charge in this discussion, and carries the implication that other ions, say six cyanide ions, whose charge, both in magnitude and sign, matches that of 30-, can, in consequence of this fact, replace the three oxide ions, This ion and yield the product sought [Fe(CN)s]'. is to be regarded as a true derivative of the hydroxylated acid, H3Fe08. The preceding speculation does not support the view that complexes actually come into being in practice through a process of direct substitution, nor does it rule out such a contingency. Instead, it urges only that through the agency of imaginary substitution one is enabled to predict what complexes are possible. In short, the argument runs as follows: If a certain hydroxylated acid is possible, then corresponding - substituted derivatives a& also possible. When complex compounds are regarded as deriva-
tives of hydroxylated acids the problem of uomenclature immediately takes on new significance, for the value of the designation can be enhanced by naming the offspring so as to show kinship with its parent. The official name of the salt which serves as the first example, and given in "Rules for naming inorganic compounds,"' hereafter referred to as the "Rules Committee," is potassium hexacyanoferrate (111). In fact the Rules Committee terminates the names of all complex anions of acids or salts with the suffix -ate. However, if the hydroxylated acid, H3Fe03,from which the salt in question appears to originate, bears the name orthoferrous acid, then a name showing this relationship of salt to acid would be potassium hexacyanoorthoferrite. When this name is used, the Roman figure (111) is not required to specify the valency of the central iron atom. The formula, 14[Fe(CN)s], has been selected for the second derivation in order to bring out additional points of interest. Since this compound contains bivalent iron, it is necessary to start with iron (11) hydroxide, Fe(OH)2 or H2FeOz. Inspection shows that this hydroxylated acid is not the one required, for the anion is not quadrivalent, and substitution of cyanide ions for oxide ions will introduce four instead of six cyanide groups. Evidently a more highly hydroxylated acid is needed. Now the acid, H2Fe02,may be said to result from the union of one mole of iron (11) oxide with one mole of water. In order to secure a still more highly hydroxylated acid, another mole of water must be added to produce the acid, H4FeOa. If now 30- is replaced by 6 CN-, the derivation of the 1 JoFSSSEN (Chairmen), "Rules for naming inorganic compounds," J.Am. Chem. SOL,63,895 (1941).
complex will be complete. The official name of the potassium salt is potassium hexacyanoferrate (11). If, however, one cares to coin a name which brings out the apparent relationship of the salt to the acid, HIFe03, the name potassium hexacyanohypoorthoferrite might a t first suggest itself. The term ortho- is required to indicate a derivative of the most highly hydroxylated acid, H4Fe03. But, accord'mg to the Rules Committee, Section D, the name, "hypo . .. ite," is barred because this name is reserved by custom for the still undiscovered hydroxylated acid whose acidforming element is univalent iron. (This interpretation of the rule necessarily follows if the acid, H3Fe03, is properly named ferrous acid, and contains tervalent iron.) The third example will be the derivation of the formula for hexaaquochromium (111) chloride [Cr(HzO)s]C13. Chromium (111) hydroxide, CI(OH)~or H3Cr03, the initial material, furnishes the orthochromite ion, CrOF. We have seen that 30- can be replaced by 6CN-. But, in complex chemistry, one cyanide ion, or one chloride ion, is equivalent in replacing power to one molecule of water. Therefore 30- can be replaced by 6HeO. However, since 30- carries a charge of minus six, and 6Hz0 has no charge whatever, the displacement of 30=, i. e., six units of negative charge, from the orthochromite ion will increase the latter's charge from minus three to plus three, while the introduction of 6Hz0 will not affect the charge. So the resulting ion will have the formnla [Cr(H20)8]+++,and the product sought will have the formula [Cr(H~O)slCla. In like manner one can derive the formula of the complex ion [Cu(NH3)4]++. Starting with copper (11) hydroxide, CU(OH)~ or H2Cu02,the cuprate ion, Cu02-, may be obtained through ionization. Now in complex chemistry 20- can be replaced by 4NH3. But, when 20- with its fonr units of negative charge is removed from the cuprate ion, the charge of the latter ion increases to plus two, and the introduction of 4NH3 leaves the new charge of plus two unchanged. So the formnla of the complex ion is [CU(NH~)~]++. An important question now arises. How is one to recognize this class of compounds which is variously designated as complex, higher order, or coordinated? If the word complex means complicated, are we to believe K3Fe(CN)6 is complicated, while H3Fe03 is not? Does the name "higher order," retain more than historic interest? Does the term "coordination," in its modem chemical sense, apply any more to Fe(CN)F than to Fe03'? If the general thesis of this paper is sound, perhaps it would be advisable to reserve a name, say "complex," for that group of compounds which may be regarded as substitution derivatives of hydroxylated acids. However, if this definition of a complex be adopted, many compounds not generally classified as complex will have to be included in the category. Several of this type will now be considered briefly. With the next example, sodium thiosulfate, Nap-
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Sz03, replace one oxide in a sulfate ion with one sulfide ion and produce [S03S]-. According to Section D of the Rules Committee, the name of the salt is sodium thiosulfate. Offhand one might be more inclined to use the name, sodium sulfosulfate, because in complex chemistry a chloride ion substitution yields a chloroderivative; so a sulfide ion substitution should give a sulfo- derivative. For potassium dichromate, KsCr2O7,staL.with two chromate ions; replace one oxide ion in one chromate ion with the second chromate ion, and have the chromatochromate ion [CrOaCr04]=. The salt should then be called potassium chromatochromate. To derive potassium triiodide, K13, start with one hypoiodite ion, 10-, and two iodide ions, 21-. Replace the oxide ion in the hypoiodite ion with 21-, and have the iodohypoiodite ion [112]-. No evidence is offered in support of the correctness of this constitution, which diiers sharply from that generally accepted, namely, that the "triiodide" ion represents some sort of union between free iodine, Is, and an iodide ion, I-. Needless to say, both points of view lead to the same quantitative results in iodimetry. This paper gives to the salt the name potassium iodohypoiodite. For a last example, let us take ammonium phosphomolybdate, listed by the Rules Committee, Section F, as ammonium dodecamolybdophosphate (NH&P04. 12Mo03. Start with one phosphate ion, PO4=, and twelve molybdate ions, 12MoOh. Replace 40- in POF with 4Mo04=, and produce the tetramolybdatoorthophosphate ion [P(Mo04)4]'. Next replace 20= in each of the fonr previously introduced molybdate ions with 2MoOa3, and obtain as the h a 1 product (NH4)3[P(Mo02)4(Mo0&], which this paper is .nclined to call, ammonium octamolybdatotetramolybdatoorthophosphate. This proposed constitution, whose correctness is subject to verification by x-ray analysis, appears capable of accounting for the "yellow precipitate's" well-known irregularity of composition. The completeness of double substitution, i. e., substitution into previously substituted ions, especially when the substituents are as voluminous as the molybdate ion, would naturally depend upon the conditions, such as temperature, concentration, and duration of time. In view of the material presented, i t appears that complex compounds differ from hydroxylated acids only in that oxygen atoms of the latter are replaced in the former by other atoms or groups of atoms, and that the first may be regarded as true derivatives of the second. When the case is considered in this light, it seems only natural that the naming of complexes should follow closely the well-established nomenclature of hydroxylated acids, certain modifying terms, like chloro-, sulfato-, aquo-, etc., being inserted in the Werner manner to call attention to the substituents. The principle of substitution, as developed, not only dissipates much of the mystery enshrouding complexes, but offers a basis upon which to formulate a definition of this type of compound.
