NOTES ON THE FORMATION AND STABILITY OF COMPLEX IONS DALLAS T. HURD General Electric Research Laboratory, Schenectady, New York
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A similar equation may be written to illustrate the formation of calcium fluoborate. Both of these fluoborates are ionic salts. We next observe that potassium fluoborate is quite stable. It melts at a red heat and can be heated to 600 to 700°C. without noticeable decomposition. On the other hand, calcium fluoborate decomposes quite readily if one heats it to 250' and it reverts to boron fluoride, which escapes as a gas, and solid calcium fluoride. We might try to derive an explanation for this difference in behavior by basing it on chemical analogy as follows: We know that the amount of energy released in the formation of a compoundi. e., binding energyincreases with the differencein electrical character of the component atoms. Compare, for example, the energy evolved in the formation of sodium chloride and in the formation of hydrogen iodide and the relative stability of these two compounds. Therefore, since calcium is less electropositive than potassium we might postulate in the case of t h e fluoborates that there is more binding energy to stabilize the potassium compound. The above qeasoning, however, is not correct in explaining the formation and stability of complex ion compounds although it is based on a sound premise and in some cases may offer a satisfactory explanation for the stability of bmary compounds. Let us look again a t the fluoborates using the formal acid-bsse concepts. Potassium fluoride comprises potassium ion, a weak "acid" in the Lewis sense, and fluoride ion, a weak "base." This duplicity of use for the common terms acid and base is confusing but what is meant is that potassium ion has only a slight tendency to accept electrons and fluoride ion has only a slight tendency to donate electrons: Boron fluoride, on the other hand, is a very strong acid and is eager to accept. a pair of electrons. Referring to equation (1) again we see that the fluoride ion base has donated a share in a pair of its valence electrons to the boron atom with the establishment of a chemical bond and has taken up a position near the boron to form the complex fluohorate ion. The very weak acid potassium ion offerslittle or no competition to the boron for the fluoride ion so the complex fluoborate ion appears to be quite stable. Note. however. that this formation of potassium fluobo.-.... rate is an equilibrium reaction, although the equilibrium lies very far over on the side of the products. The for' The reader is referred to the excellent text by W. F. Luo~q: mation is quite analogous except ~ ~ ~ ~ ~of~ calcium , ~ fluoborate S. zUPPANTI, ' c ~~ h ~ l~h~~~ of~ id^ ~ B ~ i ~ for one thing. Calcium ion is a stronger acid than poJohn Wile? & Sons, Ine., New York, 1946.
SUBJECT that rarely is touched in beginning cl~rmistry,and ninv 1~ehazy in the mind of rile grnrh~xtc stndrnt. deals with thr farr(~rs i~~flucncine rhr formxtim and stability of complex ions. To put this subject in the form of typical questions that an intelligent student well might ask: Why is ammonium sulfide less stable than ammonium fluoride? Why is potassium fluoborate stable a t red heat whereas calcium fluoborate decomposes a t 250°? Why do acids like orthophosphoric, molybdic, vauadic, silicic, etc., form condensed polyacids as the pH decreases? Some explanations for these and related questions can be found in the chemical literature but they are not easy for the student to dig out and much of the reasoning, as presented, may be difficult for a student to grasp. There also is the danger that in supplying his own answers to some of these questions the student may be guided by reasoning from other chemical phenomena to logical but wrong conclusions. It is possible to explain the phenomena that are observed in complex ion systems in a very simple fashion by using the formal concept of acids and bases introduced by G. N. Lewis.' In this system the term acid is applied to electrophilic or electron attracting groups, and the term base is applied to electrophobic or electron donating groups. Chemical reactions thus can be elucidated in terms of electronic interaction. The use of these concepts comprises a very ppwerful chemical tool. The application of this tool to the knotty chemical problems of complex ion behavior r e v a h something of its adaptability and may help to familiarize students with this way of chemical thinking. To begin with, we observe that boron fluoride can react with potassium fluoride under a variety of ronditions to form potassium fluoborate, KBF4. Similarly, calcium fluoborate, Ca(BF&, is formed from boron fluoride and calcium fluoride. These both are reactions in which a fluoride ion from the metal fluoride has shared a pair of its valence electrons with an electron-greedy boron atom to form the complex fluoborate ion BFa-, a process which may be represented by an equation:
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JULY, 1948
tassium ion. That is, it has more tendency to accept gradually acidified we observe that the orthosilicate ions electrons. Therefore, although the formation of the condense into polysilicate ions and finally precipitate as fluoborate ion is the same, we can say that the calcium highly condensed polysilicic acid. ion offers the boron more competition for the fluoride ion Let us examine this behavior in detail: We can conbase. This increased competition results in an equi- sider the orthosilicate ion as a silicon surrounded by four librium lying less far over on the side of the products oxygens, the whole unit having a charge of -4 units. and, in turn, in decreased thermal stability of the com- Each oxygen is bonded to the silicon by a pair of elecplex. Fluoborates of metals less electropositive than trons. In strongly alkaline solution there are only a calcium--i. e., stronger acids-are found to be even less very few protons and no other groups which would comstable toward thermal decomposition. pete with silicon for the attached oxygens. Thus the We can apply the same sort of reasoning to the for- orthosilicate ion is regarded as stable under these conmation and stability of ammonium compounds. The ditions.. However, if we begin to add protons to the formation of ammonium fluoride can be represented as solution in the form of acid we find that these are more the reaction of a hydrogen ion (strong acid), fluoride ion acidic than the silicon and can remove oxygen from the (weak base), and ammonia, which, since it has an extra silicate ion in the form of "basic" oxide ions to form pair of electrons to donate or share, may be considered water. The residue of the silicate ion that remains is as a strong base. Since the proton tends to coordinate acidic, i. e., it wants electrons, although it is less acidic to the group that most readily can share a pair of elec- than the protons. To satisfy this tendency it takes a trons with it the fluoride ion (weak base) offers little share in an oxygen attached to another silicate ion or competition to the ammonia (strong base) for the pro- silicate ion residue and the result is the first step in the ton, although this proton originally was accepting elec- formation of a highly condensed siloxane network. trons from the fluoride in the compound hydrogen We can represent this process by equations as follows: fluoride. Thus: (It should he remembered that these reactions are equilibrium reactions and can be reversed.) H H+ .. . .. HF=H+:&.. H + + :N:H :o:-' . .O:-% .. e H:N:H. .. ...... OR.:.
.
H
H
The equilibria lie far over on the side of the products. However, in the presence of a base stronger than fluoride ion, say hydroxyl ion or sulfide ion, the equilibrium is shifted away from the product side with the result that the complex ammonium ion becomes less stable against decomposition back into an ammonia molecule and a proton. Thus we see that the relative stability of a complex ion in its various compounds does not reside in the energy of formation of the compounds (in fact, the energy of formation of an ammonium ion from ammonia and a proton should be the same regardless of the other ions present), but in the relathe weakness as acids or bases-. e., as electron acceptors or donors-of the ions that form compounds with the complex ions. Another chemical phenomenon for which an explanation can he made with the aid of the acid-base concepts is the condensation of certain acid radicals into polyacid systems with decreasing pH. For example, in very strongly alkaline solution we can keep the silicate ion in solution as the orthosilicate ion. As the solution is
:O:Si:O: ...... + 2 H f :o ..:
H+
:O:Si " :o ..:
H:O:H
"
..
(8irOs)-4 =(more highly condensed ions)
This type of behavior also is observed for a number of other oxyacid systems and, lacking knowledge to the contrary, may he fairly general. By comparing the behavior of the silicates with the ,ahminates and the phosphates, for example, we can make the observation that as the central member of the complex ion becomes more electropositiv~'.e., less desirous of accepting electrons-the condensation reactions occur more readily and more completely. This approach to explaining the formation and stability of complex ions is, indeed, rather simple. The simplicity lies in the great value of the formalized acidbase concepts in explaining chemical phenomena otherwise difficult to interpret.
