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Llnus Pauling Institute of Science and Medicine I The Relative Stability of lsosteric 2700 Sand HN Road Menlo Park, California 94025 Ions and Molecules In the section Eco-Chem for April 1974l a statement was made that cyanate ion is CNO- and fulminate ion is NCO-. This statement, however, was wrong and was amended by an erratum in the Sevtember issue. This mis- understanding emphasizes the necessity of understanding the argument provided by Sterling Hendricks and me in 1926.2 Moreover, the argument that we used provides a simple way of remembering which structure is to be as- signed to the more stable isomer. We had shown3 by the X-ray diffraction study of crystals of potassium cyanate that the cyanate ion is linear, with in- teratomic distances close to those in the azide ion, N3-, and the carbon dioxide and nitrous oxide molecules. These studies suggested to us the possibility of developing a sim- nle aualitative and rou~hlv auantitative theom of the rela- iive &ability of isoateric isomeric ions and mdlecules, such as the cvanate ion and the fulminate ion. The normal states of these ions may he described as involving mainly reso- nance among the structures . . :&--, :A=B&:, and :~-~-ij: with contributions such as to lead to consistency with the electroneutrality rule.4 The symbols A, B, and C represent the kernels of the atoms-the nuclei plus the two K elec- trons. We calculated the repulsion energy of the kernels, with bond lengths given the observed value 115 pm, for the two ions Cyanate ion Fulminate ion NW+++--oW C4+-NS+_06+ 71384 74933 k J mole-' It is seen that the kernel repulsion energy is larger for the second structure than for the first. The difference is 3e2/ 115 pm, which is 3629 k J mole-'. We expected the distribution of the outer electrons to readjust itself in such a way as to shield the kernels from one another, but not completely. Accordingly we assigned the less stahle of the two structures, as given by this calcu- lation, to the fulminate ion, since the fulminates are less stable than the cyanates. The effectiveness of the screenine bv the outer electrons was evaluated by use of the enthaliy i f the reaction CO + N9O - N? + CO?. which involves onlv isosteric isomeric in- teiconve&ons. he observed enthaGy difference, 365 k J mole-'. is 10.1% of the chance in kernel re~ulsion enerev (which'is the same as for cyan-ate-fulminate): Assuming t& same factor to hold for other pairs, we obtain 365 k J mole-' as the expected difference in enthalpy of fulminate ion and cvanate ion. No ex~erimental value for this difference is a t hand. The observed difference for silver fulminate(c) and silver cyanate(c) is 270 k J mole-'. A rough correction can he made for the difference in partial ionic character of the silver-carbon and silver-nitrogen bonds, with the expres- sion 100 kJ mole-' (XA - xd2, with x = 1.9 for silver, 2.5 for carhon, and 3.0 for nitrogen. Assuming a single covalent bond to carhon or nitrogen, we obtain 355 kJ mole-', in good agreement with the kernel-repulsion value. It is evident that for a linear triatomic ion or molecule the structure with minimum kernel repulsion energy is that in which the kernel with smallest kernel charge is in the middle; hence cyanate ion is NCO-. The third structure, NOC-* with the atom with largest kernel charge in the mid- dle is expected to he still less stahle than fulminate ion. Similarly, we pointed out in 1926 that nitrous oxide is NNO rather than NON. There was no direct evidence for NNO at that time, but Irving Langmuir had suggested this structure several years earlier? Also, the kernel-repulsion postulate leads to the conclusion that the alkyl nitriles RCN and isocyanates RNCO should be more stahle (by about 90 and 180 k J mole-', respectively) than the corre- sponding isocyanides, RNC, and cyanates, ROCN. These and other predictions made with the postulate are in agree- ment with experiment. Campbell, J. A,, J. CHEM. EDUC., 51,269 (1974); errata in 51, 587 (1974). 2Pauling, L., and Hendrieka, S. B., J. Amer. Chem. Soe., 48,641 (192fil~ , . . . . , . Hendricks, S. B., and Pauling, L., J. Amer. Chem. Soc., 47, 2904 (1925). Pauling, L., J. Chem. Soc., 1948, (1961). Langmuir, I., J. Amer. Chem. Soe., dl, 1543 (1919) Volume 52, Number 9, September 1975 / 577
