.pi.-Bond strengths in the second and third periods

Abstract

All possible 1r bonds formed between elements C, N, 0, Si, P, and S are considered. The 1r bond strengths are estimated by cis-trans rotation barriers (where possible) and by hydrogenation energies. The ability of these elements to form strong 1r bonds is in order 0 > N C » S > P > Si. In addition, computed bond lengths and vibrational stretching frequencies are reported for both singly and doubly bound The structure of lowest triplet state of each double-bonded compound is given, along with singlet-triplet splitting. The field of p,.-p.,. bonding involving elements from third period of periodic table, and below, became an active research area only in 1970s, with efforts intensifying sharply in 1980s. Perhaps one reason for late development of field was so-called Double Bond Rule, which states that elements with a valence principal quantum number of three or greater will not participate in 1r bonding. In fact, in mid 1960s paucity of compounds containing heavy atom multiple bonds led to classification of such molecules as nonexistent compounds. 1 In 1948, Pitzer2 noted relative lack of examples of 71-bonded heavy elements. He explained this by qualitative argument that since heavier elements have longer bond lengths, their p,.-p .. overlap integrals should be smaller than those for corresponding second period elements. Since bond strengths are usually believed to be proportional to overlap integrals (see, for example, extended Hucke! theory), reluctance of heavy elements to 71 bond was explained in a simple, intuitive manner. However, in 1950 Mulliken3 tested this idea by actually computing values of overlap integrals, by using a minimal Slater orbital basis and assumed geometries. He found that in fact overlap integrals did not decrease significantly when heavy atoms replaced their second row congeners. To illustrate his result, we have computed similar overlap integrals by using Hartree-Fock quality atomic orbitals and bond distances typical of -N=Nand -P=Pcompounds. The 71 overlap is actually larger in phosphorus case, 0.65 vs. 0.62! This discovery led Mulliken to conclude the differences between second and third period atoms with respect to readiness of formation of multiple bonds ... are shown to be attributable to increased strengths of u bonds in third period. A number of workers have objected to this on grounds that, for example, Si-Si u bonds are weaker than C-C u bonds. What Mulliken meant, of course, was that difference between u and 71 bond strengths is larger in third and higher periods than in second. A preliminary feel for relative strengths4-9 of u vs. 71 bonding for second and third period elements may be obtained from Tables I and II. Table I shows that u bond strengths decrease toward right of periodic table. Because this tailing off is larger in second period, third period elements actually possess greater u bond strengths in groups 16 and 17. Table II shows that when 71 bonds are also formed, as in homonuclear diatomics, second period elements always have markedly stronger bonds. In fact, C 2 contains two 71 bonds, in preference to a u and 71 bond! In a recent landmark paper Kutzelnigg 10 surveyed chemical bonding between main group elements. He explains weakness (I) Dasent, W. E. Nonexistent Compounds; Marcel Dekker, Inc.: New York, 1965; Chapter 4. (2) Pitzer, K. S. J. Am. Chern. Soc. 1948, 70, 214G-2145. (3) Mulliken, R. S. J. Am. Chern. Soc. 1950, 72, 4493-4503. (4) Benson, S. W. Thermochemical Kinetics; John Wiley and Sons: New York, 1976. (5) Foner, S. H.; Hudson, R. L. J. Chern. Phys. 1978, 68, 3162-3168. (6) Benson, S. W. Chern. Rev. 1978, 78, 23-35. (7) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (8) Walsh, R. Ace. Chern. Res. 1981, 14, 246-252. (9) McAllister, T.; Lossing, F. P. J. Phys. Chern. 1969, 73, 2996-2998. (10) Kutzelnigg, W. Angew. Chern., Int. Ed. Engl. 1984, 23, 272-295. 0002-7863/87/1509-5217$01.50/0 Table I. Homopolar <J Bond StrengthsK H3C-CB3 88a H3Si-SiH3 74' H 2N-NH2 64b H2P-PH2 611 HO-OH 50' HS-SH 66' F-F 37d Cl-Cl 57d a Reference 4. b Reference 5. 'Reference 6. d Reference 7. e Reference 8. !Reference 9. Kkcal/mol. Table II. Diatomic Bond Strengths a Reference 7. b kcal/ mol. 74