Π Bond Strengths in the Second and Third Periods Michael W

Iowa State University From the SelectedWorks of Mark S. Gordon August, 1987 π Bond Strengths in the Second and Third Periods Michael W. Schmidt Phi N. Truong Mark S. Gordon, Iowa State University Available at: https://works.bepress.com/mark_gordon/61/ J. Am. Chern. Soc. 1987, 109, 5217-5227 5217 1r Bond Strengths in the Second and Third Periods Michael W. Schmidt,* Phi N. Truong, and Mark S. Gordon* Contribution from the Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105. Received February 2, 1987 Abstract: All possible 1r bonds formed between the elements C, N, 0, Si, P, and S are considered. The 1r bond strengths are estimated by the cis-trans rotation barriers (where possible) and by hydrogenation energies. The ability of these elements to form strong 1r bonds is in the order 0 > N "" C » S > P > Si. In addition, computed bond lengths and vibrational stretching frequencies are reported for both the singly and doubly bound compounds. The structure of the lowest triplet state of each double-bonded compound is given, along with the singlet-triplet splitting. The field of p,.-p.,. bonding involving elements from the third Table I. Homopolar <J Bond StrengthsK period of the periodic table, and below, became an active research H C-CB 88a H Si-SiH 74' area only in the 1970s, with efforts intensifying sharply in the 3 3 3 3 H 2N-NH2 64b H2P-PH2 611 1980s. Perhaps one reason for the late development of the field HO-OH 50' HS-SH 66' was the so-called "Double Bond Rule", which states that elements F-F 37d Cl-Cl 57d with a valence principal quantum number of three or greater will a Reference 4. b Reference 5. 'Reference 6. d Reference 7. not participate in 1r bonding. In fact, in the mid 1960s the paucity e Reference 8. !Reference 9. Kkcal/mol. of compounds containing heavy atom multiple bonds led to the classification of such molecules as "nonexistent compounds". 1 In 1948, Pitzer2 noted the relative lack of examples of 71"-bonded Table II. Diatomic Bond Strengths heavy elements. He explained this by the qualitative argument 74 that since heavier elements have longer bond lengths, their p,.-p .. 116 overlap integrals should be smaller than those for corresponding 101 second period elements. Since bond strengths are usually believed a Reference 7. b kcal/ mol. to be proportional to overlap integrals (see, for example, extended Hucke! theory), the reluctance of heavy elements to 71" bond was of 71" bonding in the heavier main group elements in terms of lone explained in a simple, intuitive manner. pair repulsions and isovalent hybridization. The differences be However, in 1950 Mulliken3 tested this idea by actually com tween the second period elements and their heavier congeners are puting the values of the overlap integrals, by using a minimal Slater attribut::d to the fact that the former do not have any core p orbital basis and assumed geometries. He found that in fact the orbitals. Kutzelnigg advances the novel viewpoint that it is the overlap integrals did not decrease significantly when heavy atoms second period elements which are "unusual" in their 71" bonding replaced their second row congeners. To illustrate his result, we and many other properties. have computed similar overlap integrals by using Hartree-Fock In spite of their significantly lower 71" bond strengths, it has been quality atomic orbitals and bond distances typical of -N=N possible in recent years to generate and characterize molecules and -P=P- compounds. The 71" overlap is actually larger in containing many different types of heavy atom 71" bonds. A number the phosphorus case, 0.65 vs. 0.62! This discovery led Mulliken of strategies may be employed to form such compounds. 11 The to conclude "the differences between second and third period atoms multiply bonded element may be stabilized by incorporation into with respect to readiness of formation of multiple bonds ... are shown a delocalized or aromatic 71" system, by imparting an overall charge, to be attributable to increased strengths of u bonds in the third or by ligating the 71" bond to a transition metal. As the above period." A number of workers have objected to this on the grounds methods all produce chemically altered 71" bonds, the final synthetic that, for example, Si-Si u bonds are weaker than C-C u bonds. stratagem (using bulky substituents to impart kinetic stability to What Mulliken meant, of course, was that the difference between the double bond) produces the most satisfactory examples of u and 71" bond strengths is larger in the third and higher periods multiple bonds. This method has been used to generate solution than in the second. stable and even isolable compounds containing one and, more A preliminary feel for the relative strengths4-9 of u vs. 71" bonding recently, two 71"-bonded heavy elements. for the second and third period elements may be obtained from A number of review articles on the subject of heavy element Tables I and II. Table I shows that u bond strengths decrease 71" bonding exist. Jutzi 12 has reviewed multiple bonds between toward the right of the periodic table. Because this tailing off carbon and the elements P, As, Sb, Bi, Ge, and Si. Since then is larger in the second period, the third period elements actually Appel, Knoll, and Rupert13 have updated the story of the C=P possess greater u bond strengths in groups 16 and 17. Table II bond. Compounds containing the -P=N-- bond have been shows that when 71" bonds are also formed, as in the homonuclear reviewed by Abel and Mucklejohn. 14 Cowleyl 5•16 has twice diatomics, the second period elements always have markedly surveyed multiple bonding in group 14 and group 15. Perhaps stronger bonds. In fact, C 2 contains two 71" bonds, in preference the most attention in the field of heavy atom 1r bonding has focused to a u and 71" bond! on the element silicon, because of its relationship to carbon. One In a recent landmark paper Kutzelnigg 10 surveyed chemical of the authors17 has reviewed theoretical calculations on multiply bonding between main group elements. He explains the weakness bound silicon. Gusel'nikov and Nametkin 18 surveyed the status (I) Dasent, W. E. Nonexistent Compounds; Marcel Dekker, Inc.: New York, 1965; Chapter 4. (II) van der Knaap, Th. A.; Klebach, Th. C.; Visser, F.; Bickelhaupt, F.; (2) Pitzer, K. S. J. Am. Chern. Soc. 1948, 70, 214G-2145. Ros, P.; Baerends, E. J.; Starn, C. H.; Konijn, M. Tetrahedron 1984, 40, (3) Mulliken, R. S. J. Am. Chern. Soc. 1950, 72, 4493-4503. 765-776. (4) Benson, S. W. Thermochemical Kinetics; John Wiley and Sons: New (12) Jutzi, P. Angew. Chern., Int. Ed. Engl. 1975, 14, 232-245. York, 1976. (13) Appel, R.; Knoll, F.; Ruppert,!. Angew. Chern., Int. Ed. Engl. 1981, (5) Foner, S. H.; Hudson, R. L. J. Chern. Phys. 1978, 68, 3162-3168. 20, 731-744. (6) Benson, S. W. Chern. Rev. 1978, 78, 23-35. (14) Abel, E. W.; Mucklejohn, S. A. Phosphorus Sulfur i981, 9, 235-266. (7) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van (15) Cowley, A. H. Ace. Chern. Res. 1984, 17, 386-392. Nostrand Reinhold: New York, 1979. (16) Cowley, A. H. Polyhedron 1984, 3, 389-432. (8) Walsh, R. Ace. Chern. Res. 1981, 14, 246-252. (17) Gordon, M.S. In Molecular Structure and Energetics; Liebman, J. (9) McAllister, T.; Lossing, F. P. J. Phys. Chern. 1969, 73, 2996-2998. F., Greenberg, A., Eds.; Springer-Verlag: New York, 1986; Vol. l, Chapt~r (10) Kutzelnigg, W. Angew. Chern., Int. Ed. Engl. 1984, 23, 272-295. 4. 0002-7863/87/1509-5217$01.50/0 © 1987 American Chemical Society 5218 J. Am. Chern. Soc., Vol. 109, No. 17, 1987 Schmidt et a!. of silicon multiple bonding up to 1979. This review has recently molecules. However, even ordinary single bond distances involving been brought up to date (late 1985) by Raabe and Michl. 19 third period elements such as phosphorus are normally computed Wiberg20 has reviewed the mixed multiple bonding between the to be too long, unless the basis set contains d orbitals.27 In addition, elements Si and Ge and the elements C and N. split valence basis sets without d orbitals are well known to give That the field continues to be actively pursued is indicated by in many instances incorrect bond angles around nitrogen atoms. the recent report of an allene analogue containing a -P=C-P Therefore, as will be detailed below, our computed structures are group.21 Additional7r-bonded compounds between even heavier obtained by using d orbitals located on all second and third period members of groups 14, 15, and 16, such as the solution stable atoms. Once the geometries of the molecules are known, complete Ge=P22 and Sn=P23 and an Si=Se intermediate,24 are now force constant matrices and vibrational frequencies are computed known. In fact, it would seem to be safe to stand the "Double at the SCF level. Bond Rule" on its head by predicting that eventually compounds Although meaningful results for structures and frequencies of representing all possible double bonds between atoms from groups closed shell molecules can be obtained at the SCF level with small 14-16 will be isolated and characterized. bases, the accurate computation of energy differences normally The purpose of the present theoretical work is to provide in requires both larger bases and the inclusion of electron correlation formation on double bonds that has been slow to emerge from effects.

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