REACTIVITY OF COORDINATED LIGANDS
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Reactivity of Coordinated Ligands as Studied by Molecular Orbital
Calculation. I.
Radical Polymerizability of Zinc Complexes
of Vinyl Compounds
by S. Tazuke, K. Tsuji, T. Yonezawa, and S. Okamura Department of Polymer Chemistry, Kyoto University, Kyoto, Japan
(Received April 8, 1967)
The simple LCAO approximation for an organic conjugated system was extended to include the coordination bond. This method was applied to interpret radical poly’merization of 4-vinylpyridine (4-VP), 2-vinylpyridine (2-VP), and 2-methyl-5-vinylpyridine (MVP), methyl methacrylate (MMA), and acrylonitrile (AN) complexed with zinc salts. The effects of metal salt on the rate of catalyzed homopolymerization, on spontaneous thermal polymerization, and on copolymerization were explained with a single set of parameters. Stabilization energy at the transition state as a measure of the ease of reaction to proceed and superdelocalizability of monomers as a measure of monomer reactivity were calculated. Agreement between calculation and experiment was semiquantitative but was satisfactory with an exception of copolymerization of 2-VP with styrene, in which steric effects seemed to predominate. The successful application of the LCAO model in those systems rendered the conclusion that the transmission of the electronic effect through the coordination bond would be brought about by both inductive and mesomeric effects similar to the organic substituent effect.
Introduction A study of the effects of coordination bond formation on the chemical reactivity of ligands’ would be an important field in the boundary territory of organic, physical, and inorganic chemistry in connection with the fundamental aspect of catalytic action of metal sah2 Although the published results on chemical reactions involving coordinated ligands are still fragmental and no general approach to estimate the reactivity of coordinated ligands even in a qualitative manner has been commenced, it seems to be clear that the formation of a coordination bond would bring about electronic as well as steric or configurational effects on ligands. The latter effect would be more diffcult to estimate. Let us consider the case where metal ion influences the reactivity of ligand a t some remote position from the coordination site so that the participation of steric effects would be minimized. It might then be compared with the substituent effect in organic chemistry.
The electronic effect caused by substituents would be transmitted by either an inductive or a mesomeric effect and has been quantitatively understood by the generalized treatments such as Hammett’s rule and the quantum chemical approach. When steric factors come into a reaction, explanation of the substituent effects tends to be more arbitrary and less confirmatory although considerable advancement has been made by the application of the Taft-Ingold e q ~ a t i o n . When ~ the problem of coordination bond formation affecting the chemical reactivity of the ligand could be confined to the electronic effect alone, similarities with the organic substituent effect might be found. Theories on the nature of the coordination bond indicate the possibility of (1) Q. Fernando, Advan. Inurg. Chem. Radiochem., 7, 185 (1965), and references therein. (2) J. Halpern, Ann. Rev. Phys. Chem., 16, 103 (1965), and references therein. (3) R. W. Taft, Jr., S. Ehrenson, I. C. Lewis, and R. E. Glick, J . Am. Chem. Soc., 81, 5352 (1959).
V o l u m 7 1 , Number 9
August 1967
2958
s. TAZUKE, I
