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Donor-acceptor properties of methylphosphonitriles Mah, Timothy W. J.


The methylphosphonitriles were shown by the method of Benesi and Hildebrand to form outer complexes with I₂. The electronic spectra of the complexes as well as the determined equilibrium constants indicate the use of the methylphosphonitriles as n-donors toward I₂. This is confirmed by the molecular structure of N₃P₃Meg₆∙I₂, in which the N∙∙∙I-I unit was found to be linear. The interpretation of both the electronic and proton spectra of these complexes indicate the relative base strengths of the methylphosphonitriles as being in the order N₄P₄Meg₈ > N₅P₅M₁₀ > N₃P₃M₆. This order is explicable in terms of the u-electron densities at the ring nitrogen atoms, as affected by the homomorphic Tr-system, and the effects of c-hybridization. The synthesis of inner complexes formed by the interaction of iodine with the methylphosphonitriles show further analogies of pyridine with the methylphosphonitriles. The structure of N₄P₄Meg₈I₄ was shown to be (N₄P₄Meg₈I)⁺I₃⁻ by the electronic, proton, and vibrational spectra as well as by conductivity measurements. The electronic spectra and polarographs of the methylphosphonitrilium and dimethylpyridinium iodides show that the acceptor levels of the phosphonitrilic rings lie at higher energies than that of the pyridine ring. In CHCl₃, the similarities in the spectra of the methylphosphonitrilium, dimethylpyridinium, and tetra-alkylammonium iodides indicate that all the charge-transfer to cation processes are similar, the relative energies of transition being affected only to a small degree by the specific cation involved.

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