Singlet, planar vinyl

This square planar nickel(ll) complex is stable in air in the solid state, but solutions of the complex react in air to produce oxidation products of unknown composition. For this reason, manipulations of the complex are best performed in an inert atmosphere. The complex is a nonelectrolyte and is soluble in most common organic solvents, including diethyl ether, but not in water. The infrared spectrum of the complex contains an intense broad band in the double-bond region centered at 1610 cm. The electronic spectrum of a solution of the compound in toluene contains several bands 17.9 (e = 107), 23.0 (e 1600), 24.5 (e 4600), 25.9 (e 2700), and 29.4 kK (e 5500). The PMR spectrum of the complex in CDCI3 contains four bands as expected singlet methyl at 5 1.88, methylene at 6 3.13, vinyl doublet at 6 4.51, and a second vinyl doublet at 5 6.63. The vinyl protons of the charged chelate ring are coupled, J = 3 Hz. 

These calculations show that, if vinyl alcohol and vinyl thiol indeed mainly exist in planar syn conformations, as previously stated, the situation can be very different when halogen substituents are present. In some cases, the anti conformation is more or less out-of-plane staggered and can become more stable than the syn one. The general trends of substituent effects can be discussed by molecular orbitals analysis and electrostatic interactions. The results are subject to experimental confirmation, at least in the case of enols since their conformers can be prepared by the action of singlet oxygen atom on substituted ethylen, and trapped in low temperature matrices. 

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