Cation Dependence of the Dimerization Enthalpy for A [tetracyanoethylene] (A=NMe , Mepy, NEt ) Possessing a Long, Multicenter Bond.

[TCNE] (TCNE=tetracyanoethylene) has been isolated as D π-[TCNE] possessing a long, 2.9 Å multicenter 2-electron-4-center (2e /4c) C-C bond, and as C π-[TCNE] possessing a longer, 3.04 Å multicenter 2e /6c (4 C+2 N atoms) bond. Temperature-dependent UV/Vis spectroscopic measurements in 2-methyltetrahydrofuran (MeTHF) has led to the determination of the dimerization, 2[TCNE] ⇌π-[TCNE] , equilibrium constants, K (T), [[TCNE] ]/[[TCNE] ] , enthalpy, ΔH, and entropy, ΔS, of dimerization for [Mepy] [TCNE] (Mepy=N-methylpyridinium, H CNC H ) possessing D π-[TCNE] and [NMe ] [TCNE] possessing C π-[TCNE] conformations in the solid state; however, both form D π-[TCNE] in MeTHF solution. Based on ΔH=-3.6±0.1 kcal mol (-15.2 kJ mol ), and ΔS=-11±1 eu (-47 J mol  K ) and ΔH=-2.4±0.2 kcal mol (-10.2 kJ mol ), and ΔS=-8±1 eu (-32 J mol  K ) in MeTHF for [NMe ] [TCNE] and [Mepy] [TCNE] , respectively, the calculated K (298 K) are 1.6 and 1.3 m , respectively. The observed K (145 K) are 3 and 2 orders of magnitude greater for [NMe ] [TCNE] and [Mepy] [TCNE] , respectively. The K (130 K) is 4470, 257, ≈0.8, and ≪0.1 m for [NMe ] [TCNE] , [Mepy] [TCNE] , [NEt ] [TCNE] , and [N(nBu) ] [TCNE] , respectively, decreasing with increasing cation size. At standard conditions and below ambient temperature the equilibrium favors the dimer for the NMe and Mepy cations. From the decreasing enthalpy, NMe >Mepy , along with the decrease in dimer formation K (T) as NMe >Mepy >NEt >N(nBu) , the dimer bond energy decreases with increasing cation size in MeTHF. This is attributed to a decrease in the [A] ⋅⋅⋅[TCNE] attractive interactions with increasing cation size. Solid state UV/Vis spectroscopic determinations of [NMe ] [TCNE] are reported and compared to D π-[TCNE] conformers. The feasibility and limitations of temperature-dependent electron paramagnetic resonance (EPR) measurements for the determination of K (T) are also discussed.