Cation Dependence of the Dimerization Enthalpy for A 2 [tetracyanoethylene] 2 (A=NMe 4 , Mepy, NEt 4 ) Possessing a Long, Multicenter Bond

[TCNE] .− (TCNE=tetracyanoethylene) has been isolated as D 2 h π–[TCNE] 2 2− possessing a long, 2.9 Å multicenter 2‐electron‐4‐center (2e − /4c) C−C bond, and as C 2 π–[TCNE] 2 2− 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] 2 2− , equilibrium constants, K eq ( T ), [[TCNE] 2 2− ]/[[TCNE] .− ] 2 , enthalpy, Δ H , and entropy, Δ S , of dimerization for [Mepy] 2 [TCNE] 2 (Mepy= N ‐methylpyridinium, H 3 CNC 5 H 5 + ) possessing D 2 h π–[TCNE] 2 2− and [NMe 4 ] 2 [TCNE] 2 possessing C 2 π–[TCNE] 2 2− conformations in the solid state; however, both form D 2 h π–[TCNE] 2 2− in MeTHF solution. Based on Δ H =−3.6±0.1 kcal mol −1 (−15.2 kJ mol −1 ), and Δ S =−11±1 eu (−47 J mol −1  K −1 ) and Δ H =−2.4±0.2 kcal mol −1 (−10.2 kJ mol −1 ), and Δ S =−8±1 eu (−32 J mol −1  K −1 ) in MeTHF for [NMe 4 ] 2 [TCNE] 2 and [Mepy] 2 [TCNE] 2 , respectively, the calculated K eq (298 K) are 1.6 and 1.3  m −1 , respectively. The observed K eq (145 K) are 3 and 2 orders of magnitude greater for [NMe 4 ] 2 [TCNE] 2 and [Mepy] 2 [TCNE] 2 , respectively. The K eq (130 K) is 4470, 257, ≈0.8, and ≪0.1  m −1 for [NMe 4 ] 2 [TCNE] 2 , [Mepy] 2 [TCNE] 2 , [NEt 4 ] 2 [TCNE] 2 , and [N( n Bu) 4 ] 2 [TCNE] 2 , respectively, decreasing with increasing cation size. At standard conditions and below ambient temperature the equilibrium favors the dimer for the NMe 4 + and Mepy + cations. From the decreasing enthalpy, NMe 4 + >Mepy + , along with the decrease in dimer formation K eq ( T ) as NMe 4 + >Mepy + >NEt 4 + >N( n Bu) 4 + , 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 4 ] 2 [TCNE] 2 are reported and compared to D 2 h π–[TCNE] 2 2− conformers. The feasibility and limitations of temperature‐dependent electron paramagnetic resonance (EPR) measurements for the determination of K eq ( T ) are also discussed.