Reversible (2+2) alkene binding. Styrene substrates were found to bind reversibly to 1 (Figure ). Stepwise addition of excess styrene (10 -40 equiv.) to 1 in C6D6 at room temperature led to the gradual appearance of a new species 3a suggesting an equilibrium established immediately after each addition. A Van't Hoff analysis of the reaction of 1 with excess styrene (21.6 equiv.) was conducted in toluene-d8 over a temperature range of 248-298 K. The formation of 3a was found to be slightly exergonic; ΔH°298 = -12.5 kcal mol -1 and ΔG°298 = -0.4 kcal mol -1 . The formation of 3c appeared to be more energetically favourable. Addition of 4-(trifluoromethyl)styrene (1.5 equiv.) to a solution of 1 in toluene-d8 at -35 °C resulted in an immediate colour change from dark red to bright orange and the NMR spectra recorded at the same temperature revealed that the equilibrium between 1 and 3c has been completely shifted to the product side. 3c could be crystallized from n-pentane at -35 °C and the solid-state structure was analysed by single crystal X-ray diffraction (Figure ).