Solution Calorimetric Investigation of Oxidative Addition of HEAr (E = O, S, Se; Ar = C₆H₄X, X = CH₃, H, Cl, NO₂) to (PMe₃)₄Ru(C₂H₄):  Relationship between HEAr Acidity and Enthalpy of Reaction

The enthalpies of reactions of (PMe 3 ) 4 Ru(C 2 H 4 ) ( 1 ) with a series of HEAr compounds (E = O, S, Se; Ar = aryl), leading to the formation of (PMe 3 ) 4 Ru(H)(EAr) complexes, have been measured by anaerobic solution calorimetry in C 6 H 6 at 30.0 °C. These reactions are rapid and quantitative under the calorimetry conditions. The measured reaction enthalpies span a range of some 35 kcal/mol. The relative stability scale established is as follows: (PMe 3 ) 4 Ru(H)(OC 6 H 4 - p- CH 3 ) < (PMe 3 ) 4 Ru(H)(OC 6 H 5 ) < (PMe 3 ) 4 Ru(H)(OC 6 H 4 - p- Cl) < (PMe 3 ) 4 Ru(H)(OC 6 H 4 - p- NO 2 ) < (PMe 3 ) 4 Ru(H)(SC 6 H 4 -p- CH 3 ) < (PMe 3 ) 4 Ru(H)(SC 6 H 5 ) < (PMe 3 ) 4 Ru(H)(SC 6 H 4 - p- Cl) ≈ (PMe 3 ) 4 Ru(H)(SeC 6 H 5 ) < (PMe 3 ) 4 Ru(H)(SC 6 H 4 - p- NO 2 ). A single-crystal X-ray structure of one of these complexes, (PMe 3 ) 4 Ru(SC 6 H 5 )H ( 7 ), is reported. The measured enthalpies of reaction display a dependence on the acidity of HEAr.