Thermochemistry of Slfide and Thiolato Derivatives of Iron - Carbonyl, and the Strengths of the Iron - Sulfur Bonds

Calorimetric measurements of the thermal decomposition of {Fe(CO)(3)S}(2), Fe-3(CO)(9)S-2, and {Fe(CO)(3)(SR)}(2) (R is CH3, or C2H5), have been made in the temperature range T = 450 K to 550 K. Careful analyses of the decomposition of the alkanethiolate complexes indicate that the products include R(2)S and FeS in each case. Using drop-microcalorimetric methods, values were determined for the standard molar enthalpies of formation at T = 298.15 K of the crystalline compounds: Delta(f)H(m)(o){Fe-2(CO)(6)S-2, cr} = -(1068 +/- 20) kJ . mol(-1); Delta(f)H(m)(o){Fe-3(CO)(9)S-2, cr} = -(1530 +/- 20) kJ . mol(-1) Delta(f)H(m)(o){Fe-2(CO)(6)(SCH3)(2), cr} = -(1190 +/- 18) kJ . mol(-1); Delta fHmo{Fe2(CO)6(SC2H5)2, cr} = -(1222 +/- 20) kJ . mol-1. These are combined with molar enthalpies of sublimation and standard values of the Fe-CO and, where appropriate, Fe-Fe molar bond-enthalpy contributions to provide estimates of the FeS molar bond-enthalpy contributions: D-m(Fe-SR) = (178 +/- 5) kJ . mol(-1); D-m(mu-S-Fe) = (156 +/- 5) kJ . mol(-1). A group-disruption model is also considered.