Enthalpies of formation of homoleptic dimethylamido compounds of tantlum (V), molybdenum (III), molybdenum (IV), tungsten (III) and tungsten (VI). Enthalpy contributions of metal–metal triple bonds

Abstract

Solution, combustion and other calorimetric studies on dimethylamido–metal compounds of Ta, Mo, and W have led to the following enthalpies of formation at 25°C (kJ mol^–1): [graphic omitted]

Average bond enthalpies, D(M—NMe₂) in the mononuclear compounds M(NMe₂)_n, derived from these data are: Ta(NMe₂)₅; D(Ta—NMe₂)=(328 ± 5) kJ mol^–1, Mo(NMe₂)₄; D(Mo—NMe₂)=(255 ± 5) kJ mol^–1,W(NMe₂)₆; D(W—NMe₂)=(222 ± 5) kJ mol^–1.

ΔH^°_t(g) values for the unknown compounds Mo(NMe₂)₆ and W(NMe₂)₄ have been estimated at (259 ± 30) and (174 ± 20) kJ mol^–1, respectively, from which average bond enthalpies, D(Mo—NMe₂)=(190 ± 5) kJ mol^–1 and D(W—NMe₂)=(295 ± 5) kJ mol^–1 were derived.

Transfer of D(M—NMe₂), unchanged, from M(NMe₂)₄ into dinuclear M₂(NMe₂)₆ gives D(MoMo)=(398 ± 18) kJ mol^–1, and D(WW)=(558 ± 20) kJ mol^–1; transfer of D(M—NMe₂), unchanged, from M(NMe₂)₆, however, gives much higher values, (789 ± 24) kJ mol^–1 for D(MoMo), and (995 ± 18) kJ mol^–1 for D(WW).

These latter are unacceptably high, and imply that transfer from the mononuclear hexakis-(dimethylamido) metal compounds to the corresponding dinuclear hexakis(dimethylamido) dimetals may be an invalid procedure. Steric crowding in M(NMe₂)₆ is probably a major invalidating factor; transfer from the tetrakis (dimethylamido) metal compounds is free from this objection, and the D(MM) values so obtained are considered to be more realistic.