MOLECULAR-HYDROGEN COMPLEXES OF THE TRANSITION-METALS .7. KINETICS AND THERMODYNAMICS OF THE INTERCONVERSION BETWEEN DIHYDRIDE AND DIHYDROGEN FORMS OF W(CO)3(PR3)2H2 WHERE R = ISOPROPYL AND CYCLOPENTYL

Kinetic and thermodynamic parameters have been obtained for the equilibrium system H2W(CO)3(PR3)2 ⇌ W(CO)3(PR3)2(η2-H2). The systems studied include a newly synthesized complex where R = cyclopentyl.31P NMR spin saturation transfer studies over a temperature range of 288-310 K afford values of ΔH* = 10.1 ± b 1.8 kcal/mol, ΔS* = –19.9 ± 6.0 eu, and ΔG* = 16.0 ± 0.2 kcal/mol (298 K) for the dihydrogen to dihydride exchange process where R = isopropyl. Exchange between nonequivalent hydrides in the seven-coordinate dihydride complex corresponds to ΔG* ≈ 11 kcal/mol (253 K) derived from NMR coalescence data.1H{31P} NMR equilibrium studies for the formation of the dihydrogen complex from the dihydride complex over the temperature range 265–309 K produce the following: ΔH° = –1.2 ± 0.6 kcal/mol, ΔS° = –1.2 ± 2.1 eu, and ΔG° = –0.80 ± 0.12 kcal/mol (298 K) for R = isopropyl; ΔH° = –1.5 ± 0.4 kcal/mol, ΔS° = –2.4 ± 1.4 eu, and Δ° = –0.75 ± 0.12 kcal/mol (298 K) for R = cyclopentyl. © 1990, American Chemical Society. All rights reserved.