The isotopomeric complexes trans-W(Cmesityl)[(C(H,D)3) 2PCH2CH2P(C(H,D)3)2] 2(H,D) 1 -4 were prepared. 2 (W(CmeSityl)(dmpe) 2D) was used to study the Deuterium Quadrupole Coupling Constant (DQCC) and the ionicity of the W-D bond (DQCC = 34.1 kHz; ionicity 85%). 1 (W(Cmesityl)(dmpe)2H) shows several dynamic exchange processes in solution, such as Hw/Hw, H w/ortho-Memesityl, and Hw/H2 exchanges observed by NMR in combination with deuterium labeling studies and double label crossover experiments. Except for the Hw/H2, these reactions comprise elementary steps, which also appear along the isomerization pathway of 1 into (2,3,5-trimethylphenylcarbyne)(dmpe) 2WH (5) at 60 °C. 5 was characterized by an X-ray diffraction study. In the solid state only an Hw/Mep exchange process prevails appearing at higher temperatures, which was identified by NMR and by Quasielastic Neutron Scattering. The latter also provided an activation barrier of 5 kcal/mol and a "jump width" for the moving H nucleus in agreement with the HW⋯Mep distance of the X-ray diffraction study of 1.