Compounds Cp*MH3(dppe) (M = Mo, 1; W, 2) are oxidized chemically and electrochemically to the corresponding 17-electron cations 1(+) and 2(+). Analogous oxidations of 1-d(3), and 2-d(3) provide 1(+)-d(3) and 2(+)-d(3), respectively. Complex 2(+) is stable in CH2Cl2, THF, and MeCN at room temperature. A single-crystal X-ray analysis of the PF6- salt of 2(+) shows a geometry for the cation which is intermediate between octahedral and trigonal prismatic, which is reproduced by geometry optimization of the [CpWH3(PH2CH2CH2PH2)](+) model at the B3LYP/LANL2DZ level. Identical calculations on the neutral analogue also reproduce the previously reported trigonal prismatic structure for 1. A blue shift in the M-H stretching vibrations upon oxidation for both Mo and W compounds indicates that a M-H bond strengthening accompanies the oxidation process. The DFT calculations (M-H bond lengths, BDE, and stretching frequencies) are in good agreement with the experimental results. Complex 1+ decomposes in solution at room temperature by one or more of three different mechanisms depending on conditions: Hr reductive elimination, solvent-assisted disproportionation, or deprotonation. In THF or CH2Cl2, a reductive elimination of H-2 affords the stable paramagnetic monohydride Cp*MoH(dppe)PF6 (3), which adds a molecule of solvent in CH2Cl2, THF, and MeCN. EPR studies show that the CH2Cl2 molecule coordinates in a bidentate mode to afford a 19-electron configuration. A solvent dependence of the decomposition rate [k(CH2Cl2) approximate to 7.8k(THF) at 0 degrees C] and an Inverse isotope effect [k(H)/k(D) = 0.50(3) in CH2Cl2 at 0 degrees C] indicate the nature of If as a classical trihydride and suggest a decomposition mechanism which involves equilibrium conversion to a nonclassical intermediate followed by a rate-determining associative exchange of H-2 With a solvent molecule. In MeCN at 20 degrees C, a solvent-assisted disproportionation (rate kdisp[1(+)](2), k(disp) = 3.98(9) x 10(3) s(-1) M-1) and a deprotonation by residual unoxidized 1 (rate k(deprot)[1(+)], k(deprot) = 2.8(2) x 10(2) s(-1) M-1) take place competitively, as shown by detailed cyclic voltammetric and thin-layer cyclic voltammetric studies. The stoichiometric chemical oxidation of 1 in MeCN leads to a mixture of [Cp*MoH2(dppe)(MeCN)](+) and [Cp*MoH(dppe)(MeCN)(2)](2+) by the disproportionation mechanism.