The hydrolytic instability of Cp'Mo(NO)R2 complexes (Cp' = Cp (eta-5-C5H5), Cp* (eta-5-C5Me5); R = alkyl, aryl) becomes evident during attempts to synthesize them by procedures which have previously afforded the congeneric tungsten species. Thus, treatment of Cp*Mo(NO)I2 with 2 equiv of Me3SiCH2MgCl in Et2O at room temperature results in the formation of three organometallic complexes which are separable by chromatography on alumina. These products are (a) the expected alkylation product, i.e. the 16-valence-electron dialkyl complex Cp*Mo(NO)(CH2SiMe3)2 (1), (b) a novel bimetallic bridging-oxo complex, [CP*Mo(NO)(CH2SiMe3)]2(eta-O) (2), formed by hydrolysis of 1 during workup, and (c) a dimeric reduction product of the starting material, namely [Cp*Mo(NO)I]2 (3). Each of the complexes 1-3 has been fully characterized by conventional spectroscopic methods, and a single-crystal X-ray crystallographic analysis of 2 has also been performed. Crystal data for 2: triclinic, space group P1BAR, a = 11.7721 (7) angstrom, b = 16.373 (1) angstrom, c = 18.161 (1) angstrom, alpha = 84.947 (5)-degrees, beta = 89.974 (4)-degrees, gamma = 83.367 (4)-degrees, Z = 4, D(c) = 1.367 g cm-3. The structure was solved by standard heavy atom methods and was refined by full-matrix least-squares procedures to R(F) = 0.025 and R(wF) = 0.038 for 12 489 reflections having I greater-than-or-equal-to 3-sigma(I). The most chemically interesting feature about the solid-state molecular structure of 2 is the orthogonal orientation of the two Cp*Mo(NO)(CH2SiMe3) units about the essentially linear Mo-O-Mo grouping, an aspect consistent with the existence of multiple bonding in this linkage. Five analogues of 2 have been synthesized by the hydrolysis of several Cp'Mo(NO)R2 precursors (i.e. Cp' = Cp, R = CH2SiMe3; Cp'= Cp*, R = CH2CMe3, CH2CMe2Ph, o-tolyl, Ph). These transformations of Cp'M(NO)R2 to [Cp'M(NO)R]2(mu-O) are quite general for M = molybdenum but do not occur at all for M = tungsten when R = alkyl. Related CP*W(NO)(aryl)2 complexes (i.e. aryl = Ph, p-tolyl and o-tolyl) are converted to their aryl dioxo derivatives Cp*W(O)2(aryl) when exposed to water. Clearly, the reactivity of the 16-electron dialkyl and dialkyl complexes of molybdenum with water is fundamentally different from that exhibited by the congeneric tungsten compounds. The reactivities of both the molybdenum and tungsten complexes toward water are discussed in detail.