The effect of the Grignard compounds (EtMgBr, PhMgBr, t-BuMgCl, cyclo-C6H11MgBr, and PhCequivalent toCMgBr) on the isomerization of HCequivalent toCCH(2)XMR(3) into R(3)MCequivalent toCCH(2)XH (M=Si, Ge; X=O, S) was studied. The efficiency of the 1,4-(X-->C-sp)-migration of the R3M group was shown to depend on the nucleophilicity of the Grignard compound and nature of the M-X bond.