Organometallic compounds are widely used as heterogenous and homogenous catalysts in the industrial productions of important chemicals and synthetic products. In many cases of such industrial transformation, metal carbenes are thought to play important roles as the active catalytic ingredients that bring about the transformation. Such carbenesrnare classified into two groups electrophilic and nucleophilic. Although several nucleophilic carbenes have been prepared, no general synthetic methods have been developed for the generation of electrophilic metal carbenes. We believe that oe-monohaloalkyltransitionrnmetal derivatives can be used as precursors of electrophilic metal carbenes via their interaction with lewis acids(see equation below) fLnMCCRg)]A1C14rnR = alkyl, H, aryl; In = Ligand.rnConsequently, our main goal is to synthesizerna-monohaloalkyltransition metal derivatives by thermal or photochemical decompositions of thernhaloacetyl metal derivatives.rnBrCH2COM(00)^0^ with traces of BrC0CH2M(CCO^C^H^ (M = Mo , V/) were made in good yields by allowingrnNaM(CO)5C5H^(M = Mo , W) to react with BrCH2G0Br.rnThe thermal decompositions of BrCH2C0M(C0)^ Cc,H^(M = Mo , W) were shown to involve p-halogenrnelimination. Decarbonylation followed by further 1rndecomposition produced the metal halides,rnG^H^M(C0)^Br (M =- Mo , W)rnThe photochemical decompositions of BrCH2C0M(C0)rn(M - Mo , W) gave the metal halides ,rnC^H^M(CO)jBr (M = Mo, V/). Also trace amounts ofrnB r C H2M(C0) (M = Mo , w) were obtained.rnNa5'e(C0)2CrH^ reacted with BrCH2C0Br givingrn[C^H^FeCOOIg]^ as the major product. Also smallrnquantities of BrCHgCOFeCCO)^!^ and BrCOCHgFeCCO^C^ were detected. (Ph^P)^Pt reacted with BrCH2C0Brrngiving Br(BrCH2C0)(Ph^P)2Pt and trace quantity ofrnBr(BrC0CH2)(Ph^P)2Pt. The thermal and photochemicalrndecomposition of Br(BrCH2C0)(Ph^P)2Pt resulted inrnthe production of (Ph^P)2Pt Br2>