The electrochemical properties of a-ethynyl complexes of chromium subgroup metals were studied by cyclic voltammetry and preparative-scale electrolysis. The redox cycle of CsHs(CO)3CrC~CPh was shown to give the bis-carbyne complex (qS-CsHs)(CO)2C~C--C(Ph)=C(Ph)--C,~Cr(CO)2(q5-CsHs) formed via the reduetive C1~--CI3 coupling of ethynyl moieties. The influence of the nature of the metal atom and the ligand environment on the course of this reaction was considered.

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ul. Vavilova, 117813 Moscow, Russian Federation. Fax: + 7 (095) 5085The main regularities of the reactions of l-haloalkynes RC~CX with carbonylmetallate anions [('qS-CsR'5)(CO)3M]-(R" = H (1--3), M = Cr (1), M = Mo (2), or M = W (3); R' = Me (4-6), M = Cr (4), M = Mo (5), or M = W (6)) were revealed. It was established that the first stage of the reactions of anions 1--6 with bromo-or iodoalkynes RC~CX (X=Br or I) involved the transfer of the halogen atom from the sp-hybridized carbon atom to the transition metal atom to form carbonyl halides (rls-CsR'5)(CO)3MX. To the contrary, the reactions of anions 1--6 with chloroalkynes RC~CCI proceeded selectively as a nucleophilic substitution at the unsaturated carbon atom, the reaction rate being governed by the nucleophilicity of the carbonylmetallate anions and the electron-withdrawing ability of the R group. These reaction paths are consistent with the structures of the lowest unoccupied molecular orbitals (LUMO) in the PhC~CX molecules (X = CI, Br, or 1) calculated by the MNDO/PM3 method. In the case of the reactions of l-chloroheptyne-I CIC~CCsHI]n , anions 1--3 appeared to be insufficiently nucleophilic, but these reactions can be performed as cross-coupling of the carbonylmetallate anions with chloroalkynes catalyzed by palladium complexes.Key words: e-ethynyl complexes of chromium, molybdenum, and tungsten, l-haloatkynes, carbonytmetallate anions of chromium, molybdenum, and tungsten.Bifunctional l-haloalkynes RC=CX can react with transition-metal complexes either at the carbon--halogen or at the triple carbon--carbon bond. Examples of formation ofrl2-haloalkyne, halovinylidene, o-l-alkynyl, and tetrahalometallacyclopentadiene complexes of transition metals in reactions of haloalkynes with various organometallic substrates were reported in the literatu re. ! Although haloalkynes are promising ligands, their coordination chemistry remains poorly studied. The reactions of nucteophilie substitution of the halogen atom in 1-haloalkynes are often used to form carbon--heteroatom (oxygen, sulfur, nitrogen, phosphorus, etc.) bonds. 2 However, these reactions have virtually not been used to form transition metal--carbon ,-bonds. The available examples of these processes concern the reactions of carbonylmetallate anions I(nS-CsHs)M(CO)3] -(M = Cr, Mo, or W) with PhC-=-CBr 3, and XC~CX (X =-CI; Br,or-l); 4 thereactionsof[CO)sM-(M =Mn or Re) with IC-=CI, s and the reactions of anionic phthaIocyanine rhodium complexes with RC-=CBr (R = Ph or Prn). 6 Results and DiscussionWith the aim of revealing the regularities and synthetic possibilities of the reactions of carbonylmetallate anions with l-haloalkynes-1, in this work we studied the reactions of anions 1--6 with RC~CX (R = Ph and X = CI, Br, or 1; R = CsHll n and X = CI or Br; or R = COOMe and X = CI): The reactions of anions 1--3 with phenylethynyl halides PhC=CX (X = CI, Br, or I) were studied in close detail. The course of the reaction and the composition of the reaction products are governed by the nature of M, X, an...

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