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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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A new approach to the activation of transition metal bis-vinylidene (μ-1,3-butadiene-1,4-diylidene) complexes toward nucleophilic addition is proposed, based on their two-electron oxidation to corresponding μ-2-butene-1,4-diylidyne compounds of a bis-carbyne type. The latter are more electrophilic and able to react with nucleophiles that are inert to the former. In line with this synthetic methodology the reaction of the manganese bis-vinylidene complexes [(η5-C5R5)(CO)2MnCCPh]2 (1, R = H; 2, R = Me) with oxygen nucleophiles (H2O and OH-) to form binuclear cyclic bis-carbene compounds [(η5-C5R5)(CO)2Mn]2(μ-C4Ph2O) (5, R = H; 6, R = Me) is induced by oxidation, 2 equiv of oxidant (Cp2FeBF4) being required to complete the reaction of 1 with water, resulting in 5 with 76% yield. The oxidation of 1 with 1 equiv of AgBF4 leads to the thermally unstable radical cation 7, which reacts with water to give an equimolar mixture of 1 and 5. Two-electron oxidation of 1 leads to the dicationic bis-carbyne complex (η5-C5H5)(CO)2Mn+⋮CC(Ph)C(Ph)C⋮Mn+(CO)2(η5-C5H5) (3), which exists as the solvate 3·2THF (11) in THF solution and reacts with water smoothly to give 5 in about 90% yield. Unlike 1, the bis-vinylidene complex 2 does not react with water even in the presence of oxidant. Complex 6 can be obtained in 15% yield by the reaction of the bis-carbyne dicationic complex (η5-C5Me5)(CO)2Mn+⋮CC(Ph)C(Ph)C⋮Mn+(CO)2(η5-C5Me5) (4) with Bu4NOH. The different reactivities of 1 and 2 are also confirmed by cyclic voltammetry. The crystal structures of 2, 5, and 6 are reported.