Photolysis of the halfsandwich tetracarbonylmetal complexes CpV(CO)4, Cp*V(CO)4 and Cp*Ta(CO)4 in solution in the presence of di(organyl)dichalcogenides E2R2 (E = S, Se, Te; R = Me, Ph, Fc) leads to diamagnetic doubly organochalcogenolato‐bridged compounds, [Cp()M(CO)2(μ‐ER)]2. According to the X‐ray structure determinations carried out for [CpV(CO)2(μ‐TeMe)]2, [Cp*V(CO)2(μ‐TePh)]2 and [Cp*Ta(CO)2(μ‐SPh)]2, the molecular framework consists of a folded M2(μ‐ER)2 ring with the cyclopentadienyl ligands in cis‐configuration and the organyl substituents R in a syn‐equatorial arrangement, thus forming a bowl‐shaped molecule with the four terminal CO ligands protruding into the inner sphere. The M…M distances (in the range between 305 and 330 pm) are not considered to indicate direct bonding interactions. The vanadium complexes [Cp()V(CO)2(μ‐ER)]2 are completely decarbonylated in the presence of an excess of E2R2 in boiling toluene, and in many cases the paramagnetic quadruply‐bridged products, [CpV(μ‐ER)2]2, can be isolated.

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The reaction of Cp*Ta(CO) 4 (1) (Cp* = η 5 -pentamethylcyclopentadienyl, η 5 -C 5 Me 5 ) with chlorine leads to Cp*TaCl 4 (2a), whereas the corresponding reactions with bromine or iodine give the oxo-bridged complexes [Cp*TaX 3 ] 2 (µ-O) (X = Br (3b), I (3c)). The oxygen atom apparently stems beschrieben. Die Umsetzungen und die entstehenden Produkte sind in Schema 1 zusammengestellt.

The photo-induced substitution of a CO ligand in Cp*Ta(CO)4 (1) has been used to pre pare a series of Cp*Ta(CO)3L (2) complexes, in which the two-electron ligand L includes, e.g., isonitriles, sulfanes, cyclic ethers, phosphanes and pyridines. The complexes (2 a -y ) were characterized by their v(CO) frequencies and their 'H and 13C NM R spectra, and were compared -if possible -with the corresponding vanadium complex, C p*V(C O )4 (3) andits derivatives, C p*V(C O )3L (4). The molecular structure o f the 4-picoline complex Cp*V(C O )3(NC5H 4-M e(4)) (4r) has been determ ined by X-ray crystal structure analysis.

A survey is presented on various reactions of Cp*V(CO) 4 , including CO ligand substitution and/or oxidative decarbonylation, to give complexes in which the formal oxidation state of vanadium may vary from 0 to V. Diamagnetic Cp*V complexes can be rapidly and conveniently characterized by 51 V NMR spectroscopy.