The reaction of [Cp#2NbTe2H] (1#; Cp# = Cp* (C5Me5) or Cp(x) (C5Me4Et)) with two equivalents of [Co2(CO)8] gives a series of cobalt carbonyl telluride clusters that contain different types of niobocene carbonyl fragments. At 0 degrees C, [Cp#2NbTe2CO3(CO)7] (2#) and [Co4Te2(CO)10] (3) are formed which disappear at higher temperatures: in boiling toluene a mixture of [cat2][Co9Te6(CO)8] (5#) (cat= [Cp#2Nb(CO)2]+) and [cat2][Co11Te7(CO)10] (6#) is formed along with [cat][Co(CO)4] (4#). Complexes 6# transform into [cat][Co11Te7(CO)10] (7#) upon interaction with HPF6 or wet SiO2. The molecular structures of 2(Cp(x)), 4(Cp(x)), 5(Cp*), 6(Cp*) and 7(Cp*) have been determined by X-ray crystallography. The structure of the neutral 2(Cp(x)) consists of a [Co3(CO)6Te2] bipyramid which is connected to a [(C5Me4Et)2Nb(CO)] fragment through a mu4-Te bridge. The ionic structures of 4(Cp(x)), 5(Cp*), 6(Cp*) and 7(Cp*) each contain one (4, 7) or two (5, 6) [Cp#2Nb(CO)2]+ cations. Apart from 4, the anionic counterparts each contain an interstitial Co atom and are hexacapped cubic cluster anions [Co9Te6(CO)8]2- (5) or heptacapped pentagonal prismatic cluster anions [Co11Te7(CO)10]n- (n=2: [6]2- , n=1: [7]-), respectively. Electrochemical studies established a reversible electron transfer between the anionic clusters [Co11,Te7(CO)10]- and [Co11Te7(CO)10]2in 6# and 7# and provided evidence for the existence of species containing [Co11Te7(CO),0] and [Co11Te7(CO)0]3-. The electronic structures of the new clusters and their relative stabilities are examined by means of DFT calculations.

The reactions of [Cp#2NbBH4] (1) [Cp# = C5Me5 (Cp*) or C5Me4Et] and [Cp′2TaH3] (Cp′ = tBuC5H4) with elemental selenium give complexes of the general composition [Cp2MSe2H]. These exist in three isomeric forms, A–C, depending on the metal and the chalcogen. [Cp#2NbSe2H] (2) incorporates the novel η2‐Se2H ligand (C), which is labile when exposed to light. Solutions of 2(Cp*) can be converted into [Cp*2NbSe2H] (3), which exists in two isomeric forms A or B, along with [Cp*2NbSe3H] (4) and [Cp*2NbO2H] (5). [Cp′2Ta(η2‐Se2)H] (6) is the only isomer found for the Ta system. Se abstraction by means of P(OEt)3 gives [Cp′2Ta(=Se)H] (7). Complex 7 can be reduced with Na/Hg and subsequently hydrolysed to give [Cp′2Ta(SeH)H2] (8), which has also been observed during the formation of 6. Coordination of [W(CO)5THF] at the Se lone pair of 7 affords the heterobimetallic complex 9. All compounds have been characterized by means of 1H‐ and, in some cases, 77Se‐NMR spectroscopy. Products 8 and 9 have also been subjected to X‐ray diffraction analysis.

Reaction of [Cp′2NbTe2H] (1) (Cp′ = η5‐tBuC5H4) with [W(CO)6] in decane at 140 °C gives red [{(Cp′2Nb)2WTe4}⋅W(CO)4] (2), whereas in the reaction of 1 with [Cr(CO)6] green‐black [Cp′4Nb2Te2] (3) is formed. The crystal structures of 2 and 3 have been determined. The structure of 2 contains a slightly distorted WTe4 tetrahedron to which two niobocene fragments and one W(CO)4 group are coordinated. The crystal structure determination of 3 reveals the presence of two Cp′2Nb units bridged by two Te atoms. Complex 2 represents the first example of an organometallic derivative of the unknown WTe42− anion.

Reaction of [Cp′2NbH3] (Cp′ = tBuC5H4) with Te powder in THF gives [Cp′2Nb(Te2)H] (1) and [Cp′6Nb4Te4O] (2). The yield of 1 varies between 10 and 81% depending on the degree of oxygen contamination of the reagents. Complexes 1 and 2 react with [Cr(CO)5THF] to give [Cp′2Nb(Te2)H·Cr(CO)5] (3) and [Cp′6Nb4Te4O·2Cr(CO)5] (4), respectively. The crystal structures of 2−4 have been determined. In 3 a Te2 unit and an H ligand are coordinated to a bent niobocene moiety; the Cr(CO)5 group is attached to the lateral Te atom. The molecular cores of 2 and 4 are practically identical in that they contain two planar Nb2Te2 rings connected by a nearly linear oxygen bridge. Each of the “outer” Nb atoms bears two Cp′ ligands, whereas the “inner” Nb atom only has one such ligand. An additional structural feature in 4 is two Cr(CO)5 groups, attached to one Te bridge of each Nb2Te2 ring. Thermolysis of 3 leads to the formation of diamagnetic [Cp′4Nb2Te2] (5), which also contains a planar Nb2Te2 core. The relatively long transannular Nb−Nb distance (3.647 Å) is consistent, according to DFT calculations, with a through‐space Nb−Nb coupling. Complex 5 reacts with CH3I with successive methylation of both Te bridges to give [Cp′4Nb2Te(CH3Te)]I ([6]I) and [Cp′4Nb2(CH3Te)2]I2 ([7]I2). The crystal structure of [7]I2 may be derived from that of 5, the incoming CH3 groups being fixed at the Te bridges in a trans position. 1H NMR spectroscopic investigations reveal a restricted rotation around the Cp′−Nb bonds in 2 and 5 at −90 °C and in 4 and [7]I2 at ambient temperature. Electrochemical studies have been carried out on 5, [6]I, and [7]I2, showing that all compounds undergo two reversible one‐electron reduction steps. The reduction potential decreases by ca. 1.6 V when going from 5 to [7]I2. There is also a clear linear correlation between the reduction potentials measured for 5 to [7]I2 and the energies of the corresponding LUMO’s calculated at the DFT/B3LYP level. These LUMO’s bear some 70% contribution from both Nb atoms and, consequently, the reduction processes mainly operate at both metallic centers. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

ions (m 1 ± 5; n 1, 2). Each of these intermediates is distinguished by at least one oxidation and two reduction waves, giving rise to a total of 21 redox couples and 27 electroactive species. The electron sponge character of the new compounds is particularly pronounced in 5, which exhibits charges n between 1 and À 4 corresponding to metal valence electron counts of between 146 and 151.