Niobium octahedral clusters

The structural features of most niobium oxychlorides known to-date are summarized in Table 6.1. The use of a combination of chloride and oxide hgands leads to compounds with unique structure types [41], characterized by a remarkable variety of cluster frameworks, ranging from discrete cluster units to chains, layers, and three-dimensional nets, some topologies of which are unprecedented in compounds containing octahedral clusters. Most of the niobium oxychlorides known to date have anisotropic structures (the exceptions are Cs2LuNb,5Cli70 and PbLusNbsClisOg). 

For Nb or Ta clusters MgLig units are formed when the ligands are Cl or Br for niobium, and Cl, Br or I for the tantalum. In contrast, the niobium iodides have the MeLi4 unit, owing to the important steric effect of the iodine, as discussed later. Such MeLis units can also be prepared from valence-electron-poor transition metals such as zirconium, but in this case an interstitial element located at the center of the octahedral cluster is necessary to stabilize the cluster. ... 

The octahedral metal clusters that have long been familiar features of the lower halide chemistry of niobium, tantalum, molybdenum, and tungsten represent a category of cluster different from those so far considered in that their metal-metal bonding is best treated as involving four AO s on each metal 49, 133,144,165,178). 

Thus far, dimensional reduction has not been applied in this way to any of the other parent solids listed in Table I. Of particular interest are systems that straddle the divide between the edge-bridged and face-centered octahedral geometries. Will reduced niobium-molybdenum halide mixtures simply disproportionate or will they lead to new mixed-metal clusters If the latter, then where does the crossover in cluster geometries occur An important factor in applying dimensional reduction is recognizing the variability in the stable electron counts within a particular cluster system. An extreme case occurs... 

CsNb6IuH]. A completely different niobium hydrido species has been reported recently. It apparently contains an octahedral Nbe cluster with H at its center pNb-H for this compound is at 1120 cm-1 169). 

A manganese bis(acetylacetonato)-ethylenediamine complex was employed in the preparation of supramolecular assemblies with an octahedral niobium cyanochloride cluster, [Nb6Cli2(CN)6] . These assemblies range in size from 1.5 to 2.4 nm and their charge varies between —3 and +2, depending on the number of Mn units per cluster. 

The alkali metal rhenium- and technetium chalcogenide clusters described in Section 5.6 are also octahedral.These clusters have M6Es units with eight face-capping anions on the octahedral faces in contrast to the niobium halide or oxide clusters. The A4M6S14 (A = alkali metal, M — Re, Tc), Cs6Rc6Si2, and... 

Cluster complexes that obey Cotton s strict definition such as [ Nb6 Cli8]K4 and [ Mo6 C1h]Cs2 have 16 and 24 electrons, respectively, for bonding interactions within the octahedral niobium and molybdenum clusters. In a VB picture, these electrons are consumed for eight two-electron-three-center (2e-3c) or 12 2e-2c bonds, situated in the eight triangular faces and the 12 edges of the respective octahedra. 

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