Early transition metal halide clusters

Early transition metal halide clusters, see Group 5 metal halide clusters Group 6 metal halide clusters... 

The octahedral early transition-metal-halide clusters [Mo Clg]" and [M6Xi2] + (M = Nb, Ta, Re X = F, Cl, 1 n = 2, 3, 4) contain both strongly anchored inner halide ligands that bridge metals and substitution-labile outer chloride ligands that are terminal in the six axial/equatorial positions and give rise to a rich hexasubstitution chemistry. 

The bulk of binary metal-halide clusters involve the early transition metals, however, later transition metal-halide clusters are known. [Pt6(M2-Cl)i2] is one example. 

The lack of late transition-metal halide clusters and the poor bonding which results when they are formed can be easily rationalized. In binary metal halide clusters the metal atoms are in a high oxidation state. The valence orbital contraction in these high oxidation state metals is much greater for the later transition metals than it is for the early ones. The diminished radical extension of the valence shell inhibits metal-metal bonding. 

The lower halides of niobium and tantalum consist of tightly bound clusters of metal atoms, with metal-metal distances close to those found in the metal. They contain ions with average oxidation numbers between +III and +1 (Table 43). Their size depends on the valence electron concentrations (VEC) that are available on the metal atoms for M—M bonding, and on the halide-metal ratio.644 Several reviews have been devoted to the clusters of early transition metals.3,643... 

A second feature of metal halide cluster chemistry is that the early transition metals are more prone to form metal -metal bonds than are the later noble metals and coinage metals. Again the polynuclear metal carbonyls differ in this facet of metal-metal bond behavior, and, in fact, metal carbonyl clusters become more common on going from the left to the right of the Periodic Table. 

Little is known about the oxidative addition of organohaUdes to early transition metal atoms. Nevertheless, macroscale amounts of solvated transition metal halides in lower valency states have been obtained. Re yields interesting clusters in reactions with 1,2-dibromoethane and oxalyl chloride... 

K.2 Coordination chemistry in the solid state cluster and condensed cluster halides of the early transition metals... 

II. Early Transition Metal Clusters with Halide and Alkoxide Ligands... 

III. EARLY TRANSITION METAL CLUSTERS WITH HALIDE AND ALKOXIDE LIGANDS... 

We have found that the main group metal and metalloid reductants mocury, bismuth, and antimony are highly effective " in reducing WCIe or M0CI5 at surprisingly lower temperatures than commonly used in the solid-state synthesis of early transition metal cluster halides. BorosUicate ampules can be substituted for the more expensive and less easily sealed quartz ampules at these lower temperatures, and the metals and metalloids are not as impacted by oxide coatings that inhibit sohd-state reactions with more active metals. These lower temperatures may allow access to kinetic products, such as trinuclear clusters, instead of thermodynamic products. 

John D. Corbett once said There are many wonders still to be discovered [4]. This certainly holds generally for all the different areas and niches of early transition cluster chemistry and especially for the mixed-hahde systems. The results reported above so far cover a very Hmited selection of only chloride/iodide systems and basically boron as the interstitial. Because of the very sensitive dependence of the stable stracture built in the soHd-state reaction type on parameters like optimal bonding electron counts, number of cations present, size and type of cations (bonding requirements for the cations), metal/halide ratio, and type of halide, a much larger mixed-hahde cluster chemistry can be expected. Further developments, also in mixed-hahde systems, can be expected by using solution chemistry of molecular clusters, excised from solid-state precursors. 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

Those in which the metal atoms are in somewhat higher oxidation states (+2 to +4) and the ligands are typically halide, sulfide, or oxide ions and some others of the same ilk as those in mononuclear Werner complexes. Clusters of this type are most common among the early transition elements, groups 5-7. 

Although the above mentioned face- and edge-halide-bridged clusters are characterised by 84 and 76 valence electrons respectively, it was apparent by the early 1970 s that the great majority of octahedral transition metal clusters possess 86 electrons. Braterman rationalised the 86 cluster valence electron count for octahedral metal carbonyl clusters, such as [Co6(CO)i6]75), [Ru6C(CO)17]76) and [Co6(CO)i4]4- 77) in terms of localised M-CO... 

As the field of metal carbonyl cluster chemistry was expanding rapidly many other new classes of clusters joined the fold, in particular, gold phosphine clusters, metal-alkoxide, -thiolate, and -halide clusters of the early transition elements. The theories of bonding in metal clusters rapidly advanced aided by both computational procedures and conceptual ideas. Without a doubt the most significant in the latter... 

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