Metal Halide Ratio

Mixed-Chloride-lodide Zirconium Cluster Phases with a 6 12-Metal Halide Ratio 

Further effects seen with the substitution of Cl for T in the [(Zr6Z)Ii2] type are a reduction of inter-cluster I I repulsions which allows for a reduction in Zr-I inter-cluster bond lengths. The [110] section of this rhombohedral structure is shown in Fig. 5.3 and illustrates that the clusters can be described as a cubic-close-packed array, with the 3 axes running vertically through Z. Phase widths found are 0 x 1.4 for [(Zr6Z)ClxIi2-x] (Z = B, Be) [17]. 

Close-packed layers of halogen (and interstitials) run horizontally. One pair of the short II 12 inter-cluster contacts is marked at the lower left. 

The mixed halide structure, for which the composition was refined to [(Zr6B)Clii.47(2)ll,, 53], crystallizes in the tetragonal space group P42/mnm, contrary to the only-chloride parent-type, which crystallizes orthorhombically [17]. This increase of symmetry is achieved through a random iodine substitution of 19.1(3)% of the Cl4 -site. Thereby the Zr3CF -units become planar, as can be seen from Fig. 5.5. 

---

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. 

If the assumptions of the kinetic derivation are valid, the polymerization rate Vp should, according to Equation (19-20), increase with the concentration of true active centers [C ] as well as with the fraction of catalyst surface/mon occupied by the monomer. The term ATa[A] in Equation (19-23) becomes negligibly small for constant transition metal halide surface and very small metal alkyl concentration. Then, to a first approximation, /mon is constant. The concentration of active centers, and consequently v also, should increase with increasing metal alkyl concentration, and finally—after all catalyst surface active centers have become occupied—should become constant. But with increasing metal alkyl concentration there is stronger competition between metal alkyl absorption and monomer absorption. Consequently,/mon and Vp must decrease. Thus, the polymerization rate Vp should pass through a maximum with increasing metal alkyl concentration, and there is experimental evidence for this. The polymerization rate should be proportional to the metal alkyl concentration for constant metal alkyl/transition metal halide ratios. 

The reactions of mercuric iodide, mercuric bromide, and mercuric chloride with the excited species produced in the hexafluoroethane plasma were examined first, as the expected products were known to be stable and had been well characterized 13). Thus, these reactions constituted a "calibration of the system. Bis(trifluoromethyl)mercury was obtained from the reaction of all of the mercuric halides, but the highest yield (95%, based on the amount of metal halide consumed) was obtained with mercuric iodide. The mole ratios of bis(trifluoro-methyDmercury to (trifluoromethyl)mercuric halides formed by the respective halides is presented in Table I, along with the weight in grams of the trifluoromethyl mercurials recovered from a typical, five-hour run. 

Reactions of metal halides with polysulfide dianions are useful methods for the synthesis of polysulfido complexes of main group elements and transition metals. In most of these reactions, similarly to other methods, the chain lengths and coordination types of the polysulfide ligands depend on the other ligands coordinated to the metal, on the ratio between the metal and sulfur, on the reaction temperature, and other parameters. 

As holds for other cluster systems, certain magic cluster electron counts exist, which indicates for a certain cluster-halide ratio and interstitial present the filling of all bonding molecular orbitals and therefore the thermodynamically most stable situation. For main group interstitial atoms these are 14 cluster-based electrons whereas for transition-metal interstitials the magic number is 18 [1, 10-12]. All of these phases are synthesized by high-temperature solid-state chemical methods. A remarkable variety of different structure types has been... 

Table 7.1 Radius ratios and observed structure types for the alkali metal halides...

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... 

Palladium complexes exhibit even higher catalytic activity and produce branched acids preferentially.132 133 The selectivity, however, can be shifted to the formation of linear acids by increasing the phosphine concentration.134 Temperature, catalyst concentration, and solvent may also affect the isomer ratio.135 Marked increase in selectivity was achieved by the addition of Group IVB metal halides to palladium136 and platinum complexes.137 Linear acids may be prepared with selectivities up to 99% in this way. The formic acid-Pd(OAc)2-l,4-bis(diphe-nylphosphino)butane system has been found to exhibit similar regioselectivities.138 Significant enhancements of catalytic activity of palladium complexes in car-bomethoxylation by use of perfluoroalkanesulfonic acid resin cocatalysts was reported.139,140... 

A number of other metallic halide salts have been found to condense with ethylene oxide, propylene oxide, or epichlorohydrin in a similar fashion (Eq. 662). Among them are phosphorus trichloride/ 7- a 17 bismuth trichloride,1SM arsenic trichloride,1000 silicon tetrachloride/7 1 titanium tetrachloride/ 09 16,7 beryllium chlorido/189 and Won trifluoride.401 Depending on their reactivity, on the reactant ratio, and... 

---