Halo complexes stability

In their complexes, Pd(IV) and Pt(IV) are low-spin, octahedral and diamagnetic (J ). The full range of halo complexes [MXg] " is known (e.g. equations 22.131-22.133), in contrast to Pdp4 being the only neutral Pd(IV) halide. The [MXg] " ions are stabilized by large cations. 

Although the stability constant sequence for halo complexes of many metals is F > Cl > Br > I, Cu1 and Ag1 belong to the group of ions of the more noble metals for which it is the reverse. 

In order to stabilize Os(IV), it is necessary to have either several ligands that are good tt bases (e.g., Cl- or Br-), or one ligand that is a strong7T base (e.g., O2-) (2,5,6). Many Os(IV) complexes contain one or more halo ligands. There is also a growing number of oxo- and nitrido-bridged complexes. 

A stabilization of the chalcogen(II) dihalides is possible by complex-ation with various electron pair donors (see Section III,D). A stabilization of the simple molecular species is also obtained if the halogens are substituted by pseudo-halogens. This situation is similar to the anionic halo- and pseudo-haloselenates(II) (Section III,B). As examples, Se(CN)2, Se(SCN)2, and Se(SeCN)2 could be prepared in crystalline form (182, 273). In Se(SCN)2 and Se(SeCN)2 the molecules are linked via secondary Se-"N bonds to form chains, with a distorted square planar coordination of 2 S(Se) and 2 N around the central Se (182). 

Figure 24 X-ray maps of trace elements and Ca from garnet-bearing quartzite, showing spatially coincident, pronounced spikes. The location of these spikes is systematically different in different-sized garnets, relative to Fe-Mg-Mn systematics (Chernoff and Carlson, 1997,1999). Diffusionally produced haloes around growing crystals can cause sudden stabilization or destabilization of minerals (Johnson and Carlson, 1990). The coincidence of spikes in many trace elements as well as Ca is interpreted to reflect modal changes in a mineral like apatite or allanite. A common, diffusional mechanism for intergranular diffusion of trace elements and Ca may also be required, possibly by complexing with a common, slow-diffusing carrier element (Chernoff and Carlson, 1999) (reproduced by permission of GSA from Geology, 1999, 27, 555-558).

Treatment of the alkylidyne(cyclopentadienyl)metal species 4 with halo-acids (HX X = Q, Br, I) has been described in Section I. The carbene- or acyl-ligated metal complexes so formed are stabilized by coordination to the metal of the halide ion intrinsically involved in the protonation reaction, thus highlighting the important difference between employing HX and HBF4 Et20 in the protonations. 

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