Clusters discrete

By incorporation of alkali metal halides in the reaction mix, materials of composition M4-[MfiXig] can be produced in which each MgXn unit has a further six X atoms attached to its apices, so forming discrete clusters. 

The two subunits of CODH/ACS have been dissociated to offer a clearer picture of the ACS active site 135). The holoenzyme contains 2 Ni, 12 Fe, and 14 S 120) that are organized into 3 discrete clusters, whereas the isolated a subunit contains only 1 Ni and 4 Fe and has spectroscopic properties similar to those of Cluster A in the native enzyme 186). Based on EXAFS spectroscopy of the a. subunit, the Ni site in Cluster A has been proposed to be coordinated to 2 sulfur ligands at 2.19 A and 2 nitrogen or oxygen ligands at 1.89 A in a distorted square plane 186). 

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). 

The ligand arrangement in these oxychlorides leads to equivalent coordination of all niobium atoms, which correlates with the formation of structures containing discrete clusters. 

Geometry optimization was performed on the discrete cluster unit to eliminate the effects of crystal packing and interactions with titanium ions. The calculations were carried out in local spin density approximation using a SPARTAN 5.0.3 package (Wavefanction, Inc., Irvine, CA 92612 USA). 

The serendipitous discovery of this compound has proven to be extremely important to our visions of possibilities for new metal-metal bonded structures in reduced oxide phases. In retrospect it is amazing that oxide phases containing molybdenum in oxidation states less than 4+ were essentially unknown and certainly structurally uncharacterized. The existence of the previously mentioned series M2 Mo30q and LiM Mo303 should have been a tip-off to an extensive chemistry for metal-metal bonded molybdenum oxide systems. Indeed, subsequent work has revealed a plethora of new compounds all of which (where structure has been determined) feature strong metal-metal bonding in either discrete cluster units or extended chain arrays. 

Notes on cluster phases in triel alloys. Li and Corbett (2004) have shortly reviewed the systematic and extensive experimental and theoretical work carried out by Corbett and co-workers (see for instance Corbett 1996). Considering alkali metal-triel alloys, they underlined, particularly for Ga, In and Tl, the richness of their chemistry (see also 5.3.4.4). Gallium forms many anionic network structures (and only a few phases containing isolated cluster units), indium gives several examples of both network and discrete cluster structures, thallium forms especially discrete clusters (Tl , T157A Tl , Tl , Tl(f, Tl ). 

The visible evidence in matrix bone cannot be disputed. The resting osteocyte is contained within a dense mineral environment, all of which is clearly outside the membrane. It is easy to accept the idea that the matrix is laid down by the osteoblast and subsequently mineralized extracellularly. The dynamic evidence about bone formation, however, is not so clear, since a brief examination of the literature suggests that evidence is so uncertain as to require complete re-examination. The mineral at a developing bone front appears in discrete clusters or pockets with a... 

Figure 4. (b) Overlay of the ligand backbones of 40 metal complexes of the tetradentate ligand shown in (a) (c) averaged ligand structure with metal positions of three discrete clusters (Mn11, Cu1, Cu11).19,75... 

The hidden layer parameters to be determined are the parameters of hyperellipsoids that partition the input data into discrete clusters or regions. The parameters locate the centers (i.e., the means) of each ellipsoid region s basis function and describe the extent or spread of the region (i.e., the variance or standard deviations). There are many ways of doing this. One is to use random samples of the input data as the cluster centers and add or subtract clusters as needed to best represent the data. Perhaps the most common method is called the K-means algorithm (Kohonen, 1997 Linde et al 1980) ... 

Subsequent principal components and cluster analyses were performed eliminating variables appearing in only one or two cases or using only those variables such as silver, iron, or mercury which seemed to co-vary consistently between cases. Regardless of these transformations, discrete clusters were not produced. 

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