Atoms central

Overview of the Metal Complexes Involved in Aqueous Catalysis 

This section is not intended to give a detailed description of the applications of organometallic complexes in aqueous catalysis. In Table 1 some recent catalytic applications for different metals are presented as examples. The complexes here are considered to be organometallic in a broader context. The comparatively few examples described with organometallic compounds in a stricter sense (containing an M-C bond) will be dealt with in the following paragraph. Since only the most recent applications are mentioned, for further information see the References. 

It is also evident from Table 1 that seemingly a variety of ligands and even metal oxidation states may be applied for similar catalytic purposes. However, changes of both the surrounding ligands and the metal center make comparisons often difficult and generalizations dangerous. 

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Alternatively a redistribution of groups around a central atom, e.g. 

Unlike the forces between ions which are electrostatic and without direction, covalent bonds are directed in space. For a simple molecule or covalently bonded ion made up of typical elements the shape is nearly always decided by the number of bonding electron pairs and the number of lone pairs (pairs of electrons not involved in bonding) around the central metal atom, which arrange themselves so as to be as far apart as possible because of electrostatic repulsion between the electron pairs. Table 2.8 shows the essential shape assumed by simple molecules or ions with one central atom X. Carbon is able to form a great many covalently bonded compounds in which there are chains of carbon atoms linked by single covalent bonds. In each case where the carbon atoms are joined to four other atoms the essential orientation around each carbon atom is tetrahedral. 

In each of the examples given so far each element has achieved a noble gas configuration as a result of electron sharing. There are. however, many examples of stable covalent compounds in which noble gas configurations are not achieved, or are exceeded. In the compounds of aluminium, phosphorus and sulphur, shown below, the central atoms have 6. 10 and 12 electrons respectively involved in bondinc... 

The concept of oxidation states is best applied only to germanium, tin and lead, for the chemistry of carbon and silicon is almost wholly defined in terms of covalency with the carbon and silicon atoms sharing all their four outer quantum level electrons. These are often tetrahedrally arranged around the central atom. There are compounds of carbon in which the valency appears to be less than... 

An important reason for low coordination of iodide ions is that high coordination implies a high oxidation state of the central atom, which often (but not always) means high oxidising power— and this means oxidation of the easily oxidised iodide ligands. Thus the nonexistence of, for example, phosphorus(V) pentaiodide is to be explained by the oxidation of the iodide ligands and reduction of phosphorus to the -(-3 state, giving only PI3, not PI5. 

In empirical formulas of inorganic compounds, electropositive elements are listed first [3]. The stoichiometry of the element symbols is indicated at the lower right-hand side by index numbers. If necessary, the charges of ions are placed at the top right-hand side next to the element symbol (e.g., S "). In ions of complexes, the central atom is specified before the ligands are listed in alphabetical order, the complex ion is set in square brackets (e.g., Na2[Sn(OH)+]). 

Besides structure and substructure searches, Gmclin provides a special search strategy for coordiuation compouuds which is found in no other database the ligand search system, This superior search method gives access to coordination compounds from a completely different point of view it is possible to retrieve all coordination compounds with the same ligand environment, independently of the central atom or the empirical formula of the compound. 

To ensure that the arrangement of four atoms in a trigonal planar environment (e.g., a sp -hybridized carbon atom) remains essentially planar, a quadratic term like V(0) = (fe/2) is used to achieve the desired geometry. By calculating the angle 9 between a bond from the central atom and the plane defined by the central... 

However, one of the most successfiil approaches to systematically encoding substructures for NMR spectrum prediction was introduced quite some time ago by Bremser [9]. He used the so-called HOSE (Hierarchical Organization of Spherical Environments) code to describe structures. As mentioned above, the chemical shift value of a carbon atom is basically influenced by the chemical environment of the atom. The HOSE code describes the environment of an atom in several virtual spheres - see Figure 10.2-1. It uses spherical layers (or levels) around the atom to define the chemical environment. The first layer is defined by all the atoms that are one bond away from the central atom, the second layer includes the atoms within the two-bond distance, and so on. This idea can be described as an atom center fragment (ACF) concept, which has been addressed by several other authors in different approaches [19-21]. 

I h e -M. l+ force field assigns default values for out of plane bending terms around an sp2 center. If a central atom has some out of plane parameters, then the first out of plane parameter involving th at cen tral atom is used if a specific parameter is n ot foiin d. 

An sp sp- single bond where each of the central atoms is in Group VIA (for example, hydrogen peroxide) has a two fold barrier with optirn iitn torsional an glc of 90 degrees, as described by V2=-2,0 kcal/tnol. 

Various other ways to incorporate the out-of-plane bending contribution are possible. For e3plane bend involves a cakulation of the angle between a bond from the central atom and the plane defined by I he central atom and the other two atoms (Figure 4.10). A value of 0° corresponds to all four atoms being coplanar. A third approach is to calculate the height of the central atom above a plane defined by the other three atoms (Figure 4.10). With these two definitions the deviation of the out-of-plane coordinate (be it an angle or a distance) can be modelled Lt ing a harmonic potential of the form... 

Summing over the squares of the coefficients of the lower two orbitals (the upper orbital is unoccupied), we get electron densities of 1.502 at the terminal carbon atoms and 0.997 at the central atom. The charge densities on this iteration are... 

Trigonal pyramidal molecules are chiral if the central atom bears three different groups If one is to resolve substances of this type however the pyramidal inversion that mterconverts enantiomers must be slow at room temperature Pyramidal inversion at nitrogen is so fast that attempts to resolve chiral amines fail because of their rapid racemization... 

Valence shell electron pair repulsion (VSEPR) model (Section 110) Method for predicting the shape of a molecule based on the notion that electron pairs surrounding a central atom repel one another Four electron pairs will arrange them selves in a tetrahedral geometry three will assume a trigo nal planar geometry and two electron pairs will adopt a linear arrangement... 

Like MM2, MM-t includes coupling between bond stretching and angle bending. If the angle is defined to include atoms i, j, and k, where k is the central atom, then MM-t couples stretching of the ik and jk bonds with the angle ... 

HyperChem uses the improper dihedral angle formed by central atom - neighbor 1 - neighbor 2 - neighbor 3, where the order of neighbors is how they appear in a HIN file. Not all planar atoms customarily have associated improper torsions. The order of atoms is arbitrary but has been consistently chosen by the original authors of the CHARMM force field. The templates contain equivalent CHARMM definitions of improper torsions for amino acids. Improper dihedral angles cannot be defined that do not have a central atom, as is sometimes done in CHARMM calculations. 

Other Polyatomic Anions. Names for other polyatomic anions consist of the root name of the central atom with the ending -ate and followed by the valence of the central atom expressed by its oxidation number. Atoms and groups attached to the central atom are treated as ligands in a complex. 

Exceptions to the use of the root name of the central atom are antimonate, bismuthate, carbonate, cobaltate, nickelate (or niccolate), nitrate, phosphate, tungstate (or wolframate), and zincate. 

Naming a Coordination Compound. To name a coordination compound, the names of the ligands are attached directly in front of the name of the central atom. The ligands are listed in alphabetical order regardless of the number of each and with the name of a ligand treated as a unit. Thus diammine is listed under a and dimethylamine under d. The oxidation number of the central atom is stated last by either the oxidation number or charge number. 

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