High coordination number

For trigonal bipyramidal EX2Y3, three geometrical isomers (1.32 to 1.34) are possible depending on the relative 

Atkins (2000) The Elements of Physical Chemistry, 3rd edn, Oxford University Press, Oxford - An excellent introductory text which covers important areas of physical chemistry. 

Atkins and L. Jones (2000) Chemistry. Molecules, Matter, and Change, 4th edn. Freeman, New York - This first-year text is suitable for reviewing basic topics. 

Zumdahl (1998) Chemical Principles, 3rd edn, Houghton Mifflin Company, Boston - A useful first-year text for an overview of basic concepts. 

Ebsworth, D.W.H. Rankin and S. Cradock (1991) Structural Methods in Inorganic Chemistry, 2nd edn, Blackwell Scientific Publications, Oxford - A readable text which gives details of the important methods by which chemists determine structural details of compounds. 

Barrett (2002) Structure and Bonding, RSC Publishing, Cambridge - An introductory text that includes valence bond, molecular orbital and VSEPR theories. 

Gillespie and I. Hargittai (1991) The VSEPR Model of Molecular Geometry, All and Bacon, Boston - A text with many examples that takes the VSEPR model from basic principles to a quantum mechanical basis. 

Gillespie and E.A. Robinson (2005) Chemical Society Reviews, vol. 34, p. 396 - A tutorial review Models of molecular geometry that considers the VSEPR model and the more recently developed ligand close-packing (LCP) model. 

Ha5Tward (2002) Quantum Mechanics for Chemists, RSC Publishing, Cambridge - An undergraduate student text that covers the basic principles of quantum mechanics. 

The presence of axial and equatorial sites in a pentagonal bipyramidal molecule leads to stereoisomerism in a similar manner to that in a trigonal bipyramidal species. In a square antiprismatic molecule EXg, each X atom is identical (Fig. 2.16). Once two or more different atoms or groups are present, e.g. EXgY2, stereoisomers are possible. As an exercise, draw out the four possibilities for square antiprismatic EXgY2. 

In contrast to a single a) bond where free rotation is generally assumed, rotation about a double bond is not a low energy process. The presence of a double bond may therefore lead to stereoisomerism as is observed for N2F2. Each N atom carries a lone pair as well as forming one N—F single bond and an N=N double bond. Structures 2.43 and 2.44 show the trans- and CM-isomers respectively of N2F2. 

Draw the structures of the two isomers of octahedral [Cr(OH2)4Cl2] and give labels that distinguish the isomers. 

[PtCl2(PEt3)2] possesses two stereoisomers. Is the complex square planar or tetrahedral Rationalize your answer. 

Draw the structures of mer- andyac-[RhCl3(OH2)3]. What is the coordination geometry at the metal centre  

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The fluoride ion is the least polarizable anion. It is small, having a diameter of 0.136 nm, 0.045 nm smaller than the chloride ion. The isoelectronic E and ions are the only anions of comparable size to many cations. These anions are about the same size as K" and Ba " and smaller than Rb" and Cs". The small size of E allows for high coordination numbers and leads to different crystal forms and solubiUties, and higher bond energies than are evidenced by the other haUdes. Bonds between fluorine and other elements are strong whereas the fluorine—fluorine bond is much weaker, 158.8 kj/mol (37.95 kcal/mol), than the chlorine—chlorine bond which is 242.58 kJ/mol (57.98 kcal/mol). This bond weakness relative to the second-row elements is also seen ia 0-0 and N—N single bonds and results from electronic repulsion. 

Zirconium forms anhydrous compounds in which its valence may be 1, 2, 3, or 4, but the chemistry of zirconium is characterized by the difficulty of reduction to oxidation states less than four. In aqueous systems, zirconium is always quadrivalent. It has high coordination numbers, and exhibits hydrolysis which is slow to come to equiUbrium, and as a consequence zirconium compounds in aqueous systems are polymerized. 

Bismuthides. Many intermetaUic compounds of bismuth with alkafl metals and alkaline earth metals have the expected formulas M Bi and M Bi, respectively. These compounds ate not saltlike but have high coordination numbers, interatomic distances similar to those found in metals, and metallic electrical conductivities. They dissolve to some extent in molten salts (eg, NaCl—Nal) to form solutions that have been interpreted from cryoscopic data as containing some Bi . Both the alkafl and alkaline earth metals form another series of alloylike bismuth compounds that become superconducting at low temperatures (Table 1). The MBi compounds are particularly noteworthy as having extremely short bond distances between the alkafl metal atoms. 

To the structural generalizations on carborancs (p. 183) can be added the rule that, in metallocarboranes, the M atom tends to adopt a vertex with high coordination number. M occupancy of a low CN vertex is not precluded, particularly in kineiically controlled syntheses, but isomerization to more stable configurations usually results in the migration of M to high CN vertices. 

As with other high coordination numbers, there seems to be little difference in energy between these structures. Factors such as the number of counter ions and the stereochemical requirements of chelating ligands are probably decisive and a priori arguments are unreliable in predicting... 

Table 26.2 also reveals a diminished tendency on the part of these elements to form compounds of high coordination number when compared with the iron group and, apart from [Co(N03)4], a coordination number of 6 is rarely exceeded. There is also a marked reluctance to form oxoanions (p. 1118). This is presumably because their formation requires the donation of n electrons from the oxygen atoms to the metal and the metals become progressively... 

Table 31.4 is a list of typical compounds of the actinides and demonstrates the wider range of oxidation states compared to lanthanide compounds. High coordination numbers are still evident, and distortions from the idealized stereochemistries which are quoted are again general. However, no doubt at least partly because the early actinides have received most attention, the widest range of stereochemistries is... 

Compounds with high coordination numbers sometimes eliminate a molecule of phosphine on heating ... 

Molybdenum (High Coordination Numbers) (see also Tables 5.12-5.14)... 

In Fig. 1 are assembled the most important coordination polyhedra of tin(II) compounds omitting high coordination numbers which are difficult to illustrate. In Table 7 the corresponding bond angles characterizing the deviation from ideal... 

Compounds with High Coordination Numbers at Silicon Models for the Investigation of the Nucleophilic Substitution Reaction at Silicon Centers... 

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