Variable coordination numbers

Other cations, such as Al3+ and Ni2+, have variable coordination numbers, depending on die nature of the ligand. 

Microbial resistance to established organic antibiotics is a potentially serious problem and provides an impetus for the development of novel antimicrobial metal compounds. The potency of Ag(I) ions is well known—but how does Ag(I) kill a bacterium Much current attention is focused on Bi(III) on account of its ability to kill Helicobacter pylori, an organism which prevents ulcers from healing. Bis-muth(III) chemistry has many unusual features a variable coordination number, strong bonds to alkoxide ligands, the stereochemical role of its 6s2 lone pair, facile formation of polymers, and dual hard and soft character. 

Bismuth compounds have been used for treating gastrointestinal disorders for more than two centuries (451). These include bicarbonate, nitrate and salicylate salts, and colloidal bismuth subcitrate. These are all Bi(III) compounds Bi(V) is usually a strong oxidant. Their structures are largely unknown and often contain a mixture of anionic ligands. This reflects the strong tendency of Bi(III) to hydrolyze and form stable hydroxo and oxo complexes. The first pKa of Bi(III) in water is ca. 1.5. Bismuth(III) has a variable coordination number, from 3 to 10. 

Coordination MOs are delocalized in space — three-dimensional CuCl C0CI2 (crystal) There are no distinct characteristics variable coordination number and magnetic moment, strong mutual influence of ligands... 

Electron densiity, in lead triacetate arylations, 9, 397 Electron donor number, transition metal complex ligands and unchanging coordination number, 1, 12 and variable coordination number, 1, 12 Electron energy loss spectroscopy, dihydrogen binding on surfaces, 1, 682 Electronic effects... 

More recent studies have demonstrated that silver is a versatile metal because of its variable coordination number and geometry. As a consequence of these inherent properties, a wide variety of structures, ranging from individual cage-like molecular complexes to infinite ID, 2D, and 3D architectures with specific topologies, have been... 

Table 2 presents effective ionic radii for many metal ions. For any metal ion, the radius increases with coordination number since the greater number of bonds weakens the strength of any one bond. The radius of the most common coordination number is underlined in Table 2. The alkali and alkaline earth metal ions exhibit variable coordination numbers without strong directionality in bonding. Because they are of similar size, Ca + and Na+ of differing charges...

Variable coordination numbers. The coordinating stabilization energy (about 4.18 kJ-mol ) of lanthanide ions is much smaller than the crystal field stabilization energy of transition metals (typically >418 kJ moP ). Therefore, the coordinating bonds of lanthanide complexes are not directional and the coordination number varies from 3 to 12. 

The synthetic flexibility of P-diketones has led to an explosion in the developments of related lanthanide complexes and their coordination chemistry. The [)-diketonate lanthanide complexes have been demonstrated to have variable coordination numbers ranging from six to ten, and diverse structural motifs. This family of complexes, having desirable luminescent properties... 

Clearly, the YETI force field is a very complicated approach to solving equally complicated problems. As such, it illustrates well the problems that must be addressed in inorganic modeling that simply do not occur in organic modeling. The primary problem addressed in Vendani s work is the variable coordination number of Zn and Co in carbonic anhydrase. The YETI program obviates some of these problems, but it is not apparent that the solution can be transferred to other metal systems. Nonetheless, useful results are obtained for the metalloenzymes and model systems for which the method was developed. 

Bromothallate(III) complexes also show variable coordination numbers and structural diversity for the thallium(III) ion. >591,602-605 x-ray data, supported by Raman analysis, showed that the [TlBrs] ion of 1,1,4,4-tetramethylpiperazinium and A,A -dimethyltriethylenediammonium salts adopts a trigonal-bipyramidal geometry. " Compounds derived from 4,4 -dimethyl-2,2 -bipyridi-nium cation contain unusual bromothallate units, with four short T1—Br bonds and one long T1—Br interaction. The A-methyl-l,3-propanediammonium salt of [TlBrs] is known. The X-ray... 

Low-spin d -complexes commonly exhibit a strong preference for the octahedral (i.e., six-coordinated) configuration, which disfavors ligand dissociation or addition. On the other hand, low-spin d - and d -complexes commonly exhibit variable coordination numbers as illustrated by... 

Scheme 22 Synthons of variable coordination numbers of silicon and nitrogen...

Metallocompounds with variable coordination numbers and plastic coordination stereochemistries may not behave in this conventional way the energy-distance curves for any one bond could vary with the number and lengths of the other bonds. With this variability of the primary bonding, the residual, more weakly bonded interactions could be expected to be even more variable. Thus an open mind is required in developing parameters for weakly bonded interactions for metallocompounds. Further development of fundamental theory for long, weak interactions to metal atoms is required. 

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