Coordination number solid

Complex Coordination number Solid state Solution Ref. 

Another characteristic of modem coordination chemistry is the increasing reliance upon physicochemical methods unknown to Werner and his contemporaries. Simultaneously with an introduction of these newer techniques, emphasis shifted from preoccupation with qualitative studies of stmcture and stereochemistry to quantitative studies of thermodynamics, kinetics, and reaction mechanisms. Some areas of current research interest include unusual ligands, oxidation states and coordination numbers, solid-state chemistry, photochemistry, relationship between stmcture and reactivity, variable oxidation state, chelates, heteropoly complexes, organometalhc... 

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

It can be readily confirmed thaf by decreases as the number of bonds N increases and/or llieir length (r ) decreases. This relationship between the bond strength and the number of neighbours provides a useful way to rationalise the structure of solids. Thus the high coordination of metals suggests that it is more effective for them to form more bonds, even though each individual bond is weakened as a consequence. Materials such as silicon achieve the balance for an infermediate number of neighbours and molecular solids have the smallest atomic coordination numbers. 

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identify and quantitate crystalline phases in bulk materials and at surfaces and interfaces. While NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under smdy, NMR can also provide structural insights on a molecularlevel. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly usefrd in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often fail. 

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

Monomer-oligomer equilibria. [Ni(Me-sal)2], mentioned above as a typical planar complex, is a much studied compound. In pyridine it is converted to the octahedral bispyridine adduct (/zsoo = 3.1 BM), while in chloroform or benzene the value of is intermediate but increases with concentration. This is ascribed to an equilibrium between the diamagnetic monomer and a paramagnetic dimer, which must involve a coordination number of the nickel of at least 5 a similar explanation is acceptable also for the paramagnetism of the solid when heated above 180°C. The trimerization of Ni(acac)2 to attain octahedral coordination has already been referred to but it may also be noted that it is reported to be monomeric and planar in dilute chloroform solutions. 

The third step consisted of the direct investigation of IR emission spectra for a wide range of concentrations. The investigation showed the tendency of the metals to reduce their coordination number when moving from solid to molten state. This property of the melt depends on the equilibrium between two types of complex ions, MeF72 and MeF6 ... 

If more than simple atoms are bound to a central atom, then the coordination number still refers to the number of near neighbors. For example, in solid potassium chrome alum, KCr(S04)2- 12H20, and also in its fresh aqueous solutions, the chromium-containing cation is... 

Most of these compounds are solids (though a few such as Pt(PEt3)3, Pt(PBu3)3 and Pt(PF3)4 are oils at room temperature). Their stoichiometry in solution has been studied, most particularly by31P NMR at low temperatures, to determine which species are present [47]. Table 3.8 shows that the coordination number is principally determined by the bulk of the ligand... 

Acetic add, ethylenediaminetetra-, 4,253 add-base equilibria, 2,779 in analysis, 1,522 complexes composition, 2,783 coordination numbers, 2,783 solid state structure, 2,783 cyclic derivatives complexes, 2,785 in electroplating, 6,14 heteroatom derivatives metal complexes, 2, 786 homologs... 

In an ionic solid, the coordination number means the number of ions of opposite charge immediately surrounding a specific ion. In the rock-salt structure, the coordination numbers of the cations and the anions are both 6, and the structure overall is described as having (6,6)-coordination. In this notation, the first number is the cation coordination number and the second is that of the anion. The rock-salt structure is found for a number of other minerals having ions of the same charge number, including KBr, Rbl, MgO, CaO, and AgCl. It is common whenever the cations and anions have very different radii, in which case the smaller cations can fit into the octahedral holes in a face-centered cubic array of anions. The radius ratio, p (rho), which is defined as... 

Calculate the number of cations, anions, and formula units per unit cell in each of the following solids (a) the rock-salt unit cell shown in Fig. 5.39 (h) the fluorite (CaF2) unit cell shown here, (c) What are the coordination numbers of the ions in fluorite ... 

The dominant features which control the stoichiometry of transition-metal complexes relate to the relative sizes of the metal ions and the ligands, rather than the niceties of electronic configuration. You will recall that the structures of simple ionic solids may be predicted with reasonable accuracy on the basis of radius-ratio rules in which the relative ionic sizes of the cations and anions in the lattice determine the structure adopted. Similar effects are important in determining coordination numbers in transition-metal compounds. In short, it is possible to pack more small ligands than large ligands about a metal ion of a given size. 

Mossbauer spectroscopy has also been widely used to investigate the structures of dialkylstannylene derivatives of carbohydrates in the solid state. The usual magnitude of A = 2.78-3.07 mms indicated a coordination number larger than four, with Sn centers in a penta- or hexacoordinated environment. 

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