Examples of the Coordination Numbers

Coordination number 2 is represented by the sublimeable two coordinate bent alkyl [Yb C(SiMe3)3 2] (C-Yb-C 137°) and the europium analogue. Although there are only two Yb-C a bonds, the bending is caused by a number of agostic Yb... H-C interactions 

As yet this is a rare coordination number. The three-coordinate silylamides can add two slender ligands to form five-coordinate examples Ln[N(SiMe3)2l3(NCMe)2, whilst there are a few alkyls e.g. Yb(CH2Bu03(thf)2.[Nd P(SiMe3)2 3(thf)2] has also been described. These have trigonal bipyramidal structures, as expected. 

The most common geometries encountered are capped octahedral and capped trigonal prismatic. Many of the known seven-coordinate compounds involve /1-diketonate ligands, particularly adducts of the type Ln(diketonate)3.L (L = Lewis base e.g. H2O, py). Several 

Sheer congestion of donor atoms around the metal ion and concomitant inter-donor atom repulsions makes these high coordination numbers difficult to attain. They are often associated with multidentate ligands with a small bite angle such as nitrate that take up little space in the coordination sphere, either alone, as in (Ph4As)2[Eu(N03)5] or in combination with other ligands, as in Ln(bipy)2(N03)3, Ln(terpy)(N03)3(H20) (Ln = Ce-Ho), and crown ether complexes (Section 4.3.7) such as Ln(12-crown )(N03)3(Ln = Nd-Lu). Other crown ether complexes can have 11 and 12 coordination, e.g. Eu(15-crown-5)(N03)3 (Ln = Nd-Lu) and Ln(18-crown-6)(N03)3(Ln = La, Nd). 

Polyhedra in these high coordination numbers are often necessarily irregular, but when all the ligands are identical, near-icosahedral geometries occur for the 12-coordinate [Pr(l,8-naphthyridine)6] + and [La(N03)6] ions in crystalline salts. It should also be remembered that the geometries discussed here are found in the solid state, but on dissolution in a solvent, where the influence of counter-ions is lessened, matters may be different (see the aqua ions. Sections 4.3.1 and 4.3.2). In principle, isomers are often possible, but because of the lability of lanthanide complexes they are very rarely observed. 

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Coordination numbers in this triad are again rarely higher than 6, but the univalent metals provide examples of the coordination number 2 which tends to be uncommon in transition metals proper (i.e. excluding Zn, Cd and Hg). 

Examples of this coordination number are virtually confined to linear Dock complexes of Cu Ag Au and Hg of which a well-known instance is the ammine formed when ammonia is added to an aqueous solution of Ag+ [H3N-Ag-NH3]+... 

Instead, we believe the electronic structure changes are a collective effect of several distinct processes. For example, at surfaces the loss of the bulk symmetry will induce electronic states with different DOS compared to bulk. As the particle sizes are decreased, the contribution of these surface related states becomes more prominent. On the other hand, the decrease of the coordination number is expected to diminish the d-d and s-d hybridization and the crystal field splitting, therefore leading to narrowing of the valence d-band. At the same time, bond length contraction (i.e. a kind of reconstruction ), which was observed in small particles [89-92], should increase the overlap of the d-orbitals of the neighboring atoms, partially restoring the width of the d-band. 

This, together with the Ir-H terminal bond indicates that the compound can be regarded as a further example of the growing number of compounds in which the high oxidation state Ir(v) is stabilized by coordination to a soft polyhedral borane ligand... 

For a better quantification of the coordination number, several alternative schemes have been proposed. For example, a simple procedure is to identify a gap in the list of interatomic distances and to add atoms up to this gap. A similar procedure (O Keeffe 1979) may be to add atoms to the coordination polyhedron in order of increasing interatomic distances and to stop when the next addition would... 

The presence of more than one specie in metastable equilibrium at the interface. Still the accuracy of SEXAFS can be good if the two phases have first distances sufficiently different (> 1 A). This is not the case if two silicide phases, for example, are formed. In this case the presence of more than one phase can only be inferred from the inconsistency of the coordination numbers with any one only of the silicide phases, but the relative amounts or any better details are lost. It can be quite easy... 

An interesting feature of this simple rectangular band model with ah = is that the cohesive energy in eqn (7.44) is independent of the coordination number,, so that the diamond, x — A, simple cubic, ( = 6), and close-packed, (x = 12), lattices, for example, would all be equally stable. The origin of this unexpected result may be traced back to the form of the binding energy, namely... 

With an ionic radius of 0.81 A (Pauling), the ion Sc3+ lies on the borderline between six-coordination and higher coordination numbers. In those crystal structures which are known, Sc3+ is predominantly six-coordinated, but examples of seven-, eight- and nine-coordination do occur, together with a limited number of examples of lower coordination numbers than six where ligands are very bulky. 

Tables 3.1, 3.2 and 3.3 compiled by Povarennykh (1963) specify the initial data accepted for the calculation of hardness from formulae (3.5) and (3.6). As the ratio WJWa increases, the coefficient a decreases (Table 3.1). For compounds with ratios inverse to those given in the table, i.e., for compounds having a so-called antistructure, the coefficient a will be exactly the same, e.g., 1/2 and 2/1. In both cases, x — 80. The link attenuation coefficient / varies over a relatively narrow range, usually between 0.7 and 1.0 (Table 3.2). This coefficient requires the state of lattice linkage to be considered in each case, and like coefficient a it depends on the type of compound involved. For various types of compounds, the values of the coefficient / may be lower taking as an example minerals in the pyrite and skutterudite group, they are as follows for compounds 2/2—0.60, for 3/3—0.48 and for 4/4—0.39. The values of the coefficient y grow proportionally with coordination number (Table 3.3). The constancy of the coefficient y depends on the constancy of the coordination number which is influenced by the valence ratio of electropositive and electronegative atoms. Lattice spacings, state of chemical bonds and electron-shell structure, and for complex compounds, also the degree of action of the remain-...

Table II shows the reactions in coordination chemistry coded in TAMREAC. For each we give a name, a general description, and an example. We also give the most common characteristics of each reaction the variation of the number of valence electrons (NVE), of the coordination number (CN), of the oxidation state (OS), and of the global charge of the complex (Q).

Applications of NMR spectroscopy to structural, thermodynamic, and dynamic processes have been described. A brief discussion of the types of problems appropriate for study by this technique has been included. H and 13C NMR spectroscopy has been applied to define the ligand coordination in complexes. These experiments, combined with 170-labeling experiments, allowed deduction of the coordination number of the vanadium atom. Integration of NMR spectra allowed measurement of the formation constants and equilibrium constants. 2D 13C and 51V EXSY experiments were used in a qualitative and quantitative manner to examine intra- and intermolecular dynamic processes, of which several examples are discussed. The interpretation of the rate matrix and its relationship to the chemical processes under examination were also described. 2D EXSY spectroscopy has great potential as a tool with which to probe mechanisms in complex reactions however, such uses often requires estimation of errors. The major source of error in 2D 51V EXSY NMR studies on a two- and four-site vanadate system were found to be baseline distortion and the errors were estimated. Our results suggest... 

For the same element with the same coordination number, the effective radius will decrease if the valence state increases. For example, when the coordination number is six the effective radius of Ce + is 101 pm while the effective radius of Ce + is 87 pm. When the coordination number is eight, the effective radius of Sm + is 127 pm while that of Sm + is 102 pm. The reason for this is that one more electron is present in the outer shell for the lower valence ion compared with the higher valence one. 

The directional preferences for coordination to the alkali metal and alkaline earth cations is obviously related to the number of substituents coordinated to the cation. As yet there is little predictability of the coordination number among these cations. For example, the first member of this series, the Li" cation, is the best characterized with well over 500 X-ray crystal structures containing this ion. Coordination numbers to Li ranging from two through seven and all values in between can be found. The Li cation is also found symmetrically w-complexed to the faces of aryl anions and to conjugated linear anions (see... 

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