Coordination Numbers and Structures

Another measurement, circular dichroism, CD, is caused by a difference in the absorption of right and left circularly polarized light, defined by the equation 

Even with CD, spectra are not always easily interpreted because there may be overlapping bands of different signs. Interpretation requires determination of the overall symmetry around the metal ion and assignment of absorption spectra to specific transitions between energy levels (discussed in Chapter 11) in order to assign specific CD peaks to the appropriate transitions. Even then, there are cases in which the CD peaks do not match the absorption peaks and interpretation becomes much more difficult. 

Gillard, Optical Rotatory Dispersion and Circular Dichroism, in H. A. O. Hill and P. Day, eds.. Physical Methods in Advanced Inorganic Chemistry, Wiley-lnterscience, New York, 1968, pp. 183-185 C. J. Hawkins, Absolute Configuration of Metal Complexes, Wiley-lnterscience, New York, 1971, p. 156. 

FIGURE 9-20 The Cotton Effect in ORD and CD, (a) Idealized optical rotatory dispersion (ORD) and circular dichroism (CD) curves at an absorption peak, with a positive Cotton effect, (b) Structures of tris-(S-alaninato) cobalt(IIl) complexes, (c) Absorption and circular dichroism spectra of the compounds in (b). (Data and structures in (b) adapted with permission from R. G. Denning and T. S. Piper, Inorg. Chem., 1966, 5, 1056. 1966 American Chemical Society. Curves in (c) adapted with permission from J. Pujita and Y. Shimura, Optical Rotatory Dispersion and Circular Dichroism, in K. Nakamoto and P. J. McCarthy, eds.. Spectroscopy and Structure of Metal Chelate Compounds, John Wiley Sons Inc., New York, 1968, p. 193. 1968 John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.) 

The overall shape of a coordination compound is the product of several interacting factors. One factor may be dominant in one compound, with another factor dominant in another. Some factors involved in determining the structures of coordination complexes include the following  

The isomers described to this point have had octahedral or square-planar geometry. In this section, we describe other geometries. Explanations for some of the shapes are consistent with VSEPR predictions (Chapter 3), with the general assumption that the metal d electrons are stereochemically inactive. In these cases, 3-coordinate complexes have a trigonal-planar shape, 4-coordinate complexes are tetrahedral, and so forth, assuming that 

The structure of a coordination compound is the result of several interacting factors, and structure predictions must always be supported with experimental data because the relative importance of these factors for a given complex can be challenging to deduce. These factors include  

Occupancy of d orbitals. Examples of how the number of d electrons may affect the geometry (e.g., square-planar versus tetrahedral) are discussed in Chapter 10. 

Steric interference, by large ligands crowding each other around the central metal. 

Crystal packing effects. This solid-state effect results from the sizes of ions and the shapes of coordination complexes. The regular shape may be distorted when it is packed into a crystalline lattice, and it may be difficult to determine whether deviations from regular geometry are caused by effects within a given unit or by packing into a crystal. 

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If the identity of the backscatterer is known, then the interest is in determining the number of near neighbors. In this case, one needs to compare the amplitude of the EXAFS of the material of interest (unknown) to that for a compound of known coordination number and structure. However, unlike transferability of phase, which is generally regarded as an excellent approximation, the transferability of amplitude is not. This is because there are many factors that affect the amplitude and, except for the case of model compounds of very similar structures, these will not necessarily (and often will not) be the same. As a result, determination of coordination numbers (near neighbors) is usually no better than 20%. 

The interpretation of conductance data is complicated by the labile nature of the lanthanide complexes in solution which results in ligand exchange and dissociation reactions. It is difficult to understand the nature of the complex species present in solution. A combination of conductance data and molecular weight determination may be useful in determining the coordination number and structure of the complexes in solution. However, due to the poor solubility of lanthanide complexes in suitable solvents, molecular weight data have been obtained for only a few complexes. The dissociative reactions of lanthanide complexes in solution are well illustrated by the TPPO complexes of lanthanide isothiocyanates (202). In chloroform solution, the dissociation... 

Coordination Number and Structure of liquid metals. Fiz. Metal, i Metalloved. 9, 888 (1960) bzw. Phys. Metals Metallogr. (USSR) (English Transl.) 9, 80 (1960). 

Table 10.2 Coordination number and structure types of the actinide trihalides...

While radius ratio rules generally predict correct structure types for the alkali halides in only about 50% of the cases, the MEG calculations (not even shell stabilized) correctly predict the NaCl structure to be more stable for 15 of the 16 compounds studied. Calculated structural parameters were also in good agreement with experiment, and calculated pressures for the NaCl CsCl transition were in fair agreement with the limited experimental data available. These results indicate that preferred coordination numbers and structural types in alkali halides can be accu-... 

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... 

Metal complexes have a variety of stractures. Silver complexes are often linear beryllium complexes are usually tetrahedral iron forms a carbonyl compound that has a trigonal bipyramidal structure cobalt(lll) complexes are octahedral and tantalum forms an eight-coordinated fluoride complex (Figure 3.1). Although a variety of coordination numbers and structures have been observed in metal complexes, the only common coordination numbers are four and six the common structures corresponding to these coordination numbers are tetrahedral and square planar, and octahedral, respectively. In studying metal complexes, it soon becomes clear that the octahedral structure is by far the most common of these configurations. 

The most common and important complex ions are hydrated metal ions. The coordination numbers and structures of some of these simple complexes have been determined. Isotope dilution techniques were used to show that Cr and Al are bonded rather firmly to six water molecules in aqueous solutions. The interpretation of the visible spectra of solutions of transition metal ions using CFT indicates that ions such as Mn, Fe, Co, Ni, Cr, and Fe are octahedral [M(H20)6] species. For non-transition metal ions it has been more difficult to obtain structural information. Flowever, nuclear magnetic resonance spectroscopy demonstrates that Be in aqueous solution is surrounded by four water molecules. These data support the importance of six coordination. The only exception cited here is Be, an element which obeys the octet rule. 

The complexes of a great number of metal cations with NIPA can be prepared in the solid state. Coordination numbers and structural data of many compounds were determined by de Bolster and Groeneveld, Following their procedure we prepared the complexes n g(NIPA)3]2+, [Ca(NIPA)3] [Sr (NIPA) 3] 2+ and... 

In order to understand the photochemical reactions of metal complexes at the molecular level, it is necessary to know both the number and the energy levels of the spectroscopic states of the complex. The first step in developing a state model is to know the coordination number and structure of the complex about the metal center. For complexes of the lanthanide and actinide ions the coordination number is commonly 8 or 9, but for transition metal complexes a coordination number of 6 is that most frequently observed. 

The most common coordination number is six and such complexes have an octahedral structure. The next most common four-coordinated systems have either tetrahedral or square planar structures. Other complexes are known having different coordination numbers and structures. The stereochemistry of metal complexes is a fascinating subject. Several different types of isomeric structures are possible and have been demonstrated in these systems. For our purpose here it is sufficient to cite examples of geometrical (ds-trans) and optical isomerism. This can readily be iUustrated by the cis (III) and trans (IV) isomers of QCo(en)2Cl2]+. Note that the... 

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