Polyatomic ligands

More precise structural information can be obtained when a separation of interactions can be made. The RDFs for 2.9 M solutions of erbium(III) and yttrium(III) nitrate, which are isostructural (36), are 

Structures of Complexes with Polyatomic Ligands in Aqueous Solution  

By using the intensity difference curve the light atom interactions can be eliminated and the RDF, including only interactions involving the metal ions, can be calculated. The results, referred to the erbium(III) solutions, are shown in Fig. 23 for three nitrate solutions of different compositions, including the 2.9 M solution illustrated in Fig. 22. The intramolecular N03 interactions together with all other nonmetal interactions are now eliminated and the first coordination peak appears as a separate peak. Its shape, however, depends on the composition of the solution, contrary to what was found for perchlorate solutions of similar compositions, and in comparison with these it is significantly broadened. For the 1 M solutions the difference is small but it becomes much more pronounced, when the concentration of nitrate ions is in- 

A model calculation on the basis of this structure is illustrated in Fig. 24 for one of the solutions. It leads to a symmetric or only slightly asymmetric bidentate bonding of about two nitrate ions to erbium (Fig. 24) with the Er—0(N03) bond lengths, 2.45 A, somewhat longer than the Er—0(H20) bonds, 2.32 A, in the first coordination sphere. The Er3 + ion and the nitrate ligand are coplanar as shown by the Er—03 distance of 4.1 A. This bonding of nitrate to the metal ion is very similar to that found in crystals, with the same orientation of the nitrate ion and the same difference between Er—0(H20) and Er—0(N03) bond lengths (36). 

Since the metal-sulfur distance in an inner-sphere sulfate complex appears in a region where a large number of other distances in the solution will also occur, a unique identification and structure determination of sulfate complexes is difficult to make unless difference methods can be used. Definite conclusions cannot generally be made on the basis of an analysis of a single diffraction curve only. Data for several solutions of different compositions are needed and a careful analysis 

An important consequence of the nonaxially symmetric electron distribution of the oxygen atom in an angular OX ligand is that B(OX)4 molecules do not have a regular tetrahe- 

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Among the polyatomic ligands the dithiocarbamato ligand can stabilise high oxidation states of the transition metals in its complexes. like all 1,1-dithioates the a-donation and TT-back-donation of the sulfur atoms is assumed to be of the same order of magnitude. 

Complex formation with several other polyatomic ligands have been investigated and results reported in the literature are summarized in Table VI. Some of the ligands, because of their internal structure, are more favorable for structure determinations than others. 

The internal structure of a polyatomic ligand is obtained as an additional result in these investigations. Its high concentration and the sharp intramolecular distances often result in dominant contributions to the diffraction curves and make possible a precise determination of its bonding distances. When careful analyses have been made, no significant differences from values found in crystals have been found, however. Therefore, the derived structure for the ligand can serve as an internal check on the quality of the data. Large deviations from values found in crystal structures may be an indication of errors in the data or in the method of analysis. 

Per Section 1-10.6.2.2. of [48] "In the nomenclature of polydentate chelate complexes, single ligating atom attachments of a polyatomic ligand to a coordinating centre are indicated by the italic element symbol preceded by the Greek letter kappa, k."... 

Transitions of this type are associated with polyatomic ligands which have electronic spectra in their own right. As an example, the tris(3,5-dimethylpyrazolyl)borate (tp 2-) proligand (Figure 4.2) exhibits a tc 7C transition in the ultraviolet region at about 230 nm, which is also observed in its transition metal complexes. Usually, such absorptions do not reveal much about the metal centre and so they are of less interest to transition metal chemists than the remaining types of absorption which directly involve the metal centre. 

The symbol kappa, k, was introduced, in Section IR-9.2.4.2, in order to specify the ligating atoms in polyatomic ligands. This use also applies to such ligands when they appear in polynuclear complexes. However, the symbol k then assumes a new function, namely that... 

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