Coordination Numbers 1, 2, and

FIGURE 9.25 Complexes with Coordination Number 2. (a) Linear Au in [EtjPAuTilCOjg] . Note that this structure features 2-coordinate gold and 7-coordinate titanium, (b) A )3-diketiminate complex of thallium, (c) The result of a novel strategy for low coordinate iron that features dative Fe—Fe bonding. (Molecular structure drawings created from CIF data with hydrogen atoms omitted for clarity.) 

Tetrahedral and square-planar structures are common. Another structure, with four bonds and one lone pair, appears in main group compounds—such as SF4 and TeCl4 —giving a seesaw geometry (Chapter 3). Many cf and d complexes have tetrahedral 

The trls(2-mercapto-1-f-butyl-lmldazolyl)hydroborato ligand that binds to Hg via the three sulfur atoms. This ligand is related to the pyrazolylborato ligand (Table 9.4). (Molecular structure drawing generated with CIF data. Hydrogen atoms omitted for clarity.) 

FIGURE 9.28 Compiexes with Square-Pianar Geometry. 

A number of complexes that appear to be 4-coordinate are more accurately described as 6-coordinate. Although (NH4)2[CuCLi] is frequently cited as having a square-planar [CuCy ion, the ions in the crystal are packed so that two more chlorides are above and below the plane at considerably larger distances in a distorted octahedral structure. The Jahn-Teller effect (Section 10.5) accounts for this distortion. Similarly, [Cu(NH3)4]S04 H2O has the ammonias in a square-planar arrangement, but each copper is also connected to more distant water molecules above and below the plane. 

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As shown in Table 15-2, coordination numbers 4, 5, and 6 are the common ones for all three elements, although linear 2-coordination is often seen for Hg2+ and has been attributed to relativistic effects on the 6s orbital.4... 

The scheme shown in Figure 38 was put in operation, as shown by the spectro-and electrochemical characterization of the intermediates involved. Nevertheless, complete control of the cycle—with the possibility of isolating each of the three different topographies corresponding to the different coordination numbers 4, 5, and 6—has not been possible. The penta-coordinate species could not be isolated, i.e., the motion could not be stopped at this stage, neither in the oxidation nor in the reduction step. 

These compounds are of quite unusual interest because their empirical formulae suggest coordination numbers 4, 5, and 7 for Pt. Platinum forms a number of compounds Pt(CH3)3X, in which X is Cl, I, OH, or SH, which exist as tetrameric molecules (Fig. 27.13(a)) in which Pt and X atoms occupy alternate vertices of a (distorted) cube. These compounds are notable for the facts that the hydroxide was formerly described as the tetramethyl compound and the iodide as Pt2(CH3)6. In the early study of [Pt(CH3)3Cl]4 Pt-Cl was found to be 2-48 the value, 2-58 A determined in [Pt(C2Hs)3Cl] should be more reliable. The following figures for the hydroxide show that there is some distortion from the highest possible cubic symmetry ... 

Lead(IV) compounds are known with coordination numbers 4, 5, and 6, and in some cases >6. Typically, the nuclear shielding of 207Pb nuclei increases with coordination number. This is shown for coordination numbers 4 and 5 by the example given below for compound 15 and the anion 16.61... 

Nickel(n) forms a large number of complexes encompassing coordination numbers 4, 5 and 6, and all the main structural types, viz., octahedral, trigonal bipyramidal, square-pyramidal, tetrahedral, and square. Moreover, it is characteristic of Ni" complexes that complicated equilibria, which are generally temperature-dependent and sometimes concentration-dependent, often exist between these structural types. In this Section we shall describe the characteristics of the individual structural types separately, using as examples mainly those compounds that exist completely or almost completely in one form or another. In the next Section we shall discuss the configurational equilibria. 

Other traditional scales, such as that of Goldschmidt, are more empirically based and rely upon experimental internuclear distances found in oxides and fluorides (which are expected to be particularly ionic) assumed values for the radii of F and O2- are needed. A very complete tabulation, based on a large amount of modern data for oxides and fluorides, both binary and complex, has been published by Shannon and Prewitt using values of 133 pm for F and 140 pm for O2-. One of the merits of this collection is that it recognises the dependence of ionic radii oh the coordination number. For example, in the case of Cd2+ radii are quoted for coordination numbers 4, 5, 6, 7, 8 and 12 as shown below ... 

As for other first-row elements (B-F), the coordination number of nitrogen cannot exceed 4, even though a valence of 5 is quite common for nitrogen (e.g., NH4 and NO3 ). This is believed to be simply a reflection of nitrogen s small atomic size. By contrast, coordination numbers of 5 and 6 are common for all the other group 15 elements for Bi, even 9-coordination has been documented. 

Dossi et al. [61] loaded platinum in KL zeolite by vapor deposition of Pt(hfa)2 (hfa hexafluoroacetylacetonate). The organometallic precursor was sublimated at 70 °C in a flow of argon and adsorbed on the dehydrated KL zeolite. The decomposition of Pt(hfa)2 was achieved at 350°C in a H2-atmosphere. In situ EXAFS measurements suggested the formation of small clusters (Pt-Pt coordination number of 5), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements using CO as the probe molecule indicated the formation of carbonyl clusters of the general formula [Pt3(CO)g]n (n= 1 -4). 

The stereochemistry of Mg and the heavier alkaline earth metals is more flexible than that of Be and, in addition to occasional compounds which feature low coordination numbers (2, 3 and 4), there are many examples of 6, 8 and 12 coordination, some with 7, 9 or 10 coordination, and even some with coordination numbers as high as 22 or 24, as in SrCdn, BaCdn and (Ca, Sr or Ba)Zni3. " Strontium is 5-coordinate on the hemisolvate [Sr(OC6H2Bu3)2(thf)3]. jthf which features a distorted trigonal bipyramidal structure with the two aryloxides in equatorial positions. ... 

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