Structures of the anions

Principal resonance structures of the anion of a 3 keto ester... 

Figure 13.25 (a) Structure of the anion As7 , isoelectronic with As4Se3 (p. 581). The sequence of As-As distances (base>cap>side) is typical for such cluster anions but this alters to the sequence base >side>cap for neutral species such as As7(SiMe3)3 shown in (b). 

Figure 13.26 (a) Structure of the anion Asu " note that the As-As distances involving the three 2-coordinate As atoms are significantly shorter than those between pairs of 3-coordinate As atoms, (b) Structure of the anion Asii i-e. [Asu-Asn] (see text). 

This last reaction is typical of many in which F3CIO can act as a Lewis base by fluoride ion donation to acceptors such as MF5 (M = P, As, Sb, Bi, V, Nb, Ta, Pt, U), M0F4O, Sip4, BF3, etc. These products are all white, stable, crystalline solids (except the canary yellow PtFe ) and contain the [F2CIO] cation (see Fig. 17.26h) which is isostructural with the isoelectronic F2SO. Chlorine trifluoride oxide can also act as a Lewis acid (fluoride ion acceptor) and is therefore to be considered as amphoteric (p. 225). For example KF, RbF and CsF yield M [F4C10] as white solids whose stabilities increase with increasing size of M+. Vibration spectroscopy establishes the C4 structure of the anion (Fig. 17.29g). 

MeCN. The structure of the anion (Fig 24.4b) can be envisaged as a tridentate [ReHg] ligand coordinated to ReCtriphos)" ", and, since the metal atoms are only 259.4 pm apart, is said to involve an Re=Re triple bond (in which case the [ReHg] should be regarded as tetradentate and its Re atom as 10-coordinated). 

In recent years various attempts have been made to account for the observed differences between the dissociation constants of organic acids, whose molecules differ only slightly from each other. The proposed explanations have naturally been given in each case in terms of the structures of the respective neutral acid molecules.1 In the tentative discussion of HN03 and HI03 that has just been given, the approach has been quite different we focused attention, not on the neutral molecule or on the structure of the anion, but on the condition of the solvent in the vicinity of the anion. 

Phenol, C6H5OH, is a stronger acid than methanol, CH3OH, even though both contain an O-H bond. Draw the structures of the anions resulting from loss of H+ from phenol and methanol, and use resonance structures to explain the difference in acidity. 

Of some relevance in this connection is a study216 on the structure of the anion radicals formed when diaryl sulphones react with n-butyllithium in hexane-HMPA solution under an argon atmosphere. Apparently, a dehydrogenative cyclization and a further one-electron reduction occurs to produce the anion radicals of substituted dibenzothiophene-S, S-dioxides. These anion radicals were studied by ESR spectroscopy. 

The calculated bond lengths for the 2 structure of the anion radical of heptafulvalene shown in Fig. 7 indicate that in one of the ring there exists a significant bond fixation to the same extent as that in the neutral heptafulvalene, while in the other ring bond lengths are nearly equalized. The calculated spin densities, presented in Table 3, indicate that the unpaired spin is localized essentially on the latter ring. 

Which of these options is the best Lewis structure Actually, no single Lewis structure by itself is an accurate representation of NO3. Any single structure of the anion shows nitrate with one NDO double bond and two N— O single bonds. In Section 9 1, we show that single and double bonds between the same types of atoms have different lengths and different energies. In contrast, experiments show that the three nitrate N—O bonds are identical. To show that the nitrate N—O bonds are all alike, we use a composite of the three equivalent Lewis structures. These are traditionally called resonance structures. Resonance stmctures are connected by double-headed arrows to emphasize that a complete depiction requires all of them. 

Figure 1.27 Structure of the anionic, bidentate nitrogen ligands [hpp] and [tbo] ...

With diammonium ions [H3N(CH2) NH3]2+ (2 < n < 6) as counterions, several linear and layered polymeric Cd sulfates are generated. In the chain anion oo1[CdCl4/2(S04)2/2]2 ( = 2), octahedral units are linked by both /r-chloro and /r-sulfato bridges a similar structural situation is met in the compounds with oo1[CdX4/2(S04)2/2]2 anions (X = C1, Br n = 6). A layered structure of the anion oo2[Cd2(H20)2(S04)3T (n = 3) is achieved by linking two different octahedral CdOfi units by S04 bridges exclusively. 0... 

Fig. 3 Protonation states, isomerism and mesomerism of the HBI chromophore (p-hydroxybenzi-lidene-imidazolinone). The chromophore is shown in its most stable Z ( cw ) conformation, conventionally associated to a 0° value of the dihedral angle t, while the E ( trans ) conformation corresponds to t = 180°. For model compound HBDI (4 -hydroxy-benzylidene-2,3-dimethyl-imidazolinone), Ri = R2 = CH3, for chromophore in GFP, Ri, and R2 stand for the peptidic main chains toward N-terminus and C-terminus, respectively, (a) Possible protonation states of HBI (a) neutral, (b) anionic, (c) enolic, (d) cationic, and (e) zwitterionic. (b) Two resonance structures of the anionic form of HBI...

Figure 7.9.1 The molecular structure of the anionic surfactant sodium lauryl sulfate.

Figure 41 The structure of the anionic fragment [ (2-pyr)(Ph) 6(H)Li8][ (tBu)2Me2AI 2Li] 472. Hydrogen atoms have been omitted for clarity. Only the core structure of [ (2-pyr)(Ph) 6(H)Li8] has been shown.

For bromide and iodide, the nature of the countercation influences the structure of the anionic complex. In fact, when the [(18-C-6)-K]Br and [(18-G-6)-K]I salts are used, the anionic complexes ([ 163—X]—, X = Br, I) remain mononuclear and adopt a T-shaped structure (Figure 11). In both cases, the Hg-X bonds are shorter than those observed in the corresponding dinuclear complexes in agreement with the terminal location of the anion. The reaction of bis(pentafluoro)phenylmercury 164 with [(18-C-6)-K]Br and [(18-C-6)-K]I also afford T-shaped complexes [164-Br] and [164-1]. The Hg-Br (2.93 A) and Hg-I (3.12 A) bonds found in these complexes are longer than those observed in [163-Br] and [ 163—1] indicating that 164 is a weaker Lewis acid than 163.206... 

Figure 3 Crystallographically determined structure of the anion [S2B18H19] (19)...

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

The structure of the anion [Be3(OH)3(malonate)3]3 (95) is illustrated in Fig. 18. The Be3(OH)3 ring has a flattened chair conformation with no crystallographic symmetry. Nevertheless, the arrangement of oxygen atoms around each beryllium is near tetrahedral. 

Fig. 17. Molecular structure of the anion [Be(maleate)]2. Reproduced with permission from Ref. (169). Copyright 1998, Verlag der Zeitschrift fur Naturforschung.

The interaction of beryllium with nitrilotripropionic acid (H3ntp) has been investigated in some detail (244). This acid forms a strong complex (log Kt = 9.24) that can be isolated as a solid. The crystal structure of the anion [Be(ntp)] is shown in Fig. 23. The structure confirms the coordination of the nitrogen atom along with an oxygen atom from each carboxyl group. 

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

The structure of the related osmium hydridocarbonyls has not been investigated in such extensive detail. However, the X-ray structures of the anions [H3Os4(CO)12] and [H2Os4(CO)12]2 are being determined (191). 

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