Halides bonds

Nonmetal haUdes are generally hydroly2ed to a hydrogen haUde and to an oxy-acid containing the other element. The first row nonmetal haUdes, eg, CCI4, resist hydrolysis because the nonmetal element cannot expand its octet of electrons to form a bond to water before its bond to the haUde is broken. Hydrolysis requires either an energetic water molecule to strike the haUde or ioni2ation of the covalent nonmetal—halide bond, processes that tend to be quite slow (16). 

Unlike the di-f dihalides, such compounds differ little in energy from both the equivalent quantity of metal and trihalide, and from other combinations with a similar distribution of metal-metal and metal-halide bonding. So the reduced halide chemistry of the five elements shows considerable variety, and thermodynamics is ill-equipped to account for it. All four elements form di-iodides with strong metal-metal interaction, Prl2 occurring in five different crystalline forms. Lanthanum yields Lai, and for La, Ce and Pr there are hahdes M2X5 where X=Br or I. The rich variety of the chemistry of these tri-f compounds is greatly increased by the incorporahon of other elements that occupy interstitial positions in the lanthanide metal clusters [3 b, 21, 22]. 

Fig. 2. Ligand substitution as a prodrug strategy for metallochem-otherapeutics (a) general scheme of prodrug activation by ligand substitution hydrolysis of a metal—halide bond is a typical activation pathway of metal-based anticancer drugs, as exemplified by the activation of cisplatin (b) and a ruthenium—arene complex (c).

The dehalogenases (EC 3.8.1), a subclass of the hydrolases that act on the halide bonds in C-halide compounds, catalyze reactions of hydrolytic de-halogenation (Fig. 11.3,a), i.e., the replacement of a halide atom at a sp3 C-atom with a OH group. Exceptions include thyroxine deiodinase (EC 3.8.1.4), which catalyzes reductive deiodination on phenyl rings, and the bacterial 4-chlorobenzoate dehalogenase (EC 3.8.1.6), which forms 4-hydroxy-benzoate. 

Organotellurium(II) compounds can also contain a three-center, four-electron bond as shown for 39 to 42 in Fig. 19. Typically, these molecules contain an odd number ligands around the central atom and an electronegative atom helps to stabilize a tellurenyl halide or selenenyl halide bond through chelation to form a four, five, or six membered ring. " Such molecules are described as lO-Te-3 and lO-Se-3... 

The SnI mechanism involves a rate-determining heterolytic cleavage of the alkyl halide bond to yield an intermediate carbocation which undergoes rapid reaction with available electron donors, including solvent ... 

Cadmium Halides. Cadmium halides show a steadily increasing covalency of the metal—halide bond proceeding from fluoride through to iodide. Bond lengths increase through the series F, 0.197 nm Cl, 0.221 nm Br, 0.237 nm I, 0.255 nm. The fluoride is much less soluble in water than the others (see Table 1) and the Cl, Br, and I compounds dissolve to a significant extent in alcohols, ethers, acetone, and liquid ammonia. Boiling points and... 

The Shannon-Prewitt ionic radii (r+ + r ) are based on the most ionic compounds, the fluorides and oxides for the radii of the metal cations, and the alkali hahdes for the radii of the anions of the remaining halides. The shortening of silver halide bond lengths is attributahle to polarization and covalency. 

Thus increased covalent bonding resulting from Fajans-type phenomena can lower the transition temperatures. For example, the alkali halides (except CsCI, CsBr. and Csl) and the silver halides (except Agl) crystallize in the NaCI structure. The sizes of the cations are comparable Na = M6 pm. Ag = 129 pm, K = 152 pm, yet the melting points of the halides are considerably different (Table 8.6). The greater covalent character of the silver halide bond (resulting from the electron confi J ra-... 

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