Hydrides anions, Group 13 elements

The main group duster chemistry discussed in this book can be considered to originate from two important, but apparently unrelated developments in inorganic chemistry in the 1930s. The first was the identification of the neutral boron hydrides by Stock [1]. The second was the observation by Zintl and co-workers [2-5] of anionic clusters formed from potentiometric titrations of post-transition metals (i.e., heavy main group elements) with sodium in liquid ammonia. 

Apart from the binary hydrides of Groups 16 and 17, Lowry/ Brpnsted acids in aqueous solution are nearly all oxoacids, i.e. substances containing O-H bonds which ionise in aqueous solution to give oxo-anions and H+(aq) (or H30+). Most oxoacids are molecular hydroxides E(OH) , such as B(OH)3, Ge(OH)4 and Te(OH)6, or oxohydroxides EOm(OH) . In addition, we have more complex species containing E-E bonds or E-O-E bridges. In EOm(OH) - for example, N02(0H), PO(OH)3, S02(0H)2,103(0H) - the m O atoms are held to E by bonds having at least some double bond character, via p -p or d -p overlap. Oxohydroxides may be seen as being derived from hydroxides by elimination of H20, and are favoured by elements E whose atoms form double bonds to O atoms. 

Lewis acids based on boronic acid derivatives or main group elements such as mercury, tin and silicon form strong bonds to anions with considerable covalency, exemplified by hydride sponge and the anticrowns. 

When an electron is removed from the metal-hydride a-bonding valence orbital of CpRe(NO)(CO)H and Cp Re(NO)(CO)H to yield [Cp Re(CO)(NO)H], the v h frequencies shift by 100-200 cm to lower wavenumbers, which indicates the bond lengthening (by 0.25 A) and corresponds to a substantial weakening upon ionization. This indicates also a predominantly localized rhenium-hydride bonding orbital similarly to main group element-hydride bonds. Moving from anionic to neutral or from neutral to cationic hydrides as the result of protonation increases the vmh frequency (see below) in contrast to the veh decrease observed for example upon protonation of amines. 

All monatomic anions are named by changing the ending of the element s name to -ide. For example, I, H, and are called iodide ion, hydride ion, and oxide ion, respectively. (The names of a few special anions also end in -ide among the most important are hydroxide and cyanide ions, listed in Table 6.4.) The charge on any monatomic anion is constant and, except for that on H , is equal to the classical group number minus 8 (see Figure 5.11). 

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