Ionic azides

The reaction of Te(N3)4 with ionic azides generates the [Te(N3)6] anion (Eq. 5.8). The stmeture of this anion is strongly distorted from octahedral by the stereochemically active lone pair on the tellurium atom, which gives rise to substantial differences in the Te-N bond lengths. Eour of these bonds are in the range 2.09-2.24 A, cf. [Te(N3)5] ,... 

Similar differences are found for organic azides (e.g. MeN3). In ionic azides (p. 417) the N3 ion is both linear and symmetrical (both N-N distances being 116 pm) as befits a 16-electron species isoelectronic with CO2 (cf. also the cyanamide ion NCN, the cyanate ion NCO, the fulminate ion CNO and the nitronium ion N02 ). 

Another application of the adjacent-charge rule, to the fluorine nitrate molecule, will be discussed in a following section, with mention also of the stability of covalent and ionic azides and nitrates. 

Substituted tetrazoles are formed in good yields by heating hydrazoic acid or ionic azides with nitriles.319-323... 

Ionic azides with alkali metal (e.g. NaN3)210 or large organic (e.g. NR/ ) counterions are chemically stable. Controlled decomposition occurs at higher temperatures, while they are also mechanically stable.206... 

It has been calculated that the resonance energy of ionic N3 is almost twice as high as that in the covalently bonded form. This explains wliy the latter complexes often are heat, light or shock sensitive whereas the ionic azides are reasonable stable.208... 

The ionic azides can be sensitized or desensitized to light by the introduction of foreign anions (Refs 80 114) (Table 17). Metal particles such as Au (Fig 19) sensitize Ag azide while electron donors such as Hgl2 desensitize it. These results may be explained in terms of the electronic processes that occur in the solid during decompn. The metal particles act as electron traps and thus enhance decompn, while with Hgl2 the reverse occurs and the (4) electron density is increased... 

Hydrazoic acid is about as strong an acid as acetic acid it may be prepared by acidifying ionic azides. The salt has also been prepared by nitrosation (treatment with nitrous acid) of hydrazine. Here again, the active nitrosating species is probably N2Og, with the reaction proceeding through the N-nitroso compound, A, and its tautomer, B ... 

Form C for the acid, however, contains two adjacent atoms having positive formal charges whereas form C for the anion does not. If we consider structures containing adjacent atoms of like charge as inadmissible, then covalent azides should have but two forms as compared with three for ionic azides. While such a correlation may be far fetched, azide behavior is predicted correctly. Azides of the alkali and alkaline-earth metals (ionic azides) decompose at much higher temperatures than do azides of the heavy metals or hydrazoie acid itself (which are predominately covalent). Similarly, simple organic azides such as methyl azide, CH3N3, for which only two admissible forms may be drawn tend to be less stable than acid... 

Azides can be converted by heat—or even sometimes just by a sharp blow—suddenly into nitrogen gas. In other words they are potentially explosive, particularly inorganic (that is, ionic) azides and small covalent organic azides. 

In ionic azides the vibration frequencies show little dependence on the nature of the cations, with Vg 1350cm , 2050 cm and... 

The azides of the alkali and alkaline-earth metals are colourless crystalline salts which can almost be melted without decomposition taking place. X-ray studies have been made of several ionic azides (Li, Na, K, Sr, Ba ) they contain linear symmetrical ions with N—N close to 1-18 A. (A number of azides MN3 are isostructural with the difluorides MHF2.) Other azides which have been prepared include B(N3)3, A1(N3)3, and Ga(N3)3 from the hydrides, Be(N3)2 and Mg(N3)2 from M(CH3)2 and HN3 in ether solution, and Sn(N3)4 from NaN3 and SnCl4. 

The physical and chemical properties of the inorganic azides have been extensively reviewed [4-11], Richter [12] has discussed the chemical classification of azides as (i) stable ionic azides, (ii) heavy-metal azides and (iii) unstable covalent azides. This classification is based on the percentage ionic character of the metal-azide bond, tabulated as formal ionicities in [12]. For example, the Na-Nj and Ba-Nj bonds are 70% ionic, but Pb-N, is only 34% and H-Nj is 22%. Bertrand et al. [13] have reviewed the photochemical and thermal behaviour of organometallic azides. Richter has also given [12] an excellent review of the methods of preparation of HNj and other azides. He criticizes early workers for inadequate purification and characterization of their starting materials and their neglect of allowance for the possible formation of hydrates (e.g., barium azide may be present as Ba(N3)2.1. SHjO below 284 K, forms a monohydrate between 284 and 325.5 K and is anhydrous above 325.5 K). 

A2ido coordination compounds. Kinetic studies [23] of the isothermal (370 to 420 K) decompositions of solid hexaammine-, azidopentaammine- and cis- and trans-diazidotetraamminecobalt(III) azides included investigations of the influences of ammonia and residual products on reaction rates and were supplemented by optical microscopy and X-ray identification of the phases present. Reactions were little influenced by the composition of the coordinated cation, the nature of the salt or the crystal structure. The decompositions of all four reactants were so similar that the operation of a common reaction mechanism was indicated. This similarity of behaviour was unexpected. Decompositions of simple ionic azides (Chapter 11) are believed to be initiated by an electron promotion step or exciton formation whereas... 

The first step in reaction (shown here for the hexaammine) is suggested [23] (as for some ionic azides) to be electron transfer ... 

Ionic azides are built up from cations and Ni ions. These are linear and symmetrical and their N—N bond lengths never deviate significantly from 1.17 A. The azide ion possesses two delocalized bonding n orbitals (fig. 1 a) and can be represented by the resonance formula... 

Research on the inorganic azides has been active ever since Curtius [1] discovered HN3 in 1890 he also, made a prominent contribution to the isolation and characterization of the bulk of the metal azides within the following 10 years. During the following five decades, publications continued to originate almost exclusively from the chemical community, covering most of the now-known chemical reactions and concluding, for example, that ionic azides are stable, heavy-metal azides explode on provocation, and covalent azides explode spontaneously as will be outlined in Section C, this traditional concept is oversimplified. 

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