Azide bridging

With a view to determining the equilibrium constant for the isomerisation, the rates of reduction of an equilibrium mixture of cis- and rra/i5-Co(NH3)4(OH2)N3 with Fe have been measured by Haim S . At Fe concentrations above 1.5 X 10 M the reaction with Fe is too rapid for equilibrium to be established between cis and trans isomers, and two rates are observed. For Fe concentrations below 1 X lO M, however, equilibrium between cis and trans forms is maintained and only one rate is observed. Detailed analysis of the rate data yields the individual rate coefficients for the reduction of the trans and cis isomers by Fe (24 l.mole sec and 0.355 l.mole .sec ) as well as the rate coefficient and equilibrium constant for the cw to trans isomerisation (1.42 x 10 sec and 0.22, respectively). All these results apply at perchlorate concentrations of 0.50 M and at 25 °C. Rate coefficients for the reduction of various azidoammine-cobalt(lll) complexes are collected in Table 12. Haim discusses the implications of these results on the basis that all these systems make use of azide bridges. The effect of substitution in Co(III) by a non-bridging ligand is remarkable in terms of reactivity towards Fe . The order of reactivity, trans-Co(NH3)4(OH2)N3 + > rra/is-Co(NH3)4(N3)2" > Co(NH3)sN3 +, is at va-... 

The complex ion [Au(N3)4] reacts with alkyl isocyanides according to equation (69) 291 and with CO to give [Au(NCO)2]. 294 Azide-bridged complexes such as [Au2(Ju-N3)2Me4] are also known. The ligand field and charge transfer bands in the UV-visible spectra of [Au(N3)4] have been assigned and the structure determined.567... 

A structural study of receptors 51, 52, and 49, showed three different modes of azide complexation to the binuclear copper(II) host (123) 1,1 cascaded, 1,3 cascaded, and noncascaded, respectively. These structures indicate that the nature of the macrocyclic framework of the receptor is important in determining the mode of anion coordination. Figure 5(a and b) shows the crystal structures of 52, 2 Cu(II)-azide, and 52, 2Cu(II)-chloride, respectively, for comparison (129). As can be seen, it is the length of the azide bridge that makes cascade complexation possible, whereas for the smaller mononuclear chloride anion this obviously cannot occur. Receptor 49 has also been shown to cascade bind pyrophosphate [as its bis-copper(II) complex] (130) and sulfate [as its bis-iron(II) complex](131). At about the same time, Nelson and co-workers (132) published a similar bis-copper(II) complex structure that cascaded an azide anion. 

All azide compounds of this element are complexes, namely, several mixed azide bridged complexes of Rh(I) [163], several mixed azido complexes of Rh(III) [164,165], and a hexaazidorhodate(III). 

Complex palladium azides are obtained by dissolving palladium salts in excess azide, e.g., palladium nitrate and sodium azide (1 4) yield a triazido anion, [Pd (N3)3]" [167]. Similarly, a tetraazido anion, [Pd (N3)4] , is obtained from K2 [PdCU] by ligand exchange [162]. Another all-azido anion is the azide-bridged, binuclear complex [Pd2(N3)6] [168]. All of these have been isolated as salts of large organic cations. 

Later, other systems were also synthesized, and their data are reported in Table 5. NINO is the analog of NENP obtained by replacing en with 1,3-propanediamine. TMNIN " has the formula [N(CH3)4][Ni(N02)3]. NINAZ, is obtained by replacing the nitrate anion with the azide anion. This introduces us to the interesting world of one-dimensional systems containing azide bridges. 

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