Pentazolate complexes

Figure 3 Different coordination modes in pentazole complexes.

It seems worthwhile to investigate whether or not the arylpentazoles form transition metal complexes, for example of type (7). These could be converted into pentazole complexes (8) without destruction of the pentazole system. Unsubstituted pentazole (1) would be expected to be a strong acid with a highly aromatic anion (6) which could possibly form ferrocene analogs such as M (N5)2, where M" represents a divalent metal ion. Unfortunately, it has not been possible to investigate the properties of unsubstituted pentazoles, since neither... 

T) (N, N ) mode complexes. Suspected T) -mode of coordination still remains unconfirmed. The organometallic chemistry of pentazole is the challenge for the further developments. 

The pentazolate anion. Ns (11.2), is estimated to have a half-life of 2.2 days, whereas that of the parent pentazole HN5 is predicted to be only ca 10 min in methanol at 0 Although HN5 is unknown, the cyclic anion N5 has been detected by tandem mass spectrometric studies of 4-hydroxyphenylpentazole. Similarly to its congener P5 (Section 11.2), N5 (isoelectronic with cyclopenta-dienide [C5H5] ) has the potential to form metallocene-like complexes. The acyclic (V-shaped) cation Ns has been isolated as a hexafluoroantimonate salt, which decomposes at ca 70 °C. The estimated energy density of [N5] [N5] is approximately twice that of hydrazine, a well-known rocket propellant, suggesting that this ionic polynitrogen allotrope would be an excellent monopropellant... 

The question of whether metal complexes of pentazoles similar to ferrocene are stable has been discussed since extended Hilckel calculations predicted the stability of these complexes 20 years ago (Figure 3) <1985POL1721, 1988AIC171>. 

---