Pseudohalide Complexes

Fulminato, cyanato, thiocyanato, and related pseudohalide complexes 281... 

Cesium niobium iodide complexes, 46 34 Cesium pseudohalide complexes, 46 42 Cesium salt (CS2MCy, lattice energy, 22 22 Cesium tetraiodide, structure of, 3 152 Cesium trifluoroacetates, 17 8, 31, 32 CF, 33 110... 

Among these, halide and pseudohalide complexes of the general formula AgP X (where X = halide or pseudohalide, P = phosphine and n = 1-4) have been studied in detail. 

Comparison of the titanium-pseudohalide complexes (86) and (88) shows nicely the different bonding of the pseudohalide, i.e. bent Ti—N—N vs. linear Ti—N—C of the N-bonded isocyanate and isothiocyanate. Table 9a shows that in isostructural M(tj5-Cp)2(NCO>2 (M = Ti, Zr)222 the N—C (b) bonds tend to be shorter than the C—O (c) bonds. This may point to a larger contribution of canonical structure (89) which has some C—O triple bond character in the bonding. 

A large amount of structural information is available for the pseudohalide complexes of Cu. Terminal,303-304 317 l,3- i304,318,319 and l,l- i3l8,319b 320 bound azide ligands have been found. For example,... 

A variety of preparative methods are available for the synthesis of metal pseudohalide complexes.202 209 The use of a large counterion like NEt4+, PPh4+ or (PPh3)2N+ is often important for the isolation of the water-free complexes. Furthermore, the presence of bulky counterions can have a stabilizing influence on the more or less explosive complexes containing azide anions. 

A new line of studying the metal-pseudohalide complexes comprises on the one hand the synthesis of the pseudohalide in the coordination sphere of a metal. On the other hand a coordinated pseudohalide is used as a building block for a new molecule which thus is constructed in the metal coordination sphere. A short outline of the first aspect will be given below, while for examples of the second aspect the reader is referred to refs. 344-347. 

Other coordination modes in pseudohalide complexes are comparatively rare. Amongst them, we note the structure of complex [AgLSCN] 0.25L (L = bipy), in which two silver ions are simultaneously bound with N atoms from NCS groups [166], The pseudohalide complexes with simultaneous different kinds of coordination of the NCS group are also rare. In particular, the complex compound [CuL(HL)2][Cu(L)(SCN)(p-NCS)], where LH is 2-dimethylaminoethanol, contains within its coordination sphere both kinds S-terminal (108) and -bridge (109) thiocyanate groups [178],... 

Other synthetic techniques, described below, are applied to produce pseudohalide complexes, for example reactions of exchange of perchlorate anion to... 

An interesting cycle of transformations, (4.5) and (4.6), including preparation of pseudohalide complexes and their use as metal-containing ligands (Sec. 3.3.2.3) was reported [51] ... 

At the same time, it is necessary to take into account that the approach described has a number of exceptions, related for example to the nature of other ligands forming pseudohalide complexes. A series of classic examples of inversion of the bond M — N —> M — S —> M — N have been reported [6,8,11,42-44,59] and are presented in Sec. 2.2.3.5. In this respect, we especially emphasize the capacity of other ligands for soft or hard metals, related with symbiotic [60] and anti-symbiotic [61] effects. Thus, Pearson [61] emphasized that soft ligands, which are placed in a trans position to SCN ion, contribute to N-binding of thiocyanate ions, and hard bases contribute to S-coordination of these ambidentate ligands. Metal oxidation number (Table 1.4) is important in this problem and it regulates soft hard properties of complex-formers. 

The precise nature of the product obtained depends on the cation used and also on the halide and the solvent. There is a variety of halide complexes with other ligands present such as Mnl2(thf)3, trnns-[MnCl4(H20)2]2 , and amine and phosphine species. Homoleptic pseudohalide complexes are represented by [Mn(NCS)6]4 and [Mn(CN)6]4, as well as Prussian blue-like analogues, e.g., K2Mnu[Mnn(CN)6], that are ferrimagnetic.3... 

Neutral and anionic Pseudohalide complexes have been reported. While most of the thiocyanates were considered to be N-coordinated, Nh NMR spectra of Nb(NCS)6 were reinterpreted in terms of a mixture of N- and S-coordinated isomers. 

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