Complex pseudohalogens

Iodine isocyanate was used to synthesize the first steroidal aziridine, 2, 3 -iminocholestane (95). from 5a-cholest-2-ene (91). This reaction sequence which is believed to proceed through a three-membered ring iodonium ion (92) illustrates the limitation of pseudohalogen additions for the synthesis of -aziridines. The iodonium complex forms from the least hindered side (usually alpha) and is opened tmK5-diaxially to give a -oriented nitrogen function. The 3a-iodo-2 -isocyanate (93) is converted by treatment with... 

Pseudohalides of Se in which the role of halogen is played by cyanide, thiocyanate or selenocyanate are known and, in the case of Se are much more stable with respect to disproportionation than are the halides themselves. Examples are Se(CN)2, Se2(CN)2, Se(SeCN)2, Se(SCN)2, Se2(SCN)2. The selenocyanate ion SeCN is ambidentate like the thiocyanate ion, etc., p. 325), being capable of ligating to metal centres via either N or Se, as in the osmium(IV) complexes [OsCl5(NCSe)], [OsCl5(SeCN)], and trans-[OsCU(NCSe)(SeCN)]2-.920) Tellurium and polonium pseudohalogen analogues include Te(CN)2 and Po(CN)4 but have been much... 

Using the tri-iodide/iodide redox couple and the sensitizers (22) and (56), several groups have reported up to 8-10% solar cell efficiency where the potential mismatch between the sensitizer and the redox couple is around 0.5 V vs. SCE. If one develops a suitable redox couple that decreases the potential difference between the sensitizer and the redox couple, then the cell efficiency could increase by 30%, i.e., from the present value of 10% up to 13%. Towards this goal, Oskam et al. have employed pseudohalogens in place of the triiodide/iodide redox couples, where the equilibrium potential is 0.43 V more positive than that of the iodide/iodide redox couple.17 Yamada and co-workers have used cobalt tris-phenanthroline complexes as electron relays (based on the CoII/m couple) in dye-sensitized solar cells.95... 

Addition of halogens and pseudohalogens to the cyclopropylthiocarbene chromium complexes 122 affords the 1,4-dihalo-1-phenylthio-l-alkenes 123 stereoselectively [65]. Electrophilic halogen is likely to activate the carbene complexes, followed by the homo-Michael addition of halide anion. (Scheme 44)... 

Since the discovery of hexacarbonylvanadium(O) and hexacarbonylvanadate( 1-) by Calderazzo and co-workers in 1959 and 1960, these substances have been key precursors to a variety of vanadium compounds, including inorganic noncarbonyl species, organovanadium complexes," and other vanadium carbonyls. Neutral V(CO)6 is of special interest in that it is the only isolable 17-electron homoleptic metal carbonyl and exhibits fascinating chemical properties that are often reminiscent of iodine and classic pseudohalogens. ... 

Phosphino complexes of nickel(O) such as Ni(PPh3)4, Ni(PEt3)3 and Ni(PCy3)2 have been found to react easily at room temperature with PhX (X = Cl, I), Mel, EtBr and other organohalogens or pseudohalogens affording nickel(II) compounds (equation 88).139,270-272... 

Another example of an eight-membered metallocycle containing Au(II) centers is the dithiocarbamate complex (9) (20), obtained from the gold(I) dithiocarbamate complex (8) with halogens or pseudohalogens at -78° in C. However, these compounds disproportionate to form mixed-valence salts (10). It was shown that the vCs band of the coordinated dithiocarbamate is sensitive to the oxidation state of the gold, increasing in the order 1495, 1523, and 1547 cm-1 for the Au(I), Au(II), and Au(III) complexes, respectively (20). 

Figure 30 shows schematically the concept of HOMO level tuning in Ir pseudohalogen complexes of the type TBA[Ir(ppy)2(CN)2] (46), TBA[Ir(ppy)2... 

NCS)2] (51), and TBA[Ir(ppy)2(NCO)2] (52). These complexes were conveniently synthesized under inert atmosphere by reaction between the dichloro-bridged Ir dimer [Ir(ppy)2(Cl)]2 in dichloromethane solvent with an excess of a pseudohalogen ligand such as tetrabutylammonium cyanide, tetrabutyl-ammonium thiocyanate, or tetrabutylammonium isocyanate, respectively, with over 70% yields [77]. 

While bifunctionality is known for the halogens and many pseudohalogens, it is most pronounced for cyanide and influences the structures, properties and chemistry of many of its derivatives. Cyanide bridges were present in the first recorded synthetic inorganic complex, Prussian blue (ca. 1700), and cyanide linkage isomers were often proposed in the old literature but reasonable evidence for the existence of linkage isomers and the structure of Prussian blue is very recent. 

The azide ion is a good ligand, and it forms numerous complexes with metal ions. Chlorazide (C1N3) is an explosive compound prepared by the reaction of OCT and N3. As in the case of CN, the azide ion is a pseudohalide ion. Pseudohalogens are characterized by the formation of an insoluble silver salt, the acid H-X exists, X-X is volatile, and they combine with other... 

A preponderance of the transition-metal pseudohalogen complexes reported in the literature are prepared in aqueous media. Several oxidation states of many transition metals are either unstable in the presence of water or form only oxygen-coordinated species. Thus, these metal ions will not form pseudohalogen complexes in the normal manner. The following method, using polar, nonaqueous solvents is suitable for the preparation of isothiocyanate complexes of several of these ions. As an example of the preparation of such complexes, the synthesis of potassium hexakis(isothiocyanato)niobate(V) is described. 

Burmeister and colleagues have described the related pseudohalogen derivatives MfterpyjXj (X = SCN or SeCN) (90-92). The platinum compound exhibits the two thiocyanate stretching frequencies expected for a square-planar complex, and is formulated [Pt(terpy)(NCS)][NCS], However, the palladium complexes are less easily formulated, exhibiting absorptions due to coordinated ECN (E = S or Se) only. These observations were interpreted in terms of a square-planar structure, with a bidentate terpy ligand in view of the known ability for palladium and platinum diimine complexes to form five-coordinate species, this formulation must also be considered. In the absence of definitive structural evidence, the formulation as five-coordinate species must be regarded as speculative. 

Most Au1 complexes are linear and of the type L—Au—X, where L is a 2-electron donor ligand (PR3, R2S, CO) and X is a halogen or pseudohalogen. The phosphine complexes are particularly stable, whereas R2S is more readily displaced hence R2S complexes are useful preparative intermediates. 

In covalent azides, the pseudohalogen azide RN3 has an angular structure as in HN3. Triazidoborazine [H3N3B3(N3)3] and other boron azides, for example, salts of the tetraazidoborate ion B(N3 (4 have been considered as boron nitride precursors (see Boron-Nitrogen Compounds). The M(Ns)3 azido complexes of the other group 13 elements (Al, Ga, In, Tl) and their M(N3)4 tetraazido anions are all known. They and their derivatives are also used as precursors for the nitrides. 

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