Selenocyanates metal complexes

Various reducing agents have been used for the generation of selenolates from diselenides or selenocyanates. Alkali metals M (M = Li, Na, K)149,150 or alkali hydrides MH (M = Li, Na, K)151,152 can generate the corresponding selenolate anions such as 80 these are more reactive than the borane complexes of type 77 (Scheme 15). Diaryl diselenides are easier reduced than dialkyl diselenides, but the mechanism for the reduction of selenocyanates is complex and can lead to either diselenides or selenolates.153,154... 

Table XLI summarizes the structures of known, homogeneous, anionic selenocyanate complexes. Although fewer examples exist than for the corresponding thiocyanate complexes, it is apparent that a similar pattern exists for the two sets of complexes. Class a metals are coordinated by the nitrogen atom, whereas selenium is the donor for the class b metals. In mixed metal complexes containing bridging seleno-oyanate groups the nitrogen atom coordinates to the harder or class a metal, and the selenium atom bonds to the softer or class b metal, as was observed with the corresponding thiocyanate complexes.

AHR/AVS] Ahrland, S., Avsar, E., Kullberg, L., Thermodynamics of metal complex formation in aqueous solution. VI. Equilibrium and enthalpy measurements on the zinc and cadmium selenocyanate systems, Acta Chem. Scand, A 28, (1974), 855-865. Cited on pages 206, 266, 276, 277. 

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... 

These results explain previous observations of the greater stability of zirconium thiocyanate and selenocyanate complexes compared with their hafnium analogues, and the greater stability of zirconium and hafnium complexes in MeCN compared with DMF in terms of competition between the ligand and solvent molecules for co-ordination sites on the metal. Zirconium alkoxides have been prepared from ZrCl4 and aliphatic alcohols158 but with salicylaldehyde a Meerwein-Ponndorf... 

The preceding sections have shown that the behavior of the thiocyanate and selenocyanate groups in homogeneous complex anions is identical in both cases the nitrogen atom coordinates to the class a metal, and the sulfur or selenium atoms coordinate to the class b metal. Although fewer comparable cyanate complexes are known, it is quite clear that the coordination of the cyanate group is not governed by the same factors. 

Since an ambidentate ligand possesses at least two groups capable of coordinating to metal ions, polynuclear complexes may also exhibit linkage isomerism. The selenocyanate complex of cadmium , [(C4H9)4N]2 [Cd2(CNSe)6], contains the dimeric anion on the next page,... 

K(18-crown-6)]2K[Bi(SCN)6], which exists in two isomeric forms." In the yellow form, octahedral [Bi(SCN)6] " anions are surrounded by four K[K(18-crown-6)]2 units in a layered arrangement all the thiocyanates are A -bound to potassium. In the yellow form, the thiocyanatobismuth anions have four 5-bonded and two trans Ai-bonded thiocyanate ligands, and are arranged in parallel columns separated by K+ cations each potassium has two trans 5-bound and four Ai-bound thiocyanate ligands. Two [K(18-crown-6)] units are located between the columns." Structurally characterized selenocyanate complexes of the i-block metals are much rarer, but as befits the softer nature of selenium relative to sulfur, all known examples are Ai-bound." " " ... 

Oxyselenation of alkenes (8, 119-120). Details of this reaction have been published. The role of the metal catalyst is considered to be polarization of the Se— CN bond of the selenocyanate, and in fact the pyridine complex of the metal salt is less effective. An episelenonium ion is postulated as the intermediate. CuBr2, CiCl, NiBr2, and NiCl2 are similar to CuCl2 in activity. CrCl3 and CoCl2 show limited activity, whereas halides of many other metals are inactive. 

Ion interaction coefficients, for the selenocyanate ion and its metal ion complexes are not available. Data for the thiocyanate ion have therefore been used as an approximation for extrapolation by SIT of selenocyanate equilibrium constants to standard conditions. 

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