Selenocyanate complexes

Therefore, if an acceptor coordinates thiocyanate and selenocyanate ions via their S and Se donor atoms, respectively, the selenocyanate complexes should be considerably stronger than the thiocyanate complexes, and moreover formed in considerably more exothermic reactions. If, on the other hand, an acceptor prefers to coordinate these ligands via their harder donor atom N, the bonds formed will be less covalent and the reactions much less exothermic than in the case of S or Se coordination. The strength of the bonds will be little influenced by an exchange of S for Se and much the same value of AH°n may therefore be expected for thiocyanate and selenocyanate complexes. 

Selenocyanate complexes, 17 354, 356 homogeneous anionic, 17 354, 355 infrared spectra, 17 255 mixed-ligand, 17 354, 356 electronic effects, 17 355 steric effects, 17 354, 355 Selenocyanate ions, see also Chalcogeno cya-nates... 

Zirconium and hafnium cyanate, thiocyanate, and selenocyanate complexes that contain N-heterocyclic ligands are discussed in Sections 32.4.2.5.iii, 32.4.2.5.iv and 32.4.2.5.V. 

The selenocyanate ion is much less stable than the thiocyanate ion. However, silver selenocyanate complexes are in general more stable than the corresponding thiocyanates.148 Complex species of the type [Ag(SeCN) ]1 rt and [Ag SeCN)]"-1 (n = 1-4) have been predicted from potentiometric and solubility studies in aqueous solution. 

Tabic 1 Typical Palladium(II)-Thiocyanate and -Selenocyanate Complexes and IR Data ... 

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... 

Mixed complexes of the type Ln(bipy)2(NCS)3 have been isolated containing both coordinated and uncoordinated groups. Because of the overlap of the bipyridine and thiocyanate band, unequivocal assignment is not possible [238], Analogous selenocyanate complexes Ln(bipy)3(NCSe>3 with N-bonded selenocyanate and a coordination number of 9 have been reported. 

Previously, reviews have appeared on the crystal chemistry of thiocyanate and selenocyanate coordination compounds (612), selenocyanate complexes 334), the stability and formation of thiocyanate complexes 637), the chalcogenocyanates in coordination chemistry 474), and the infrared spectra of thiocyanate and related complexes 57). Other reviews that are largely devoted to aspects of the chemistry of the chalcogenocyanates have appeared on organometallic pseudo-... 

The reactivity of ambidentate ligands toward organic centers has been reviewed (679). The review by Beck and Fehlhammer (56) includes a most useful section on methods used for the preparation of these complexes, and this aspect will not be emphasized here. The principles of formation of selenocyanate complexes have been reviewed (682). 

The previous discussion has been concerned with thiocyanate complexes, but a similar situation obtains with selenocyanates [437, 438). Table XI includes the frequency ranges for different modes of seleno-cyanate coordination. Kharitonov et al. (439) obtained similar results for selenocyanate complexes as for thiocyanate complexes in their theoretical treatment outlined previously, and their conclusions are the same in both cases. Thus, frequency shifts can be due to electronic effects or to changes in the bonding mode. The relative contributions of the resonance forms of NCSe (Table X) are comparable to those of NCS so that integrated intensities can be used in a similar way, and with the same possible dangers. Table XII contains some infrared data... 

The lack of compounds means that generalizations cannot be advanced for cyanates in the same way that they have been developed for the thiocyanate and selenocyanate complexes. Such data as are available suggest that for 0-cyanates lie within the range for N-cyanates and that the degeneracy of the deformation mode is apparently removed for some 0-cyanates as well as for some iV-cyanates. 

Compound K[PhgSn(NCSe)2] resonates at +215 ppm, and NCSe at +136 ppm (relative to NOg") 111) so that the same behavior as was described in the foregoing seems likely for selenocyanate complexes. The absence of further data precludes this tentative generalization from being more firmly based at present. 

The compounds (Et4N)4[M(NSCe)a] (M = Pa, U) have been characterized by comparing their spectral and crystallographic properties with the corresponding thiocyanate complexes (26). Some thorium(IV) complexes have been prepared containing DMF and N-bonded seleno-cyanate groups (326). Although various uranyl selenocyanate complexes have been reported, no structural data are available (686). 

Tetrahedral [Mn(NCSe)4] and octahedral [Mn(NCSe)g] anions are known (162, 295, 6i7, 655, 656), and [Re2(NCSe)8] has been tentatively identified (377). There seem to be no reports of similar Tc compounds, nor indeed of any mixed-ligand selenocyanate complexes of either technetium or rhenium. Some mixed-ligand complexes of manganese(II) selenocyanate have been prepared, and in every case so far described the selenocyanate is N-bonded. Analogous to the thio-... 

Theanions [FeCNCSe) ] (162,295,647,663,665), [Fe(NCSe)g] (295, 663), and [Fe(NCSe)g] (162, 663, 665) have been characterized by spectral methods. No anionic selenocyanate complexes of ruthenium or osmium appear to have been reported. 

An asterisk indicates that the corresponding selenocyanate complexes are also discussed. Octahedral Co(bipy)2(NCSe)2 is also known (655). 

Tetrahedral [Zn(NCO)4] and [Cd(NCO)4] have been reported as their Mc4N + salts by preparation in nonaqueous media (292, 296) force constants have been determined for the former and compared with the corresponding thio- and selenocyanate complexes (301). Even in the presence of excess of KNCO (158), K[Cd(NCO)g] precipitates from water and has been shown to contain... 

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.

Most palladium (II) selenocyanate complexes have a Pd—Se bond, but the compound [Pd(Et4dien)(SeCN)]BPh4, isolated at low temperatures, isomerizes in a number of different solvents via a dissociative process, whereas [Pd(dien)(SeCN)]BPh4 shows no signs of such isomerization. Further, if [Pd(Et4dien)NCSe]BPh4 is isolated it reisomerizes to the Se-bonded form at room temperature in the solid state 153, 156). This behavior parallels that of the thiocyanate group under similar circumstances and provides evidence for a steric effect modified by the nature of the anion. 

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." " " ... 

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