Chromium complexes thiocyanate

Chromium, tris(hexafluoroacetylacetonato)-optical isomerism, 28 Chromium, tris(l,10-phenaiithroline)-ligand field photochemistry, 398 Chromium, tris(trifluoroacetylacetonato)-isomerism, 28 Chromium complexes geometric isomerism, 11 Chromium (0) complexes magnetic behavior, 273 Chromium(I) complexes magnetic behavior, 272 Chiomium(II) complexes magnetic behavior, 272 spectra, 252 thiocyanate... 

Chromium in the trivalent state forms a variety of salts, the most important and the simplest being the violet salts, which liberate in aqueous solution chromium cation Cr" A green series of chromic salts, isomeric with the violet salts, liberate in aqueous solution some chromium cation, whilst part of the chromium is present as a complex ion. With weak acids, sulphurous, hydrocyanic, or thiocyanic acids, the chromic ion forms complex ions of great stability. Finally, a very large group of salts exists where chromium associated with ammonia forms the complex ion, the chromi-ammines. 

The vast majority of known chromium(III) complexes with the ambidentate chalcogenocyan-ate ligands NCO-, NCS-, NCTe- are those containing JV-bonded thiocyanate. 

The products of reduction of Fe(NCS)2+ by Cr2+ in aqueous solution containing thiocyanate ions (equation 36), include the isomers of [Cr(NCS)(SCN)(H20)4]+, which contains both bland S-bonded thiocyanate ligands.641 These isomers undergo spontaneous decomposition by parallel aquation (loss of the S-thiocyanato ligand) and isomerization (Cr—SCN— Cr—NCS) reactions. Details of the solution visible spectra of these isomers as well as those of similar chromium(III) complexes for comparison are listed in Table 68. From the compiled data it is apparent that the S-bonded thiocyanate ligand lies very close to Br- in the spectrochemical series while the N-bonded form lies between Nj and NO. 

The addition of en or 1,3-diaminopropane to a mixture of chromium(III) acetate hexahy-drate and diacetyl under reflux, followed by addition of NaC104, has given the complexes [Cr(diacen)(H20)2]C104 and [Cr(diacpn)(H20)2]C104, which contain the tetradentate Schiff bases (234).1119 The thiocyanates [CrL(H20)(NCS)](NCS)2 were also isolated. From their IR spectra, and because the coordinated Schiff base would not react with another molecule of diacetyl to form a macrocycle, they have been assigned cis configurations. 

PROBLEM 20.4 What is the formula of the chromium(III) complex that contains two ammonia and four thiocyanate (SCN-) ligands ... 

Reducing substances, hexavalent chromium and colored matter, interfere with the test. Reducing substances that may reduce Fe3+ to Fe2+ and thus prevent the formation of thiocyanate complex, may be destroyed by adding a few drops of H202. Add FeS04 to convert CC+ to Cr3+ at a pH below 2. Cr3+ and Fe3+ so formed are precipitated out when the pH is raised to 9 with NaOH. 

Ganescu et al. used thiocyanato Cr(III)-complexes in the titrimetric and spectrophotometric determination of dipyridamole [16]. For titrimetric analysis of the drug (1.36-27.2 mg), sample solutions (25-50 mL) were acidified with hydrochloric acid and treated with excess of a 3% solution of ammonium dianilinetetrakis (thiocyanato) chromium (III) in aqueous 25% ethanol. The red-violet precipitate was collected and washed, decomposed with NaOH, and the mixture again acidified. The liberated thiocyanate was titrated with potassium permanganate, potassium bromate, or potassium iodate. 

Tris(diamine) complexes of chromium(III) have been important from several viewpoints. First, the tris(ethylenediamine) complex is valuable as a synthetic intermediate,1 the action of heat on the chloride salt giving the cis-dichlorobis(ethylenediamine) complex and on the thiocyanate salt giving the irans-bis(iso-thiocyanato) complex. Second, the cations are resolvable, and studies2 of their optical activity have been fruitful in establishing relations between signs of Cotton effects and absolute stereochemistries. A large number of other studies, including kinetic and equilibrium measurements, have shown these complexes to be readily hydrolyzed to bis(ethylenediamine) complexes.3... 

Infrared and electronic spectra confirm the nature of the hexa-N-thiocyanatometallate anions of chromium(III) (293, 639, 647, 656, 663, 665) and molybdenum(III) (293, 489, 647, 663, 664), and molyb-denum(II) (534) a variety of salts have been reported containing these anions (see the foregoing and also Refs. 30 and 528). Infrared spectra and magnetic measurements have been used to characterize [M(NCS)e] (M = Mo, W) and [W(NCS)g] (386). A large number of mixed-ligand thiocyanate complexes of chromium(III) have been reported, and many of these are of the type M[Cr(CNS)4L2], similar to the original Reinecke s salt where L = NH3. 

Bautista and Hard (B8) made a comparative study of the extractability of. several of the first-transition metals from thiocyanate solutions using methyl isobutyl ketone as the organic solvent. The transition metals readily extracted were scandium (III), iron (III), and cobalt (II) while chromium (II) and manganese (II) were not. The principal extractable species were found to be the neutral scandium and iron trithiocyanate complexes, while the extractable cobalt complex was the negatively charged tetrathiocyanate radial Co(SCN)4 . The distribution ratio for scandium, iron, and cobalt decreased with increase in metal ion concentrar tion but increased with increasing ionic strength of the solutions. 

SNC, Thiocyanate, gold complex, 26 90 SNOjCjH, 1,2-Benzisothiazol-3(2//)-one 1,1-dioxide, chromium and vanadium complex, 27 307, 309 SOOsPCvH45. Osmium, carbonyl(thiocar-bonyl)tris(triphenylphosphine)-,... 

The anionic thiocyanate complex of chromium(III) has been extracted with MIBK or cyclohexanone [10], and with 5% TOA in CCI4 [11]. The complexes of Cr(III) with NTA [12] or DCTA [13,14] have been extracted with solutions of Aliquat 336 in CHCI3 or 1,2-dichloroethane. 

The basic process can be exemplified by the reaction of the cobalt(III) complex [Co(NH3)5(NCS)]2+ (the oxidant) with the chromium(II) complex [Cr(OH2)6]2+ (the re-ductant), where thiocyanate acts as the bridging ligand. The overall reaction in aqueous acidic solution is shown in Equation (5.58). 

Various complexes of Rh , Ir " and Pt" with 2-guanidiniumbenzimidazole and related ligands have been synthesized and characterized. A tris-chelated chromium(III) complex with 2-guanidinobenzimidazole has been prepared by complete replacement of thiocyanate groups in K3[Cr(NCS)g]. The 10 Dq value (20 200 cm ), obtained from the T2g - A2g transition, is slightly lower than those of Cr " chromophores in general. ... 

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