PHYSICO-CHEMICAL STUDIES ON THE REACTIONS OF DYES AND ORGANIC BASES WITH METAL CYANIDES

Abstract

interest in the chemistry of complex metal cyanides may be traced back to the discovery of Prussian blue in 1704. Since then cyanide complexes of a large number of transition metals except lanthanides and actinides have been extensively investigated inspite of the inherent difficulties in obtaining some of them in pure and stable form. By and large, most of these studies refer to solutions, although in guite a few cases compounds in the foxm of crystalline solids have been prepared and subjected to x-ray. spectral and magnetic measurements in order to throw light on the geometry of these compounds. Structural Characteristics * Considering the typical and most popular case of the cyanide complexes of Fe(II) and Fe(XXX># the following facts regarding their structural characteristics have been established* t (i) The molecular orbital configuration of the cyanide ion is KK <cr-^3)2 ( cr-|3)2 ( o-2p)2 ( -y^)* ( 7f2p)° (li) The cyanide ion has two lone pairs, one on the carbon and the other on the nitrogen atom and undergoes coordination through overlap with metal d-orbital forming L —> M, o—-coordinate bond, (ill) The bonding1"3 of the cyanide ion is usually through carbon except when the cyanide acts as a bridge. In such a case bonding takes place through both, carbon and nitrogen. The cyanide ion can act both as or- *—** as well as Tr bonding ligand. But out of these two bondings, dTT *, PTT bonding through back donation is of great significance since it avoids the accumulation of excess of negative charge on the metal atom thereby strengthening the metal-carbon bond. 4-7 (iv) Recent x-ray, neutron diffraction measurements and infrared8"11 studies support the view that the cyanide ion is bonded to the metal through carbon atom. M^nHBtra n^MP^ITION OF iM**OUS SOIUTIO^ OF COMPLEX TROH CYANIDES. Potassium hexacyanoferrates (II and III) give stable solutions in water. These are, however, very sensitive to light, especially in acidic medium. The chemistry of their solutions thus undergoes drastic change if solutions are not kept in dark. Photo-chemical decomposition of potassium hexacyano- «errate(H) is expected to take place in two ways* hi) 4— u) fcmf'*'V —> [•*«•*»«••] +HCH ltd [Fe<CN)6]- ♦ *V> -S2U Q«(CN)5(H2O)]3"+Ha.+0ir Bie latter scheme of photo-equation is considered to he more probable in view of the fact that Irradiation of j|( |,IW III IWIII 1 t— ««ults in amarked increase in pH12"25. «* reaction is completely reversible if the irradiated solution is kept in dark overnight21"25. Prolonged exposure to light results in the development of ablue colour due to the formation of Iron blues. The chemistry of the above reaction has been reviewed by Mamson et-al?ofi and Balzani and CaraS3itti27. The appearance of the aquopentacyanide has been checked by means of nitrosobenaene as a reagent28-32. It has been shown^ that photolysis of [Fe(CN)5(H2orf" eventually produces Fe(OH)3' 24 34 35 in alkaline solutions and prussian blue ' ' in acidic solutions. The photolytic decomposition is pH»dependent, possibly due to the protonation of [Fe(CN)6] to give [HFetOOj . At pH's of 1,3 and 10, the species should be [HjlMCN)^] , [HFe(CN)6"l " and [1te(ai)6] respectively. Photo-substitution of cyanide groups in Fe(CN)^ by ligands other than water has been reported. The irradiation of aqueous solution of [Fe(CN)6] with ultra violet light, in presence of 1, lo phenanthroline or 2,2' bipyridine, shows the 20,36-38 substitution of cyanide groups by these bidentate ligands According to Balzani et al36"36, [Fe(CN>4(AA>f" and^(CNJ^AA)^ are the main products with small amounts of [Fe(AA)J where AA represents the bidentate ligands, 1,10 phenanthroline or 2,2' bipyridine. The interesting feature of these substituted products is that, they are themselves photoactive and undergo photoaquation as follows* [Fe(CN)4(AA)l2' ^— [»«|U*) <«2°f +^ 2 3- The photo-chemical behaviour of pe(CN)^ is difficult to understand. The irradiation of JeCOOJ solution in 254-405nm region, strongly suggests that fFe(CN)5(H2o)f is the first photoproduct33'39 and in add solution prussian blue35 is eventually formed. There are many other aspects of the photo-chemistry of iron cyanides which are of equally importance. REACTIONS INVOLVING THE REPLACEMENT OF CATIONS, These reactions started with the studies on the iron blues and a large number of heavy metal ferro- and ferricyanides have been prepared and their composition determined, Kolthoff40'41,for the first time, used conductometry and potentiometry for determining the composition of these compounds. He also demonstrated the importance of potassium ferrocyanide as a reagent for the estimation of metal ions, especially zinc. His work was followed by extensive investigations on the use of potassium ferro- and ferricyanides In quantitative 42-47 analysis by other workers Bhattacharya and Malik48 determined the composition of zinc, manganese, nickel and cobalt ferricyanides. Malik et al49'50 studied the interaction of cr(li) with ^[FeCOi)^ and K3JFe(CN)61 and showed that Cr(II) ferro- and ferricyanides exist in the form of soluble complexes. Recently K4[Fe(CN)^] has been used as a reagent for the colorimetrlc estimation of Cr(IH) by Malik and Bembi . Potassium ferrocyanide has also been used as a reagent for the 52-54 volumetric estimation of a number of metal ions by some workers Pani and Rao55 reported the formation of molybdenum ferrocyanide and its composition was found to be (Mo02) [le (CN>^ by Malik56. REACTIONS INVOLVING THE DISPLACEMENT OF CYANIDE GROUPS. Another aspect of the chemistry of hexacyanoferrate (II) and (III), worth investigating, is the displacement of cyanide group by different nucleophiles. Many compounds, mostly monosubstitution products of iron (I I) are known, in which only one cyanide group is replaced by nucleophiles , such as, H20, NO,NO~, N0+, asO-, 30*, CO, NHj etc. or In which two cyanide groups are replaced by 1,10 phenanthroline, 2,2* dipyridyl etc. 58 The Fe (III) substituted cyanide products are not so abundant as the corresponding Fe(II) compounds. The more important compounds arci [Ye(CN)5(H2ori2" I [FetCN^NRjJj f 3- 3— [Fe(CN)5(N02)1 and [Fe(CN) 5(MO)] MIXED IRON CYAHXPE CO^LEXEg. Barbieri59 carried out extensive studies on the compounds [Fe(CN)2(C10H8N2)2], 3H20, K2 Je (CN^C^H^)] .3^0 f Pte<CN)2(C12H8H2)2] 3H2° and K2[Fe(GN>4(Cl2W] ' 2H2° which were formed by the interaction of ferrou3 bipyridyl or phenanthroline halides with KCN. Many workers60*"64 have studied a number of mixed cyanoferrates. S Scbllt63'64 established the existence of mixed ligand complexes of iron(II) and (III) with cyanide and 2,2* bipyridine, and with 1,10 phenanthroline and cyanide. He also isolated the protonated species of the neutral, mixed ligand complexes of Fe(II) and suggested that the protonatlon of tfre iron(II) complexes involved the central ferrous ion rather than the ligand. No such distinction could be observed for the Fe(III) complexes. Hamer and Orgel65 studied the structure and spectra of ^Phenanthroline)2 Fe(CNMe)^ and lPhenanthroline)2 •HFe( CN)-Hj and gave evidence which strongly suggested that 2 2] protonatlon occurred at cyanide group. Recently Malik et al66""69 studied the substituted complexes of hexacyanoferrate(II) and (III) witho(-amino acids on irradiation with ultra violet light. REACTIONS WITH ORGANIC BASES. wurster and Roser70 prepared hydroferrocyanides of p-nitroso- and p-bromo-dimethylaniline and m-toluldine, as well as the salts of dimethyl toluldine3 from strongly acid solutions of their sulphates. All the hydroferrocyanides have the general formula (Base)2 H4¥e(CN)6]» xH20. Wurster and Roser also isolated salts of tetramethylm- and-p-phenylenediamines, having general formulas Base H4rFe(CN)^, *H20» Eisenberg71 while discussing the preparation and properties of the acid hydroferrocyanides of aniline and dimethyl aniline, found that a neutral salt of the general formula (base)4 HJFe(CN)^] is formed when the base is treated with alcoholic hydroferrocyanic acid. Barth72 prepared tetramethylammonium ferrocyanide (NMe4) TMaOg-1 x^O. Similar other substituted ammonium 73 compounds have been reported by Fischer . Cumming74""77 carried out detailed studies on the hydroferro- and ferricyanides of organic bases in between 1922 and 1924. He prepared the hydroferrocyanides of aniline, o-toluidine, dimethylaniline, pyridine, benzidine etc. and gave the general formula. (Base) H^tCN)^] . YH-jO where x * 1,2,3 or 4 He74 further studied the influence of the nltro, sulphonic, hydroxyl, carboxyl and phenyl groups on the formation of organic base complexes and found that whenever a strongly acidic group was present in the molecule, no hydroferrocyanide was formed. It would be interesting to mention that as early as 1C79, wurster and Roser80 suggested that the basicity of the dimethylaniline was not materially affected by the introduction of the nitroso- or bromo-groups in the para position. Alkyl groups have a marked influence on the physical properties of the hydroferrocyanides. In the aliphatic series, the introduction of an alkyl group decreases the solubility in water. The reverse rule applies in the case of aromatic series, where the alkyl group is substituted in the nucleus or in the side chain. The introduction of an alkyl group into the nucleus or side chain reduces the stability but the introduction of the second alkyl group into the side chain increases the stability, the salts of the secondary amines are the least stable. 8 The introduction of a second amino group in the nucleus increases the solubility but decreases the stability. The salts of the diaminodiphenyl compounds are usually very insoluble in water and slightly unstable. Barbier?8 prepared a number of addition compounds of hexamethylenetetramine with hydroferro- and ferricyanides. Krohnke79"*82 studied a number of ferro-, molybdo- and tunqstocyanides of organic compounds and derived a correlation between 81 colour and constitution of these compounds. He showed that almost all hexacyanoferrate (II) with electrophillc cations exhibit oxidation- reduction bathachromism. Electrophillc substitution in the cation, which decreases the basicity, deepens the colour and decreases the stability and vice-versa. He prepared the hexacyano ferrates (R CH2Y)4[Fe(CN)2 (Y-pyrldinium) with the following R groupst