PHYSICO CHEMICAL STUDIES ON THE REACTION OF POTASSIUM CHROMICYANIDE WITH ORGANIC BASES AND HEAVY METAL IONS

Abstract

Before introducing the subject matter of my thesis on the organic base complexes of Cr(III) cyanides, it appears to be very appropriate to mention a few general ities about the co-ordination compound* of Cr(III) and the chemistry of the cyanide ion. -2- The Cr(III) complexes are relatively easily prepared and are quite stable with respect to decomposition by heat, moisture and atmospheric oxygen. A large number of compounds of this metal ion, obtained by co-ordination through oxygen , nitrogen, halogen, carbon, sulphur and even selenium atoms, have been reported in the literature. The chemistry of many such compounds has been surveyed and critically discussed in a number of monographs ' , reviews 'and text books ~ . From the structural point of view these complexes, both as solids and as solutions, are of octahedral symmetry and those, that are not, are usually treated as distorted octahedra. A highly interesting feature of the chemistry of Cr(III) complexes is that, they undergo photoaquation, although photo- 11 12 decomposition has been claimed by some xjorkers • on irrad iation by light of wavelength 254 nm. The quantum yields for photoaquation reactions are generally high, wavelength and temperature independent (except in the case of the photoexchange13 of Cr(H20)g3 with water) and are found to increase with ligand field strength except in the aquation • , of fcr(CN)|" Jm^ Cr(CN)5(H20)3"J . On the other hand, the quantum yields in the case of complexes of non-octahedral symmetry are variable, ranging from near unity to below 10~*. Unlike the complexes of octahedral symmetry, they are temperature and wavelength dependent. The existence of fluoroscent emission in Cr(III) •1 £. TO complexes has been exhibited. It has been attributed to 4T2 —4A2_ transition. The emission is broad and considerably shifted to the red-relative to the thermal 19—21 absorption band. The cyanide complexes of chromium, although not so extensively investigated as that of other ligands, offer many potentialities for physico-chemical, structural, synthetic and analytical investigations due to the powerful coordination tendency of the cyanide ion and its capacity of stabilizing a wide range of stereo-chemical configurations and metal oxidation states. The molecular orbital configu ration of this ion is KK|(o2s)2(o2s)2|(o?p)2(x2p)4(«*2p)°j, which allows the two lone pairs, one on the carbon and the other on the nitrogen (lying on the carbon nitrogen axis) to undergo coordination through overlap with the metal d-orbital. Liter- 22 2A ature survey *"***, however, reveals that bonding of the cyanide ion is usually through carbon except when the cyanide actB as a bridge. In that case, the bonding takes place through both, the carbon and the nitrogen ends. The cyanide ion can act both as a o-bonding ligand as well as a w-bonding ligand. But out of these two modes of bondings dn, pu bonding through back donation is of great sig nificance since it avoids the accumulation of excess. negative charge on the metal atom thereby strengthening the metal carbon bond and weakening the metal nitrogen 25 bond. Recent X-ray and neutron diffraction measurements have provided the evidence for such bondings. Infrared 26—29 studies reveal that transition metal cyanides show a sharp and intense C B N stretching frequency at 2100 cm" and for bridging cyanides at 2200 cm""1. Most of the struc tural studies3 33 lend support to the view that the cyanide ion is bonded to metal through carbon atom. There exists a great similarity in the cyanide and the carbonyl complexes. Grey and Beach have compared the d-d and the charge transfer absorption bands in their d metal complexes. The charge transfer transitions were assigned by them as metal (d) to ligand (nm). The most striking similarities between the cyanide and carbonyl complexes lie in the isoelectronic pair, Cr(COl)g and Cr(CN)g and their mono substituted products; Cr(C0)cA and Co(CN)cX. The similarities are both in the spectra and in the excited state assignments. The d-d transitions in Co(CN)|" are similar in type to those for Cr(C0)g. Cr(III) complexes typically show at least two quartetquartet transitions, usually in visible region, and at least one of these is spin forbidden, which is sometimes not resolved. The energy of first quartet-quartet transition, 4A2« * 4t2«» £ives directly the crystal field parameter 10 Dq and thus the position of the ligand in the spectro- 5- chemical series! ClT >NH3> SCN~ >HgO cf c2°4~ >^rea> Cl"1. The molecular orbital treatment shows the electron in the first two excited quartet states to be in or - antibonding orbital. The lowest doublet state 2Bg involves only spin pairing within a nonbonding set of orbitals. The tran sition 4 A~ —^2Sg is relatively insensitive in energy to the type of ligand and the doublet typically occurs in the region 700-650 nm. It is fairly narrow, but may be 35-38 structured, indicative of vibronic coupling," * The structure of Cr(CN)|" ion in K5Cr(CN)6 was 39 proved by Kalandiya and was represented as given below. NC. ^CN NC ~^Cr CN NC ^ ^CN Reactions of complex metal cyanides. The earliest investigations, involving replacement of cations, began with studies on iron blues. A large number of heavy metal ferro and ferricyanides have been prepared and their compositions determined. Bhattacharya and Malik40 determined the composition of zinc, manganese, 41.42 nickel and cobalt ferricyanides. Malik and Abubacker studied the interaction of Cr(II) with K4Fe(CN)6 and -6- K3Fe(CK)6. They have shown that Cr(II) ferro-and ferricyanide exist in the form of soluble complexes instead of familiar insoluble ones. Recently Malik and Bembi43 used KAFe(CN)g as an analytical reagent for the colorimetric estimation of Cr(III). Potassium ferrocyanide has been used as a reagent for the volumetric estimation of Cr(lII). Potassium ferrocyanide has been used as a reagent for the volumetric estimation of a number of metal ions. Deshmukh44'4'> has estimated lead, cadmium and cerium. AC Fujita estimated lead, silver, manganese, cobalt,nickel, cadmium and mercury(II), volumetrically, using starch iodine (Ag,Mn,M, Co), phenol red (Pb) and potassium dichromate (Hg and Cd) as internal indicators. Pani and Rao4' reported the formation of molybdenum ferrocyanide. Malik4 has found its composition to be (Mo02) CFe(CN)g3 by chemical analysis and physico-chemical methods. Malik and Iftikhar Ali49**51 have determined the composition of the freshly precipitated molybdocyanides of Cu(II), Co(II) and Ni(II) by conductometry, potentiometry and amperometry. 52 Olsson^ has described a number of compounds of general formula M4£ W(CN)8H xB^O, where M=Ag+, Tl, Mn, Cd and Zn. All the compounds were yellow and crystalline. Malik and Khandelwal53 found that Ni(II) and Co(II) ions react with KAW(CN)8 to give insoluble complexes which have the tendency -7- to pass over readily to the colloidal state. + + + + + The hexacyanochromates(III) of Li , Rb , Na ,K ,Cs , NHA+, PhAN+, Ph4As+ and Tl+ have been prepared.54 The JUray diffraction patterns were recorded. The diffuse reflectance spectra show a hitherto unknown electron transfer band at 45 kilokaysers which possesses vibrational structure in some cases. The phosphoroscene intensity strongly depends on cation. km Recently^ pure potassium chromicyanide has been prepared and its purity checked by thermogravimetry. It is stable both, in solid state under dry atmosphere, and aqueous solution and hence can be used as an analytical reagent. Reactions of complex metal cyanides with organic bases, •56 57 58 59 Cumming , Barbiesi^ and Gadrean prepared hydroferrocyanldes and hydroferricyanides of anilines, otoluidine, dimethylaniline, pyridine benzidine, hexamethyfiO— fi2 lene tetrammine and alkaloids. Krohnke prepared a number of ferro, molybdo and tungstocyanides of organic compounds and derived a correlation between colour and constitution of these complexes. Herington -?~ studied reactions of primary amines with pentacyano amminoferrate (II,III) and demonstrated the utility of these reactions V -8- in spot tests and paper chromatography. 6ft Spacu and Costin prepared compounds of some alkaloids with H^CrfCSOg, H5Cr(SCN)6, H^OsClg and H^OsBrg. An aqueous solution of K,Cr(CN)g was added to an aqueous solution of HC1 or HgSO. salts at 0°C to get the respect ive compounds. Statement of the problem From the literature survey on complex metal cyanides, described in the preceding pages, the various important aspects of the chemistry of these compounds have been brought out and the significant developments in recent years have been indicated. Researches have been mainly done on the following aspects of complex metal cyanides? the physico-chemical aspects, composition and structure, spectral studies, photoaquation reactions with transition metals and their utility as analytical reagents; but all these aspects have mostly been investigated with potassium hexacyanoferrate (II and III) and in a few cases for the octahedral complex (cyanides of Mo and W, Similar investigations on hexacyano- JJchromates(II) and (III) are scarce. Systematic and compre hensive studies on some of the above mentioned aspects of hexacyanochromates(III) were, therefore, considered worth undertaking. The subject matter of this thesis is spread over the following areas! -9- 1. Preparation of some new hydrochromicyanides of organic bases and their characterisation by ampere'ometrie and radiometric titrations (in solutions) and by spectral and magnetic studies (in the solid state). 2. Spectrophotometric (in the visible region) and polaro graphic studies on the composition and stability of the reaction products f Bfi(II), Hg(II), Sn(II), ve(ll), Cr(III) and Mo(VI) with potas ium hexacyanochromate(III) after irradiation with sunlight. 3. Comparative studies on the use of hexacyanochromate(III) and its organic base complexes as analytical reagent for the estimation of metal ions. 4. Lastly, the following less familiar aspects of the chemistry of potassium chromicyanide were studied. (i) Polarographic behaviour of the complexes in different supporting electrolytes, and (ii) analytical aspects of the study of the redox system Cr(II)+Cr(CN)|" ^ Cr(CN)4" +Cr(III). (ill) Mossbauer studies of the compound of potassium hexacyanochromate(III) with Fe(II).