PHYSICO-CHEMICAL STUDIES ON THE IRRADIATED HEXACYANO AND OCTACYANO COMPLEXES IN PRESENCE OF ORGANIC BASES AND HEAVY METAL IONS

Abstract

Alkali metal cyanogen compounds, both hexacoordinated and ^etacoordinated have been extensively investigated from different chemical and physico-chemical aspects since the turn of the present century. Most of the researches on these compounds have centered around their stoichiometry, their reactions with metals, structure and geometry of the complexes formed, redox behaviour and their use as analytical reagents in titrimetric and colorimetric methods. Recent literature on these compounds reveals the interest in the mechanism of nucleophilic substitution and addition reactions, In this context the aqua and hydroxy substitution reactions catalysed by certain metal ions, e.g., Cr(lll), Be(ll), Hg(ll) etc. deserve special mention since they seem to imbibe the coarse of corresponding reactions initiated by exposure to ultraviolet or sunlight. In recent years a large number of photochemical reactions involving photolysis of coordination compounds including hexacyanoferrate(H,IIl), octacyanomolybdate(lV) and tungstate(IV) have been investigated. These studies though useful from kinetic stand point are not comprehensive enough to account for variations in quantum efficiency in the light of accompanying dark reactions. A possible approach in this direction can be to carry out comparative studies of hydrolytic decomposition reactions in presence of metal ions and photochemical reactions initiated on ii exposing the cyanogen compounds to ultraviolet light. Besides water and hydroxyl ions, organic bases can also compete with H20 and OH" as nucleophiles, light irradiated reactions in their presence can also prove useful to know the genesis of these photochemical reactions. The investigations incorporated in the thesis have been divided into five chapters. The first chapter is an introductory chapter giving the literature survey on the reactions of metal cyanides and the statement of the problem. In the second chapter the methodology of the researches undertaken is described. The major part of the research work which is based on the comparative study of dark and photochemical reactions in presence of metal ions and organic bases is described in chapter III. Two aspects which had emerged out as a consequence of the mechanistic studies given in chapter III. viz., possible identification of intermediates and the end product of these reactions by coulometric, potentiometric and polarographic studies and use of irradiated potassium octacyanomolybdate(IV) and tungstate(IV) as analytical reagents. Investigations carried out on these two aspects form the subject matter of chapters IV and V. The quantum efficiency of primary photoproduct of hexa as well as octacyanide systems, were studied by molecular absorption spectroscopy. The photolysis was iii done by monochromatic light of wavelength 350 nm. Several parameters such as time of irradiation, concentration >f the reactants, temperature, solvent composition and pH of the solution affect the quantum efficiency. Presence of metal ions and organic bases also brings about a decrease or increase in the efficiency of the reaction. The bases used were phenylhydrazine, pyridine, morpholine, lysine and glycine while the metals employed were chromium, beryllium, gold, aluminium and vanadium. Photochemical studies : In all the above three systems the optimum time of irradiation was found to be approximately 20 minutes (chapter III PP 57-58). The quantum efficiency of primary photoproduct was constant over the concentration range 0.25 x 10"2M to 1.5 x 10"2M (chapter III p 58). Since the photochemical equilibrium involves a backward dark (thermal) reaction also, lowering uf temperature would normally increase the quantum efficiency. The quantum efficiency in the different solvents varies in the same descending order as the dielectric constant of solvents used. In both the hexa-and octacyanides quantum efficiency was found to be highest in water, followed by water-methanol, water-ethanol and water-acetonitrile mixtures (chapter III PP 59-60). But the different compos itions show that the lowering of the dielectric constant IV cannot be the snle factor responsible for bringing about a decrease in the quantum efficiency values. Solvation of the ground and excited species to different extent could be another important factor contributing to the observed difference in quantum efficiency values (chapter III pp 59-60) The low pH values are found to be most suitable for the present studies, due to the minimum backward (thermal) reaction (pH 4.0 in Fe(ll) and pH 1.5 in Mo(lV) and W(IV)). Therefore, in pH range 1 to 10 the quantum efficiency value decreases with increase in pH and maximum quantum efficiency values/^0.80 are realised only in the acidic medium (chapter III pp 60-65). The photolysis studies in presence of chromium, beryllium, gold, aluminium and vanadium metal ions were found to exert a quenching effect on the primary photoprocess as evident by a lesser quantum efficiency value as compared to that found in the absence of metal ions. The quenching ability of metal ions was confirmed by the linear plot of ratio of quantum efficiency in absence and presence of metal ions versus concentration of metal ions. The quantum efficiency for the studied metal ions shows the following order for the production of the reaction product as a result of primary photoprocess: Cr(IIl) > Be(ll) > AI(III) > Au(lll) > V(V) (chapter III PP 66-68). Contrarily, enhancement in the absorbance of primary photoproduct was found to take place in the presence of phenylhydrazine, morpholine, pyridine, lysine and glycine. Although the spectral changes upon irradiation in the presence of these bases were quite similar to those of parent compounds the production of aqua penta-and aquahepta cyano complexes as a result of the photolysis of the above metal complexes was dependent on the concentration of the bases (chapter III pp 68-70). The red coloured intermediate complex of the primary photoaquation of molybdo-and tungstocyanide, undergoes secondary reactions in alkaline medium to give the blue compound on further irradiation. The polarographic analysis of the blue compound gives two waves. The first wave is ill defined while the second is well defined (chapter III pp 70-71). Similar polarographic behaviour was observed on irradiating the mixtures of octacyanides and bases. The i of the second wave increases manifold in presence of the organic bases (chapter III p 71). This may be attributed to the substitution of the base in the coordination sphere, resulting in the further release of cyanide ion. The amount of cyanide ion released is in the following order : morpholine > pyridine > lysine > glycine, (chapter III pp 70-71). vi Colorimetric estimation of Au(lll) and V(V) ions with irradiated octacyanomolybdate(IV) and tungstate(IV) : Determination of gold and vanadium is important from both the biological and technological point of view. Several colorimetric methods of estimation of both the metal ions are available in the literature. Nevertheless the metal cyanide method can be used for routine microestimations more conveniently and at a much lower cost. The irradiated alkali metal cyanide compounds i.e., octacyanomolybdate( lV) and tungstate (IV) was used for colorimetric estimation of Au(lll) and V(V) metal ions. The intensity of colour was found to be dependent on pH and irradiation time. The optimum pH was found to be 1.5 for V(V) and 2.5 for Au(lll) metal ions. Both the metal ions can be estimated in the concentration range 0.008 x 10~3M to 1.0 x 10"3M (chapter IV p 131). The interference by several cations and anions and their tolerance limits were also determined (chapter IV pp 131-132) The development of colour and enhanced sensitivity on irradiation can be attributed to the formation of soluble complexes of the type Au[Mo/W(CN)6(H20)OHj and KV[Mo/W(CN),(H20)(0H)]2 which through back dark reaction are likely to give hydroxy or aquasubstituted octacyanides. Vll Evidence of the existence of various metal complexes in the hydrolytic decomposition of metal cyanide complexes in presence of certain metal ions : The coulometric and potentiometric titrations of potassium ferrocyanide with chromium, beryllium and mercury complexes show 1:1 ratio in the acidic pH range. In these titrations the potassium ferrocyanide was consumed in excess to that required from theoretical consideration, showing that the aquapent aayanoferrate09obtained from the decomposition of potassium ferrocyanide reacts with metal ions to form complexes of the type K/K2M[Fe (CNLH^] . H20 with all the three metal ions in 1:1 ratio (chapter V p 153). The coulometric titrations of potassium aquapentacyanoferrate(ll) confirm that the generated ferrocyanogen species further react with the metal ions in approximately 1:1 ratio showing that metal ions in hexacyanoferrate(II) systems are not present as simple metal complex species (chapter V pp 154-156).