PHYSICO-CHEMICAL STUDIES ON MIXED LIGAND COMPLEXES

Abstract

Before taking up any research project on complex metal cyanides, one is naturally reminded of the first few compounds of the series, named as iron blues, discov ered some two and a half centuries ago: . Ihese compounds were not only found to be industrially important as pig ments, but posed intricate problems regarding their composi tion and structure, which in later years laid a sound foundation for the planned development of the chemistry of transition metal cyanides. The powerful coordinating power of cyanide ion and its capability of stabilising a wide range of stero-chemical configurations and oxidation states is now well established. Physico-chemical investigations on these complexes can be traced back to the fag end of the last century. Obviously early researches were based on chemical analysis. 'ihese were followed by more critical and precise studies involving the use of electro-analytical techniques(first initiated by I.k.Kolthoff), spectrophotometric methods such as U.V., I.R.,E&B. and N.M.fi. etc. with the introduction of Mossbauer spectroscopy renewed interest on the struct ure of iron blues and related compounds has been exhibited in recent years. K'or studying the solution chemistry of these compounds, polarography and amperometry are some of -2- the new techniques worth applying more widely than hither to emphasised by various workers. Inspite of the large amount of data available on various transition metal cyanides, there are many fields which require better assessment, deeper probe, and more comprehensive experimentations. J^'rom purely structural point of view, the results of magnetic and spectral studies often pose problems which defy simple and straight-forward solu tion. While from the view point of the analyst, the focus of attention needs a shift from their use as analytical reagents in volumetric and electrometric titrations to their applicability as colour("metrie reagents. Even new direct ions are to be given by finding greater use of these compounds in radio-analytical teenniques. The fact that these compounds undergo photo-chemical decomposition came to light some twenty five years back, in recent years not only the qualitative information but also the quantitative data are available on this aspect. Lot only nexacyanides of iron but also the octacyanides of metals like molybdenum and tungston can be subjected to photo chemical studies and thus offer fresh avenues for research. On reviewing back as far as 1923 one comes across some isolated references on the reactions of organic bases and other organic reagents such as alicaloids and dyes, ihis -3- study has also reamined in oblivion for many decades. It is on_y recently that concrete steps to study these interesting reactions have been taken up. For the physical chemist, complex metal cyanides nave a special charm. It was from one amonst these comp ounds that a membrane was prepared to demonstrate the phenomenon of osmatic pressure.That it is these compounds which act as semi-permeable membranes, ion exchangers and media for the transport of cations and anions is no more a hidden-fact. Their existence as stable colloidal solut ions as well as thixotropic gels (pointing towards poly merisation) is something which is well known now. Existing literature offers many references on the mixed ligand complexes of 0,h and S, containing ligands. It is not equally true for complexes having cyanides as one of the complexing agents. This field of study, although confronted with various theoretical and practical diff iculties, is worth emphasising for more comprehensive and critical studies. Tne technologists have found little use of complex metal cyanides in industry except tne possible use as mordents in dye industry1. The high tinctorial power of trie compound 'Inorganic Maroon' with the approximate composition k2Cu[ile(Cn)6 J suggests further investigations _4- of the heavy metal salts of the complex iron cyanides which may find application in dyeing of the newer synthe tic fibres. The use of newly discovered organic derivat ives for this purpose is indicated. Recently, metal hexacyanoferrates(II) have been proved to be superior to other Inorganic complexes as ion-exchangers. Whether complex metal cyanides as such or mixed complexes in diff erent oxidation states can be investigated for exhibiting semi-conducting properties is a problem worth giving due consideration. The influence of hexacyanoferrates in biology and medicine e.g. in various prophyrin type iron complexes, vitamin E,2, metal phthalocyanin etc., is also being reco gnised. In general, complex cyanides do not appear to have markedly poisonous properties unless one or more cyanide groups are removed from the coordination sphere. The replacement of easily removable cyanide groups by amino acids and the formation of mixed ligand cyanides may also be of great biological importance. The proteins are life sustaining materials and are known to react with metal complexes. These reactions are found in some biological processes3,4, e.g., tne action of enzymes on substrate in presence of metals can be explained on the basis of such reactions. In view of tne importance of complexes as a means -5- of activating physiological and biochemical reactions, their studies have become of great importance. Complexes of the type (RnfcuJ jh^QTe(d06 J were used for protecting organic materials and agriculture crops from bacteria and fungi. Bonding properties of cyanide ion Any attempt to highlight, the structural aspects of complex metal cyanides would meet with little success unless one acquaints himself with the bonding properties of the cyanide ion^ oased on molecular orbital considera tions. On that basis the cyanide ion can be represented as KkC (o-2s)2(o-*2s)2j (tf2p)2U2p)4U*2p)°j _] The above molecular orbital configuration depicts two lone pairs of electron, one on the carbon and the other on the nitrogen atom, iheir overlap with the metal d-orbitals results in a ©-coordinate bond which is predominantly a carbon-metal rather than a metal-nitrogen bond due to greater electronegativity of nitrogen. Furthermore, such an efficient ligand metal a bonding provides conditions conductive to u bonding through back donation from the metal d* electrons to the vacant (u 2p) orbitals on CN""(dis-pw bonding). This effect is further augmented by the lower oxidation states of the central metal ion. -6- Spectral, magnetic and x-ray diffraction techniques have been widely employed for characterising these compl exes. Several compounds such as KgUfifGE), J etc. have been found to exist as dimers as evidenced from their diamagnetic behaviour. Further evidences of polymerisation including existence of bridge structure in metal cyanides has been obtained from x-ray studies. Existence of polynuclear structures in the case of metal ferro- and ferricyanides in 1937 and dipropyl gold cyanide was proposed in 7 the late thirties. For the latter a tetrameric structure given below was suggested on the basis of x-ray studies. R * / / o -_ Au.-c = N •— Au.- R I ' N C 111 ill C N I / R — £u,— M = C— Au.—R k i Similarly silver and gold cyanides have been shown to contain infinite chain of metal held together by cyan ide bridges. X-ray studies on the copper, cobalt, nickel and lead ferrocyanide studied by Weiser, Milligam and Bates8 had also provided positive evidence about the polymeric nature of complex metal cyanides. X-ray diffraction -7- and electron diffraction methods were employed by hulme and Powell as well as by Riganontic u to pure cubic structures of B-tetramethyl ferrocyanide. Since poly meric structure involves a metal carbon bond, invariably the x-ray studies elegantly support the theoretical con cepts on metal cyanides developed on molecular orbital consideration. Infrared spectroscopy has also played an important part in elucidating the structure of cyanide complexes. A number of metal substituted ferro- and ferricyanides of Cu,I,i, Co,Mn have been subjected to infrared studies . Tne far infrared spectra of hexacyanoferrate(II and III) 12 have been measured, in the crystal state. The assign ments of the measured absorption bands to low frequency normal vibrations was made and previously accepted values of bending force constants 6(C-L-0) have been modified. Shilt13 carried out studies on the cyanide stretching frequencies of the mixed ligand complexes of iron, ruthen ium and osmium. Re observed the interesting regular IE behaviour viz., the splitting and shifting of the fre quencies to lower frequency region as the atomic number of the metal increases. The structure of potassium cuprocyanide has been studied with the help of Raman spectro scopy14". The low intensity absorption bands observed in -8- the case of octahedral and square planar transition metal cyano complexes have been treated with tne help of group theory and the absorption spectra of these complexes have been interpreted, i-iossbauer spectra of various ferrocyanides have been studied in detail . Spectra of ferrocyanic acid and its various derivatives have also been reported. The nossbauer effect has been used to deter mine the electronic structure of iron atoms in complexes with cyanide and other ligands. Recently Hossbauer studies of the anhydrous ferro and ferricyanic acid has confirmed tne presence of intermolecular unsymmetrical hydrogen bond- W1-22. The additional chemical evidence supporting the x-ray diffraction as well as spectral datas that the metal is joined to carbon of the cyanide group has emerged as a result of studies by Rolzl on methylation reactions of complex cyanides. He found that alkyl isocyanide complexes were formed which yielded alkyl isocyanides on hydrolysis and not alkyl nitriles. All these studies support the Hypo thesis that the metal is attached to carbon. The cyanide ion has unshared electrons both on the carbon atom and on the nitrogen atom and as such one might expect to find isomeric series of complexes corresponding to the nitriles and isonitriles of organic chemistry. Existence of such compounds has not be observed; so it is concluded that tne -9- attachment of the cyanide ion to any given metal ion takes place through the same atom. The preponderance of the evidence indicates that in complexes of the type _y- [M(GS) J , union is always through carbon. Substituted complexes of metal cyanides Barbieri , for tne first time in 1934, succeeded in preparing compounds intermediate between hexacyanoferrate( II and III) with 1,10 phenanthroline and 2,2', dipyridyl, Schil/t and others nave studied a number of 2S—28 29 mixed cyanoferrates. Schilt J showed that the neutral dicyano-complexes with l,10,phenanthroline and 2,2',dipy ridyl display reversible reactions with strong acids and suggested the protonation of trie central metal ion. hairier and Orgel gave evidence which strongly suggests that protonation occurs at the cyanide to give rise to isocyanide ++ [Je(Phen)2(0=?NE)2 J , these xindings were later confirmed by Schilt himself51. 32 Margerum and Morgenthraler studied the reaction of cyanide ion with FePh, where (Ph) is for l,10,phenanthroline, wnich according to them followed two paths. The first one corresponds to the first order dissociation of 1,10, Phenanthroline from the complex followed by rapid -10- addition of two cyanide ions, 2+ 0'erh3J2+ ~ > ^ePhgl + Ph [FePh2;]2++20ir fast > CFePh2(Ci02H The second path is a second order reaction with dir ect cyanide attack followed by the rapid addition of another cyanide, 2+ „ _ + jPePh^ ]] + ON" - > [JePhgCN _] + Ph Q'erh20xj ++ CN~ —ffist •> fePhg (CO 2_] It was shown by Balzani et.al * that (JeCCN) 4(AA}/]] and Q?e(CN)2(AA)2 33 were the main products while small amounts of [fe(AA)* J were also produced. AA represents 33—35 1,10, phenanthroline or 2,2', dipyridyl. kinetic studies show that the above products are formed through thermal substitution reactions inserted on the photostationary equil ibrium of the ferrocyanide and its aquation products.ihe substitution is supposed to take place by the followin mechanism: T _ 2- |Je(CH')4(AA") (^") J Taa 4- . _3- , I 2-, 2+ HFe(C,.)6J fJ^ [?e(CN)5H203 ^ ffe(CH)4(H20)2] ^ J?e(H20;6] 3-:^n. 3_ 2- sU' ye(Cx05(AA~) _] Q,e(CN)4(AA)] JXAA)^-2+ 4> 2- [|e(CN)4(AA) _] -11- Bue to its rigidity l,10,phenanthroline cannot behave as a monodentate ligand which accounts for the difference in quantum yields for 1,lO,phenanthroline and 2,2', dipyridyl. The value of 0 was found to be of the order of 10 to 10 and dependent on the concentration of [Pe(Cl06 ~2^~ and light intensity. Mixed ligand cyanoferrates( rll) with l,10,phenanthroline and 2,2', dipy ridyl and their tris-base ferrate(III) have also attracted the attention of a number of workers ' u. a few of these tris-base ferrates have been used for colourimetric esti- 41,42 mation of iron ' . The nature of the mixed complex cyanides, contain ing the NO group has been the subject of much discussion. In these compounds the NO group could be bonded through + — nitrogen or oxygen and may be present as BO , NO or BO . It has been possible to distinguish between these formula tions by measuring the magnetic susceptibility. Recent studies on iw[Cr(0N)5B0J .h20 gives the magnetic moments 43 for Or in this complex as 1.87 B.M. . This is close to the theoretical value of 1.73 B.M. for one unpaired electron hence the compound is written as a Or(I)-NO complex. Reactions with organic bases A number of hydroferrocyanide and ferricyanides of organic bases (aniline, o-toludine, dimethylaniline,pyridine, -12- benzidine etc.) were prepared by Cumming ' . Gadreau and Barbieri studied the reactions of ferrocyanides with alkaloids and hexamethylene tetracaine. It is well known that ferrocyanic a.cid forms a number of oxonium salts 49-51 and methylderivatives' . Except for some isolated cases these studies nave been of qualitative nature. Krohnke prepared a number of ferro-, molybdo- and tungstocyanides of organic compounds and derived a correlation between 52-56 colour and constitution of these complexes, herington studied on the reactions of primary amines with pentacyanoamminoferrate( II,III) and demonstrated the utility of these reactions in spot test and paper chromatography. An extensive study of the organic base ferro and ferri cyanide has been done in this laboratory by Bwivedi and Bhargava. Prussian blue analogs The early x-ray studies of Keggln and Miles have rev ealed the structure of the ferro- and ferricyanide pigments. Davidson extended the concept of super complex formation proposed for Prussian blue in explaining the existence of complexes like ferric berlinate and ferrous berlinate. A structure similair to this is probably common to all the heavy metal ferrocyanides, variations being introduced,as the nature of second metal ion is changed. The complex -13- AS4_0le(Gi«)5 J 'should be formulated as Ag, j~AgPe (CB)gj . Assuming that the coordination number of silver is two and its covalences are linear, each of the coordinated silver atom must share electrons with the nitrogen atoms of two different Pe(CN),- units, thus forming a giant polymer. There is a revival in the studies of Prussian blue 59—61 analogues during the last two decades. Shriver et al. published a number of papers and studied the problem from different angles using x-ray, infrared and nossbauer spectro scopy. Re developed a new concept of linkage isomerism in which interchange of metal ions between the carbon and nitrogen sites takes place M(NC)6M'(S) >iu'(^C)6M(S) The studies were extended to a number of metal ferro cyanides and it was observed that these Prussian blue ana logues undergo rearrangement and interchange of sites. Bonding properties of l,10,phenanthroline and 2,2', dipyridy