PHYSICO-CHEMICAL STUDIES ON METAL CYANO-COMPLEXES AND THEIR INTERACTION WITH HEAVY METAL IONS

Abstract

The cyanogen complexes of molybdenum and tungsten belong to the class of compounds in which the metals in their higher oxidation states get stabilized through coordination. Unlike the familiar cyanide complexes, they are octa coordinated and owe their stability due to the special steriochemical arrangement of the ligands around the metal atom. A few compounds of this class are known and the only characterized complexes, besides molybdenum and tungsten, known are those of rhenium. The compounds exist in different oxidation states. For example, Mo(IV) and Mo(V) compounds have been prepared in stable form with the molecular formula K4Mo(CN)8.2H20 and K3Mo(CN)g respectively. The Mo(lV) compound possess an effective ionic radius corresponding to Mo(CM)o" and water molecule attached to it lies in spaces between cyanide groups. Mo(V) compound is extremely photosensitive and is readily reduced to the octacyanide molybdate (IV). Mo(III) cyanides complexes have also been prepared but both solid and its solution are very readily oxidized to Mo(IV) and Mo(V). The cyanide complexes of tungsten resemble those of molybdenum except for the fact W(III) complex is not known. Both the W(IV) and the W(V) have been obtained in the stable form, the octacyano molybdate (V) being extremely photosensitive. The cyanide complexes of molybdenum and tungsten also differ from the corresponding hexacyano compounds in given mixed complexes with increased coordination numbers. Compounds, with the molecular formula Mo(CN)qR2", W(CN)gR2 (where R is H20, NH3 or N2H4) have been reported (1,2) besides the usual K^MofCN^OH)^ , Mo(CN)4(MeNC)4,2MeOH, K4[Mo(CN)5(OH)2No] , K4(W(CN)5(0H)^).6H20, (3) K4[w(CN)4(0H)J etc. Conductivity measurements and reaction with metals (vide infra) support the existence of such deca-coordinated complexes. Structural considerations show that all the known eight-coordinated complexes involved metal atom with one or two d-electrons, and are usually confined to transition elements of the second and the third row, though recently some eight coordinated titanium complexes have been reported. Two arrangements are favoured for this coordination number, the antiprism and the dodecahedron, and in both cases the crystal-field forces would lead to the stabilisation of one d-orbital over the other four (the d 2 in the case of the z square antiprism and the d in the case of dodecahedron)(4). xy 1 2 The tendency of d - and d -complexes to adopt eight coordina tion is, therefore, understandable. Only one cyanide complex with this coordination number has been studied by X-ray diffraction, and from this it appears that K4fMo(CN)^has a dodecahedral configuration (5), although it would be expec ted that the antiprismatic structure might be more stable. Orgel(6)has explained the favouring of the dodecahedron for [Mo(CM)gj "on the grounds that d orbital has the correct symmetry for metal-carbon"TT -bonding, whereas the d 2-orbital would provide poor back-bonding facilities. On this basis, four of the ligands would be in a better position to form "ff"- bonds by receiving d -electrons from the metal than the xy other four, so it would be expected that four of the cyanide groups would be less strongly held to the metal atom and could be replaced by four non-IT-bonding ligands to give such complexes (_Mo (CN)4(0H)J "(7). Recent Raman spectros copic study of the [Mo (CN)J1 " ion indicates that the structure in solution is that of an Archimedean antiprism (8), in which case the possibilities of metal-ligandIf-bonding would seem to be greatly reduced if the d 2 were to be the only metal d-~ -orbital available. The cyanide complexes of molybdenum are also known to have coordination numbers 7-, 6-, and 5, No hepta coordinated cyanide complexes have been definitely establish ed. The black compound K4rMo(CN)_j ,H20 contains molybdenum III (9) and has an unpaired electron per molybdenum atom(10), so that it may be [Mom(CN)7]4~ or (wio(CN)7(H20))4". The existence of 7-coordination compound is also borne out on the basis of nine-orbital rule. Hexa coordinated molybdocyanide have not been reported, although Tc(IV) cyanide has been shown to exist in the presence of an excess of cyanide ions (11,12). Little evidence for the existence of the penta coordinated cyanides in the solid state is found; most of the complexes alleged to have this structure exist only in solution where they probably take up a solvent molecule as the sixth ligand, K[Mo (CN)^J has been briefly reported but incompletely characterised (13), Mo(IV) and W(IV) form stable cyanide complexes like most other metals, Bjerrum (14) has calculated some free energies of complex formation (relative to the values for the corresponding aquo-complexes) computed from mean complexity constants, and thus demonstrated that most cyanide complexes have a metal-ligand affinity greater than those of aquo-, ammino-, or halogeno-complexes. The stable complexes resembles Chatt's'group1 "bH of coordinate with carbon, their 'class1 "b", character depending on the availability of lowercl -orbitals forTT-bonding with the ligand. Standard oxidation potentials have been determined for both the molybdo-molybdicyanide and tungstotungsticyanide complexes. The values coated in the literature are not uniformly the same. For example, Latimer (15) has reported the value of -0.57vfor W(CN)4"/W(CN)g~ while Baadsgaard and Treadwell (16) have coated a value of-0,457V for this very couple. Many authors have investigated the oxidation potential of molybdo-molybdicyanide couple, Collenberg(17) reported a value of 0,939 V. in acidic medium, while Kolthoff and Latermer(18), have found 0.73 V. as the oxidation potential of the couple in the neutral medium. . In some cases, the presence of neutral salts influenced the oxidation potential values. Mikhalevich(19) has recently determined the value of the molybdo-molybdicyanide couple and found to be 0.82 V, Recently Malik and Ali(20) have found the E° of the molybdo-molybdicyanide couple at various ionic strength and in presence of different concen tration of acids. The oxidation potential values given by them, range between -0,852 to -0,872 V, in fairly acidic to strongly acidic solutions. Little work has been done on the replacement reactions of metal cyanides. This is especially true for molybdenum and tungsten octacyanides. The replacement of water in hydroxy or aquo-complexes by various ligands such as NH-", SCNT, Br" and other nucleophilic groups can be expected to take place in these complex cyanides also. Just like oxidation potential and replacement reaction studies, kinetic studies on these metal cyanide have not been adequate, Ligand exchange studies carried out with radiocyanide provide exchanges either too fast or too slow to be measured. Recently Goodnow and Garner(21) observed that no exchange of radiocyanide with W(CN)g" and W(CN)g" ions in aqueous solution of neutral pH occur in dark even after 100 days. However, these exchanges are strongly light accelerated and the rates under the illumination condition used are approximately independent of the concentration of free cyanide of complex ion