Transition Metal Cyanides and Their

Transition Metal Cyanides and Their Complexes B. M. Chadwick and A. 0. Sharpe... 

Gerard R. Dobson, Ingo W. Stolz, and Raymond K. Sheline Transition Metal Cyanides and Their Complexes... 

The attraction that the polynuclear transition metal cyanides have always had for chemists may be ascribed mainly to the characteristic deep colors of some members of this class of compotmds. These inorganic poisoners offer a rich variation in their stoichiometries and hence in their electronic properties. The broad range of different compounds having the general composition Mt[M (CN) i]j H20 is due to the large number of possible combinations of transition metal ions and M , particularly by considering the different possible oxidation states. As special cases we will find a number of mixed valence compounds, a class of compounds which has attracted widespread attention during the last few years (77, 18). 

Cyanides or nitriles, RC=N, have two regions for electrophilic attack, namely the triple bond TT-electrons and the lone pair on nitrogen. The two possibilities of attack are illustrated by the behaviour of cyanides with respect to transition metal ions. In their gas-phase studies on the interaction between cyanides and transition-metal ions Chen and Millerconcluded that both side-on and end-on interaction is present. In this respect cyanides resemble the isoelectronic dinitrogen. 

The field of transition metal cyanide chemistry has a remarkable history that spans almost three centuries, dating back to the early eighteenth century. The wide availability of transition metal cyanide complexes together with their diverse bonding and structural chemistry has led to their widespread applications in the field of materials chemistry. Several detailed reviews on metal cyanides describing their structures, reactivity, and physical properties have been published over the years.A fully comprehensive review highlighting the most important advances in this area was written by Dunbar and Heintz in 1997. ... 

Environmentally important anions such as sulfide and cyanide can be determined very sensitively via amperometric detection. For their separation on a conventional lonPac AS3 anion exchanger (Fig. 3-85), a mixture of sodium carbonate and sodium dihydrogen borate is used as an eluant. A small amount of ethylenediamine is added to the mobile phase to complex traces of transition metal ions, which could be present in the eluant [88, 89]. While the detection of these two anions is very easy, the interpretation of experimental results for the investigation of real-world samples is very difficult. These samples normally contain transition metal ions in their presence, sulfide and cyanide do not exist or only partly exist as free ions. However, only free ions are detected under the chromatographic conditions listed in Fig. 3-85. 

Hence it is more appropriate to view noble-metal carbonyls and their derivatives as coordination complexes of CO rather than as organometallic compounds. A comparison to metal cyanide complexes is far more appropriate, in particular since [Au(CO)2]+ and [Au(CN)2] as well as [Pt(CO)4]2+ and [Pt(CN)4]2- are isoelec-tronic and isostructural pairs. Strong bonding similarities have been established for the first pair (9). It is unlikely that cationic metal-carbonyl complexes will ever form so many different species as the cyanide complexes, which are known for most transition metals (86). With useful and facile synthetic routes available, further examples of cationic metal carbonyls with similar bonding and spectroscopic features as described here should be prepared and characterised in the future. 

The first polyphosphino maeroeyeles designed speeifieally for use as transition metal binders were reported in 1977 in back-to-baek eommunications by Rosen and Kyba and their eoworkers. The maeroeyeles reported in these papers were quite similar in some respeets, but the synthetic approaches were markedly different. DelDonno and Rosen began with bis-phosphinate 18. Treatment of the latter with Vitride reducing agent and phosphinate 19, led to the tris-phosphine,20. Formation of the nickel (II) complex of 20 followed by double alkylation (cyclization) and then removal of Ni by treatment of the complex with cyanide, led to 21 as illustrated in Eq. (6.15). The overall yield for this sequence is about 10%. 

The complex cyanides of transition metals, especially the iron group, are very stable in aqueous solution. Their high co-ordination numbers mean the metal core of the complex is effectively shielded, and the metal-cyanide bonds, which share electrons with unfilled inner orbitals of the metal, may have a much more covalent character. Single electron transfer to the ferri-cyanide ion as a whole is easy (reducing it to ferrocyanide, with no alteration of co-ordination), but further reduction does not occur. 

Zinc, cadmium and mercury are at the end of the transition series and have electron configurations ndw(n + l)s2 with filled d shells. They do not form any compound in which the d shell is other than full (unlike the metals Cu, Ag and Au of the preceding group) these metals therefore do not show the variable valence which is one of the characteristics of the transition metals. In this respect these metals are regarded as non-transition elements. They show, however, some resemblance to the d-metals for instance in their ability to form complexes (with NH3, amines, cyanide, halide ions, etc.). 

The cyanocarbonylchromates(O) [Cr(CO)5CN]- and cis-[CifCOMCN) ] -, which are formed by the oxidation of [Cr l,(CO)s]2-with aqueous solutions of KCN or with (CN)2 or ICN have been discussed earlier (55, 56). Further studies have shown that the reaction of metal carbonyls or their derivatives with solutions of KCN in liquid NH3 is a most advantageous method for the synthesis of new cyanocarbonyl metalates of transition metals, since, in liquid NH3, alkali cyanides are not subject to solvolysis, and also since the broad temperature range of -78 to +120°C may be employed. Thus, we have been able to obtain quantitative yields of the tricyano-tricarbonyl metalates (0)/ac-[M(CO)3(CN)3]3 (M = Cr, Mo, W) by direct reaction of the hexacarbonyls (21) ... 

Based on their easily tunable photophysical and redox properties, transition metal complexes are versatile components to be used in the construction of photochemical molecular devices. The studies presented in this article show that the combination of the Ru(bpy)22+ photosensitizer and cyanide bridging units allows the synthesis of a variety of polynuclear systems that exhibit interesting photochemical properties. Depending on the nature of the attached metal-containing units, supramolecular systems can be obtained that undergo efficient photoinduced intramolecular energy or electron transfer processes. 

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