Tri/polynuclear complexes

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

As it was shown in [15-18], such compounds, with bridging atoms of deprotonated amino alcohol, are formed when aminoalcoholate complexes of metal (III) react with bivalent 3d-metal ions. Many representatives of these compounds were synthesized in crystalline state the polynuclear compounds were also found to form in aqueous and methanol solutions. The structure of 2Co(III) - Ni(II) tri-nuclear complex, according to [17,18], is shown below in Figure 2. 

In a number of polynuclear complexes various modes of bi-, tri-, and even tetrametallic connectivity (Scheme 5) may occur and several examples are known for each one. 

Tris-carbamato-iron(III) complexes [Fe(02CNR2)3] have been prepared for R = ethyl, isopropyl, cyclohexyl (cx), and benzyl, and binuclear //-oxo derivatives (for R = Et, cx) and various /U3-OXO, //4-0X0, and /i-carbamato polynuclear complexes also obtained. Iron(III) chloride reacts with potassium 2-prqpanenitronate, K(pn), to give [Fe(Me2C=N02)3] (mean Fe—0 = 2.019 A, mean bite angle 66.0 A), which in ethanol gives binuclear [(pn)2Fe(//-OEt)2Fe(pn)2]. ... 

Ruthenium has a considerable propensity to form polynuclear complexes, particularly with carboxylate ligands which as bridging ligands span the Ru centres, sometimes accompanied by a bridging 0x0 ligand. Preparation and properties of bi- and tri-nuclear acetato complexes of Ru have been reviewed [552]. 

The tetraanionic [H2O Al3(/r-OH)(/r-OR)3]" [R = C(CH2C02)2C02] moiety has been achieved by the combination of (H20)9 Al(N03)3 with citric acid [101], with the synthesis establishing the existence of polynuclear complexes of a type previously postulated [102, 103], Al NMR spectroscopic studies have established that the tris(aluminium) cluster (Section 3.2.5) is not fluxional in D2O but instead retains three distinct metal environments over a wide pH range (3.0-9.0) with acid hydrolysis occurring below pH 3.0 and the resultant spectrum revealing the presence of [(H2O), (RO)Al3] and/or [(H20) (ROH) A13] " and also [(HjOlg Al] "" [101]. 

Pavlishchuk, V., Birkelbach, F., Weyhermuller, T., Wieghardt, K., and Chaudhuri, P. Polynuclear complexes of the pendent-arm ligand l,4,7-tris(acetophenoneoxime)-l,4,7-triazacyclononane, Inorg. Chem. 41 (2002), 4405-4416. 

Dimetalcyclo-forming ligand systems are represented by ligands of tri- and higher dentacity. A permanent interest in these compounds is related mainly with the possibility of creation on their basis of bi- and polynuclear complexes with anomaly magnetic properties [85,86]. Tridentate azomethines of type 640, an im-... 

In preparation of a Co(XII) complex by oxidation of an ammoniacai solution of CoCl2, there are three possible coordinating groups present in solution, Cl, H20, and NH3. Ignoring the possibility of hydroxo complexes, peroxo complexes, and polynuclear complexes, show that there arc 57 possible octahedral complexes which could be formed using these ligands alone. (Don t try to draw all of them )... 

In this chapter we have tried to summarize the current status of the chemistry of quadruply bridged dimetal complexes of the group 10 elements. The area is closely related to that involving similar compounds of the group 9 elements and also the doubly bridged di- and polynuclear complexes of platinum and related metals, which are not included here. We have been able to include the latest developments in this subject area. We have not attempted to be comprehensive, and some omissions may have resulted. 

Chemisorption is very pronouneed if the radionuelide is able to react with the surface. An example is the sorption of hydroxo complexes of tri- and tetravalent elements by the silanol groups on glass surfaces. Sorption increases with the formation of mononuclear hydroxo complexes in solution and deereases with the eondensation to polynuclear complexes at higher pH values. Several measures may be taken to suppress ion exchange or chemisorption of traces of radionuclides on glass surfaees ... 

Clathrochelate ribbed-functionalized tris-dioximates have attracted interest because they offer scope for the synthesis of polynuclear complexes with targeted structural parameters and physicochemical properties (see above). In most instances, it is not necessary to functionalize all a-dioximate fragments, and it appears to be sufficient to modify only one of the three ribs in the clathrochelate framework to alter the properties significantly. Several feasible synthetic routes to clathrochelate monoribbed-functionalized tris-dioximates have been proposed in Ref. 68. A direct template condensation of the mixture of a-dioximes with Lewis acids on a metal ion (Scheme 15, Route I) leads to the formation of a poorly separable mixture of nonsymmetric and symmetric products, in which the latter predominate. Halogenation of the initial clathrochelate... 

In the first part of this chapter we have tried to present the general features of chirality in transition metal complexes. In the remainder, we shall discuss a number of polynuclear complexes where chirality is important, either as a factor in analysing the structure or as a means of studying the properties. 

The importance of metal clusters stems from our efforts to understand the clusters-to-bulk problem, i.e. the correlation between properties of molecular and solid states of matter (mononuclear complex => polynuclear complex (cluster) => bulk metal). A considerable research effort (theoretical and PES)26 has been expended on trying to obtain the following information about gold clusters ... 

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