Polynuclear cluster complexes

Gates and Lamb (64) found that, by heating either Os3(CO)i2 or Os3(CO)i2 bound to MgO under a CO -I- H2 atmosphere at 200-280°C, thermally stable clusters, [H30s4(C0)j2] and Osio(CO)24(C) , were formed in high yield and extracted as the PPN salt. The remaining solid had an IR spectrum characteristic of the red complex [OsioC(CO)24] , which was also extracted as the PPN salt. Similarly, several oxide-promoted syntheses (64-67) have been reported for specific polynuclear cluster complexes using the smaller carbonyl precursors grafted on metal oxide supports ... 

Another noteworthy consequence of the metal-ligand interaction in polynuclear cluster complexes is the reduction of the metal-metal bond strength in a ligated cluster compared to the bare metal core without ligands. To illustrate this effect we consider again Ni clusters and in particular a series of Nig clusters stabilized by terminal CO and 4-X ligands where X = PR, S, GeR, Te, As, etc. Several clusters... 

While discrete two- and three-coordinate Cu(I) compounds have been reported, the favored coordination number of the metal is four. Quite frequently, two or more copper atoms will share ligands, resulting in the formation of polynuclear clusters. Complexes of stoichiometry CU4X4L4 (X = Cl, Br, I L = PR3, ASR3) for example, exist as tetrameric units whose structure depends upon the steric demands of X and R. In... 

Metal thiolate complexes will reduce elemental sulfur or red selenium via the oxidative elimination of RSSR. In a similar manner, metal selenolate complexes ean be used to reduce elemental selenium. The resulting E ligands favor the formation of polynuclear cluster complexes, due to the greater tendency of E (vs. RE ) to form bridging interactions between metal centers. Originally developed in the synthesis of [Fe4Se4(SPh)4], this method has been well utilized in the synthesis of a number of iron thiolate/sulfide clusters, as well as their selenide counterparts (Equation (5)). More recently, sulfur- and selenium-containing lanthanide clusters (see Section 7.2.5.5) have been synthesized by the displacement of ER from Ln(ER)3 ... 

A fascinating variety of discrete (or occasionally polymeric) polynuclear halogeno complexes of As, Sb and Bi have recently been characterized. A detailed discussion would be inappropriate here, but structural motifs include face-shared and edge-shared distorted (MXe) octahedral units fused into cubane-like and other related clusters or cluster fragments. Examples (see also preceding paragraph) are ... 

Lewis LN (1998) In Adams RD, Cotton FA (eds) Catalysis by di- and polynuclear metal cluster complexes. Wiley-VCH, Weinheim p 373... 

Balch et al.560 prepared the cluster [Pd3(/u-dppm)3(//3-I)(//3-PF3)]PF6 [dppm = bis-(diphenylphos-phino)methane] (shown in Figure 115), which contains a PF3 molecule triply bridging a central triangular Pd3 core. However, exceptions are not limited to PF3, an atypical phosphine often considered closer to carbon monoxide. For further examples, see the section on polynuclear Pd1 complexes. 

Moiseev, I. I. Vargaftik, M. N. Catalysis with palladium clusters. In Catalisis by Di- and Polynuclear Metal Cluster Complexes Adams, R. D. Cotton, F. A. Eds. Wiley VCH New York, 1998, p 395. 

In compliance with the nuclearity principle, polynuclear clusters are subdivided into a number of other subgroups, e.g. hexanuclear, octanuclear, etc. The binuclear clusters of technetium may be classified according to the electronic structure of their Tc-Tc2 bonds. Then, the d4-d4 complexes with quadruple M-M bonds are the father of all binuclear complexes with Tc-Tc bonds. The addition or removal of electrons from Tc-Tc bonds [1,11] should result in a decrease in the formal multiplicity of M-M bonds. Thus, for instance, the formal multiplicity of Tc-Tc bonds of d3-d3 and d5-d5 binuclear complexes equals3 3, that of d4 d5 and d4-d3 complexes equals 3.5, etc. 

It is noteworthy that all these reactions are reversible, but under reducing conditions the equilibrium shifts to the right-hand side, and under oxidizing conditions it shifts to the left-hand side. For all resulting binuclear ions at rather low concentration in solutions, reactions take place, accompanied by the rupture of the M-M bond and the formation of unstable mononuclear complexes, which are readily oxidized even by the solvent. At high concentrations of binuclear ions cycloaddition reactions of multiple M-M bonds (these reactions are considered in section 3.5) occur to form stable, under the given conditions, polynuclear clusters. 

The paper electrophoresis experiments carried out to study the mobility of polynuclear technetium clusters in aqueous solutions of HX of varying acidity, as a mobile phase, showed that these clusters were also characterized by reversible reactions such as (5) without leading to destruction of M-M bonds. On the other hand, an autoclave recrystallization of the polynuclear clusters at 200-220°C in an atmosphere of argon from concentrated solutions of HX led to a partial destruction of M-M bonds and the formation of binuclear complexes [Tc2X8]3 and [Tc,X6]2. This indirectly shows that reactions (6) and (7), leading to the destruction of M-M bonds, are likely in solutions of polynuclear clusters [15]. 

For polynuclear clusters similar reactions were not obtained These reactions proceed in the forward direction only at high temperatures ( 120 °C) and in the presence of an excess of the bidentate complex-forming agent, whereas the reverse reactions (20-22) also take place at room temperature in the presence of excess HC1 [11,46,49],... 

It has been our goal for some time to run photochemical energy storage reactions without relay molecules or separate catalysts. We have concentrated on the photochemistry of polynuclear metal complexes in homogeneous solutions, because we believe it should be possible to facilitate multielectron transfer processes at the available coordination sites of such cluster species. 

Manganese represents the epitome of that characteristic property of the transition element namely the variable oxidation state. The aqueous solution chemistry includes all oxidation states from Mn(II) to Mn(VII), although these are of varying stability. Recently attention has been focused on polynuclear manganese complexes as models for the cluster of four manganese atoms which in conjunction with the donor side of Photosystem(II) is believed involved in plant photosynthetic oxidation of water. The Mn4 aggregate cycles between 6 distinct oxidation levels involving Mn(II) to Mn(IV). 

Braunstein, P. and Rose, J. (1998) Heterometallic clusters for heterogeneous catalysis, in Catalysis hy Di- and Polynuclear Metal Cluster Complexes (eds R.D. Adams and... 

Much of the impetus for the investigation of polynuclear manganese complexes has come from a desire to produce structural and/or functional models of the manganese cluster of the OEC. Many of the results of these studies have been described in Section 5.1.2 and in reviews. ... 

In the extreme class III behaviour,360-362 two types of structures were envisaged clusters and infinite lattices (Table 17). The latter, class IIIB behaviour, has been known for a number of years in the nonstoichiometric sulfides of copper (see ref. 10, p. 1142), and particularly in the double layer structure of K[Cu4S3],382 which exhibits the electrical conductivity and the reflectivity typical of a metal. The former, class IIIA behaviour, was looked for in the polynuclear clusters of copper(I) Cu gX, species, especially where X = sulfur, but no mixed valence copper(I)/(II) clusters with class IIIA behaviour have been identified to date. Mixed valence copper(I)/(II) complexes of class II behaviour (Table 17) have properties intermediate between those of class I and class III. The local copper(I)/(II) stereochemistry is well defined and the same for all Cu atoms present, and the single odd electron is associated with both Cu atoms, i.e. delocalized between them, but will have a normal spin-only magnetic moment. The complexes will be semiconductors and the d-d spectra of the odd electron will involve a near normal copper(II)-type spectrum (see Section 53.4.4.5), but in addition a unique band may be observed associated with an intervalence CuVCu11 charge transfer band (IVTC) (Table 19). While these requirements are fairly clear,360,362 their realization for specific systems is not so clearly established. 

Adams, R.D. and F.A. Comm Catalysis by Pi- ami Polynuclear Metal Cluster Complexes, John Wiley Sons, Inc.. New York. NY. 1998. 

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