Cluster tetranuclear

The apparatus developed for trinuclear clusters is general enough to be successfully applied for clusters of higher nuclearity. Therefore only the key steps will be presented for tetranuclear and polynuclear systems. 

The bilinear isotropic exchange Hamiltonian for a tetranuclear cluster is written in the form 

In both coupling schemes we have the same reduction (i S M st 5, /5A/  

The reduced matrix element of the compound operator is expressed as follows 

Owing to one zero element, all 9/-symbols collapse into 6/-symbols. The tensor rank k = 0 implies a factor s so that the isotropic exchange cannot mix states of different total spin. The simplified reduced matrix elements are collected in Table 11.17. These formulae can be simplified further by substituting the relevant 6/-symbols (Appendix 3). 

Another dimer-of-dimers complex, [Mn404(tmdp)2(H20)2] (50), was reported by Uehara and co-workers, where they used a similar polydentate pyridyl-based ligand, tmdp. This 

Proposed PSII active site based on X-ray Dimer-of-dimers absorption studies 

The bond valence of Re4(/u-3 -H)4(CO) 12 is 8, and the tetrahedral Re4 skeleton can be described in two ways (I) resonance between valence-bond structures, leading to a formal bond order of lj, and (II) four 3c-2e ReReRe bonds. Since there are already four 1x3 -11 capping the faces, description (II) is not as good as (I). 

In other examples listed in Table 19.3.2, the calculated bond valence b is just equal to the number of edges of the corresponding metal skeleton, signifying 2c-2e M-M single bonds. 

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For tetranuclear cluster complexes, three stmcture types are observed tetrahedral open tetrahedral (butterfly) or square planar, for typical total valence electron counts of 60, 62, and 64, respectively. The earliest tetracarbonyl cluster complexes known were Co4(CO)22, and the rhodium and iridium analogues. The... 

We are pleased to accept the invitation of the editors to present a review on metal carbonyl clusters which affords us the opportunity to discuss the tetranuclear clusters and to complete the previous discussion of 3-dimensional clusters60. ... 

A knowledge of these enthalpic terms, and therefore of the relative bond energies, would be expected to considerably clarify many of these fundamental aspects. The data in Table 4 show that, with the main exception of rhenium and osmium, the metal-metal distances in the tetranuclear clusters and in the pure metals are quite similar this relationship is generally valid for all the polynuclear carbonyls60. The metal-metal bond energies in clusters are therefore expected to be of the same order as those in the metallic state for a close-packed arrangement, these are given by the formula Z m-m = A//f M(g)/6. 

Table 4. Comparison of distances found in metals229 with average values found in tetranuclear clusters and with values of AH M(g)/6227 ...

The synthesis of tetranuclear clusters is generally carried out using one of the following general methods, which we will discuss only briefly because they have already been described in previous reviews54,60 but it is appropriate to emphasize that we are not yet in a position to forecast the result of any of these syntheses. 

Some post-transition elements (or the corresponding radicals) containing 3 or more electrons in their valence shell are able to assist the formation of clusters by bonding to several metal atoms. Typical examples of this behaviour are the extraordinarily easy syntheses of large series of compounds such as Co3 (CO)9 (p3-E) (E = Al, CR, CX, GeR, P, As, PS, S, Se, PR, SR) 201 209) and Fe3 (CO)9 (p3 -E)2 (E = S, Se, Te, NR, PR). This type of stabilization is usually found in trinuclear clusters although a few examples in tetranuclear clusters are known, for instance ... 

The study of the properties and applications of clusters requires them to be accessible on a preparative scale and this is often conditioned by the ease with which they can be separated from other reaction products. Because their molecular weights are not too high, several tetranuclear clusters are volatile and/or soluble in organic solvents, and therefore the first methods of separation to be considered are sublimation under high vacuum (if the compound is thermally stable) and crystallization. When the solubility is low, continuous extraction with low boiling solvents can be tried50. ... 

Table 7. S7Fe Mossbauer data on tetranuclear clusters...

Metal-hydrogen bonds generally give a high field signal in the 1H NMR spectrum in the region 15—35 r as illustrated by the data on tetranuclear clusters, which is sum-... 

The number of 13C NMR studies on metal carbonyls has greately increased over the last few years as shown by the presently available data on tetranuclear carbonyls (Table 11) compared with that available in 1974 when the last review232 on this subject appeared and contained only two references to tetranuclear clusters. 

Co and 31PNMR have not yet been much used for the characterization of tetranuclear clusters but, in conjunction with NMR studies involving other nuclei, appear to offer great potential. 

As already shown in Eqs. (3), (4) and (5), fragmentation of carbonyl cluster anions, on reaction with carbon monoxide becomes easier with increasing negative charge. It is therefore not surprising that reduction of tetranuclear clusters with alkali metals in excess, gives rise to simple mononuclear derivatives [M(CO)4]-, as found for Co4(CO)12 in liquid NH3 13, e Rh4(CO)12 in THF under C056) and Ir4(CO)12 in THF under CO 17S ... 

In the tetranuclear clusters substitution reactions are complicated by the large number of different possible isomers. For instance, in the structure of Co4(CO)i2 there is the possibility of three different monosubstituted derivatives (apical, equatorial or axial), and seven different di-substituted isomers. It seems possible that the extensive use of 13C, and where appropriate 31P NMR66 could make an important contribution to this area. 

The only substitution in a tetranuclear cluster which has been studied kinetically is the progressive substitution in Ir4(CO)i2 by triphenylphosphine, and these results have not yet been published in full. The reported experimental data are the follow-... 

In the tetranuclear clusters significant steric control of substitution with tertiary phosphines and phosphites is generally only observed in the fourth substitution step. The synthesis of tetra-substituted derivatives, therefore, requires more severe conditions22,46,86,248 and then breaking of the cluster can become important. 

Despite the early discovery of [Fe4(CO)13]2, 119 inspection of Table 1 shows that the majority of tetranuclear clusters of this trial are formed by ruthenium. 

The related tetranuclear cluster Os2Co2(CO)i2H2, has been prepared, [Eq. (41)] ... 

In comparison with the remarkable stability of the anions [Co(CO)4], [Co6(CO)is]2- and [Co6(CO)14]4-, the low stability of the anionic tetranuclear clusters of cobalt seems extraordinary. Thus the low stability of the anion [Co4(CO)n(COOi-Pr)]- has prevented its isolation in a pure form61 ... 

Gyclopentadienyl nickel dimers with bridging ER groups can be obtained either from nickelocene or from Cp2Ni2(CO)2 (Equation (85),66 Scheme 25).68 For Cp2Ni2 EC(SiMe3)3 2, the Ni-Ni bond distances are 244.85(7) pm for E = Ga and 249.1(1) pm for E = In.68 A tetranuclear cluster is obtained when GaCp is reacted with Ni(GO)4 84.41... 

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