Tetranuclear clusters, transition metal

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 ... 

In practice, however, already with a comparatively small number of metal atoms it is no longer feasible to investigate all possible spin states with all potential realizations by various local spin distributions. Assumptions on the interaction of the metal centers on the basis of their structural arrangement and experimental susceptibility measurements have to be made. For example, for the BS state of a tetranuclear transition-metal cluster, one has to decide which of the four metal atoms couple in an antiferromagnetic fashion with each other. Prominent coupling schemes are, e.g., the dimer-of-dimers 2-plus-2-type or the 3-plus-... 

The bonding capabilities of transition metal clusters (no nonmetals in the framework), based on molecular orbital calculations, has been nicely summarized by Lauher14 (Table 16.3). Within this table we see three structures (tetrahedron, butterfly, and square plane) for tetranuclear metal clusters. The tetrahedron is a 60-electron cluster, while the butterfly and square plane clusters have 62 and 64 electrons. respectively. When we go from a tetrahedron to a butterfly, one of the edges of the tetrahedron is lengthened corresponding to bond breaking. 

To date only one such cluster has been reported. The tetranuclear cluster 33 is unstable in solution and over a period of several days eliminates the elements of Pt(COD)2, giving the Pt bridged cluster Pt[Ru3(/i-H)(ji4- 72-C=CtBu)(CO)9]2 (75) in reasonable yields (40-50%) (152). The C2 metal core (Fig. 15) is chiral, and a variable-temperature 13C NMR study showed that the cluster undergoes enantiomerization, with AG 66 = 57 kJ mol-1. The enantiomerization presumably proceeds via an intermediate or transition state with a planar coordination at the Pt atom. 

Ru and Os tetranuclear clusters, 6, 874 in transition metal complex electron counting, 1, 2-3 Eight-membered rings, via ring-closing diene metathesis,... 

III. A Survey of Magnetochemical Data on First-Row d-Block Transition-Metal Tetranuclear Cluster Complexes... 

As for tetranuclear clusters, pentanuclear transition metal clusters are only capable of partially encapsulating a main-group heteroatom. The c/o5o-pentanuclear cluster, the trigonal bipyramid, has an interstitial cavity of similar dimension to that of the tetrahedron, and must therefore open-up if it is to accommodate a semi-interstitial atom. The most commonly observed M5E clusters (E = main-group atom) adopt square based pyramidal structures, which can be derived from the trigonal bipyramid by M-M bond cleavage or can, alternatively, be viewed as /i/Jo-octahedral species (Scheme 2). 

A second effect is to be found in the acceleration of the initial activation of the nitroarene. We could observe this effect on the reactivity of both Ru3(CO)i2 and Ru(CO)s. We have previously mentioned that Han and others could not evidence any accelerating effect of chloride in the reaction between Ru3(CO)i2 and nitrobenzene [157]. However, from several studies, partly conducted by us yide infra), it has become now clear that the initial aetivation of nitroarenes from transition metal complexes always proceeds through an intermediate electron transfer from the complex to the nitro compound. Thus, any modification of Ru3(CO)i2 which increases its oxidability, specifically the introduction of the anionic ligand chloride, should increase its reactivity towards nitroarenes. However, the reaction between Ru3(CO)i2 and unsubstituted nitrobenzene requires a relatively high temperature even in the presence of chloride and at temperatures over 50°C and under a nitrogen atmosphere the initially formed [Ru3(CO)u(Cl)] is rapidly converted to a less reactive tetranuclear cluster, [Ru4(CO)i3(p-Cl)] [179]. 

A quantitative evidence for the cr-antiaromatic nature of saturated tetranuclear metal-carbonyl clusters [M4(CO)i2] (M = Fe, Ru, Os), which are similar metal-carbonyl analogs of cyclobutane was recently provided by Corminboeuf et al. [124], The isotropic NICS of the C2v [M4(CO)i2] (M = Fe, Ru, Os) clusters are about one-third less negative than those of their trinuclear congeners. Some differences in the magnetic response between cyclobutane and transition metal four-membered rings were identified and analyzed in detail using CMO-NICS (Fig. 37). 

Endicott, J. F. Ramasami, T Tamilarasan, R. Lessard, R. B. Brubaker, G. R. Structuie and Reactivity of the Metal-Centered Transition Metal Excited States, Coord. Chem. Revs. 1987,77,1. Ferraudi, G. Thotochemical Properties of Copper Complexes, Coord. Chem. Revs. 1981,36,45. Ford, P. C. Vogler, A. Photochemical and Photophysical Properties of Tetranuclear and Hexanuclear Clusters of Metals with and Electronic Configurations, Acc. Chem. Res. 1993,26,220. 

This chapter covers the literature on tetraruthenium and tetraosmium organometallic compounds from 1995 until late 2005. The literature search was carried out with SciFinder Scholar. Compounds containing other transition metals have for the most part been omitted. Although not strictly cluster compounds, compounds containing a chain of four Ru or Os metal atoms are included. Also included for their intrinsic interest are tetranuclear cage compounds with two or less metal-metal bonds. 

Cluster complexes of the composition M (mtc)g with M = Cu and Ag and mtc = di- -propylmonothiocarbamate are also luminescent. The emission is certainly related to that of the tetranuclear Cu(I) complexes [86, 87] (see above). The lowest-energy transitions of these d clusters may be MC ds transitions with LMCT contributions. Accordingly, the emissive states should be of MC/LMCT character,but modified by metal-metal interaction [91], e.g.. 

Other candidates for emissions from LMCT states are complexes of the d ° metal ions Cu(I), Ag(I), and Au(I) with donor ligands. However, in these cases LMCT and MC ds transitions occur at comparable energies. These LMCT and MC states may even mix and clear distinctions are not possible. Such complications are frequently encountered. Suitable examples are tetranuclear Cu(I) clusters [86,87,118],e.g.,... 

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