Pentanuclear metal clusters

Solid angles and distortion degree in open structures depend on a number of factors as, for instance, the number of electrons, the nature of disposable orbitals, and the properties of the ligands. 

VCp units form a trigonal bipyramid face-capped by S-atoms. d(V-V) = 3.062 (eq-ax) and 3.206 A (eq-eq). 

Pentanuclear bow-tie frame (see Fig. 2.17d) with a central Cr(II) atom. Angle between the triangle planes 90°. Sulfur atoms cap the Cr3 triangle. Thiolate groups are bridging peripheral Cr atoms which are each other double-bonded (1655 A). d(Cr-Cr) (between central and peripheral atoms) = 2.889-2.933 A. 

Fe atoms define a square pyramid with the carbide just below the center of the basal plane. 

Fe atoms define the vertices of a 5 square. The fifth is terminal to the square lying above the Fe4 plane and forming with an edge of the square an open Fe3 unit capped on each side by CS group is capping the Fe4 plane. d(Fe-Fe) = 2.46 (capped edge) to 2.78 A. 

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Selected examples of pentanuclear metal clusters are listed in Table 19.4.1. In these examples, the calculated bond valence b is exactly the same as the number of edges of the metal skeleton, indicating 2c-2e M-M bonds. In general, electron-rich species have lower bond valence and more open structures than... 

Table 2.4. Selected examples of pentanuclear metal clusters...

The compound [Mn°(bpy)2]4[Mo (CN)g]2] (194), with an octahedral cyanide-bridged core, can be considered as an extension of the pentanuclear TBP clusters described in Section II. D. The equatorial plane of the SBP (octahedron) is formed by four Mn(II) ions, each of which has four out of six coordination sites protected by two bidentate bpy ligands (Fig. 41). The other two sites are coordinated by bridging CN hgands provided by [Mo (CN)g] ions that occupy axial positions of the SBP. The formation of the SBP instead of the TBP stmcmre is explained by the presence of octacyanometalate fragments in the axial positions and the sufficiently low steric constraints of the bpy ligands that cap the equatorial metal sites. 

Interestingly, aU cyanide SMMs reported thus far are characterized by the presence of ions with appreciable spin-orbit coupling. In the prototypical Mnj2 SMMs, the orbital angular momenta of Mn(III) centers are quenched due to the tetragonal Jahn-Teller distortion. This is not the case for the compounds listed in Table XI. In the molecules based solely on 3d transition metals, important orbital contribution results from the presence of low-spin Fe(III) centers. The only exception is the pentanuclear TBP cluster [Mn (tmphen)2l3[Mn (CN)6]2 (177). This compound was subjected to a detailed theoretical treatment that... 

The structure of the pentanuclear gold-cluster [Au5(dppm)3 (dppm-H)] + is related to that of the Au4 clusters, with the fifth gold atom appended (20). This Aus cluster can be synthesized by reduction of [Au2(dppm)](N03)2 with NaBH4 by the evaporation of metallic gold into ethanohc solution of dppm and NH4NO3 or by reaction of [Au9(PPh3)g] + with dppm hgand. 

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). 

Fig. 7 Molecular structure of the pentanuclear heterothiometallic metal clusters MS4Cu4X2Py6 with M = Mo, W and X = Br, I.

Some pentanuclear osmium cluster complexes show structures with edge-bridged tetrahedral geometries (c in Fig. 2.17). These structures have the same number of metal-metal bonds as the square-based pyramid but only triangular faces, so the stabilization of intersticial atoms is not possible there. 

A related pentanuclear / 3-oxo species 146 with a highly distorted bow-tie structure was isolated in low yield. " The mixed-metal clusters [Ru2Mo2(/i3-S)2(/ -SR)(/i-X)(GO)4Gp2] [X = SR, R = Et, Pf 147 or PPh2 148 ] were prepared by the treatment of 149 or 150 with Ru3(GO)i2, which involved the dealkylation of the thiol ligand at the dimolybdenum center. 

There is only one example of a pentanuclear cluster, namely [Au5(dppm)3(dppm-H)](N03)2 (622), where the metal core adopts a spiked-tetrahedral geometry of gold atoms. 3... 

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