Square-based pyramidal metal clusters

Indium clusters have also recently been characterized, notably in intermetallic compounds. Thus, the Zintl phase, Rbzinj, (prepared by direct reaction between the two metals at I530°C) has layers of octahedral closo-lnf, clusters joined into sheets through exo bonds at four coplanar vertices. These four In atoms are therefore each bonded to five neighbouring In atoms at the comers of a square-based pyramid, whereas the remaining two (Irans) In atoms in the Ine cluster... 

For five-metal clusters, two "parent polyhedra may be considered, viz., the trigonal bipyramid and square-based pyramid. 

The relationship between boranes and metal-carbonyl clusters can be extended by considering the compound Fe5(CO)i5C, which has the square-based pyramidal structure shown in Fig. 13, with the carbide carbon atom just below the center of the Fe square, clearly contributing all its valence shell electrons to the cluster 24). The metal-carbonyl residue FeB(CO)i4 formally left by removal of this carbon as has the nido structure appropriate for a cluster with 5 skeletal atoms and seven skeletal bond pairs. 

HOs6(CO)18] and H2Os6(CO)18 marks one of the highlights of the early period of osmium carbonyl cluster chemistry [162]. While both the dianionic cluster and the monoanionic system have the expected octahedral metal core, a capped square based pyramidal structure was found for H2Os6(CO)18. This turned out to be the archetypal example for the capping rule , a concept which proved to be very successful in the analysis of the structures of metal carbonyl clusters of the iron and cobalt triads [166],... 

The condensation reactions of carbonyl metallates with neutral species, which are either coordinatively unsaturated or which will readily generate coordinatively unsaturated fragments, have also yielded a variety of mixed-metal clusters. The square-based pyramidal dianion [Fe5C(CO)i4] , for example, has been shown to react with a number of such species to yield octahedral FcjMC cluster compounds (Scheme 9) 269, 381). In some cases. 

Addition of protons to anionic clusters to generate hydrides leaves the cluster electron count unaffected, yet the process is sometimes accompanied by structural changes. For example, both [Os6(CO)i8] (87) and [Os6(CO)igH] (88) have octahedral arrangements of metal atoms as predicted by Wade s rules. In the dihydride Osg(CO)i8H2, however, the metal atoms describe a capped square-based pyramid (87). 

Recent molecular orbital (MO) calculations by Wade et al. (408) using the series [B Hg] , [BgH,]", and BgHg as models for the protonation of hexanuclear metal carbonyls have attempted to rationalize these findings. The charge distribution is symmetrical in an octahedral [BgHg] but asymmetrical in the capped square-based pyramidal isomer. It was found that upon protonation, significant charge redistribution occurs. This results in a substantial decrease in the symmetry of the octahedral cluster framework, which is disfavored in comparison with the capped square-based pyramidal structure much less affected by the protonation process. 

Dynamic behaviour involving a rearrangement of the metal cores of clusters containing square-based pyramidal AU2RU3 units... 

The metal core of the recently synthesized hexanuclear cluster [Au2Ru4(/i3-H)-(/z-H)( ti-l,2-Ph2PC6H4PPh2)(CO)i2] (4) contains a square-based pyramidal AU2RU3... 

Figure 6. The restricted Berry pseudo-rotation mechanism proposed for the intramolecular metal core rearrangements observed in solution for the clusters [M2Ru4H2(CO)i2L2] (M = Cu, Ag or Au L = a variety of monodentate phosphine ligands or L2 = a variety of bidentate diphosphine ligands). The mechanism exchanges the two coinage metals in sites M(l) and M(2) of the trigonal bipyramidal M2RU3 unit in the metal skeletons of the clusters via a square-based pyramidal intermediate (reprinted by permission of the Royal Society of Chemistry from ref. 36).

One of the equatorial Au-Ru distances is too long (Au Ru 3.446 A) for any significant bonding interaction, so the AU2RU3 unit in the AU2RU4 metal core of the cluster is distorted towards a square-based pyramidal geometry.P l... 

Figure 15. The molecular structure of one of the two molecules in the asymmetric unit of the cluster [AU2RU3 (/r3-S)(/r-l,2-Ph2PC6H4PPh2)(CO)9]. In each of these molecules, one of the equatorial Au-Ru [eg. Au(2)- Au(3)] distances is too long (Au- Ru 3.487 and 3.515 A) for any significant bonding interaction, so the AU2RU3 metal core of the cluster is distorted towards a square-based pyramidal geometry. Therefore, the gold atoms in the metal framework of the cluster adopt a structure, which is intermediate between gold arrangements A and B (Fig. 13).

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). 

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