Complexes with unusual structural features

In recent years, many examples of compounds with tmusual coordination numbers, with novel structures, and with metal-metal bonds have been reported. Examples of complexes with odd coordination numbers and with coordination numbers greater than six are presented in Table 3.2. 

The ion, structure 1, occupies little space in the coordination sphere of a metal while acting as a bidentate chelate ligand and, therefore, can produce complexes in which the metal has a large coordination number. (The ligand is said to have a small bite .) With the large thorium(IV) ion it yields the ten- 

7 Trigonal prism with one F at center of a square face  

A variety of compounds with metal metal bonds have been prepared. Mercurous compounds which contain the [Hg-Hg] unit have long been known. More recent examples include compounds in which carbon monoxide is a ligand such as (Co)sMn Re(CO)s and (C6H5)3PAu-Co(CO)4. A number of complexes with transition metal tin bonds have been prepared. One of these, [Pt(SnCl3)5], contains five-coordinated platinum(ll) and is an excellent hydrogenation catalyst. 

Another example Br4Re-ReBr4, structure III, has two unusual structural features. The bromines attached to the two different metal atoms are not staggered to minimize repulsive interactions, but are eclipsed. Moreover, the rhenium-rhenium bond is very short. These structural features and the magnetic properties have been interpreted as indicating that a quadruple bond links the metal atoms. The bonds include one o--bond, two r-bonds, and a S (delta)-bond. A S bond can be visualized by placing the bonded atoms on a z coordinate axis and having overlap between two orbitals (one from each metal atom), structure IV. 

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Six members of this series could be isolated in modest yields as highly air-sensitive, dark blue or dark purple crystalline solids for which analytical, spectroscopic, and single-crystal X-ray analyses were fully consistent with the side-on-biidged N2 structures shown in Scheme 102. These complexes show unusual structural features as well as a unique reactivity. An extreme degree of N = N bond elongation was manifested in rf(N-N) values of up to 1.64 A, and low barriers for N-atom functionalization allowed functionalization such as hydrogenation, hydrosilylation, and, for the first time, alkylation with alkyl bromides at ambient temperature. ... 

In the last few years, computer graphics with colour display are being more commonly used not only to visualize complex structures better, but also to examine unusual structural features, defects and transformations as well as reactions. In Fig. 1.45, we show the presence of a Nal" cluster within the sodalite cage of zeolite Y as depicted by computer graphics the cluster fits well within the cavity bounded by the van der Waals surface (net) of the framework atoms. The immense power of computer graphics has been exploited widely in recent years. Structural transitions in solids and sorbate dynamics in zeolites are typical areas where computer simulation and graphics have been used (Ramdas et al., 1984 Rao et al., 1992). 

A number of unusual structural features have been noted. The ruthenium porphyrin formed by reaction of triruthenium dodecacarbonyl with an N, AT-vinyl-bridged tetraphenylporphyrin involves disruption of a pyrrole C-N bond to give a product in which the ruthenium is bound to two pyrrole N atoms, the C and N atoms of the ruptured pyrrole ring, and two mutually cis carbonyl ligands. The remaining pyrrole N is uncoordinated.613 Mutually cis configurations have also been found for the dicarbonyl complex of molybdenum tetraphenylporphyrin and for some dinitrosyl porphyrins. 

In this volume we have collected 10 review chapters from distinguished scientists who have contributed extensively to the study and development of supramolecular assemblies that contain metals and metal-like elements with unusual structures and morphologies and possess potentially useful (and applicable) physical and biological properties. The first chapter by K. Ariga et al. is a general discussion of supramolecular structures that contain inorganic building blocks for hybrid lipid thin films, layer-by-layer assemblies, structure transcription, and functional mesoporous hybrids. This is followed by two chapters, the first by M. L. Kistler et al., who describe the self-assembly of hydrophilic polyoxometalate (POM) macro-anions and examine the structure and behavior of POM macro-ions in solution. This is followed by a chapter by S. K. Das, who provides an overview of the supramolecular features of POM-supported transition metal complexes, POM-crown ether complexes with supramolecular cations, and supramolecular water clusters associated with POMs. 

Silver(I) coordination chemistry produces a stream of unusual structures, and a great number of complexes with all type of donor ligands have been prepared. An important feature of the... 

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