Coordination block metal compounds

In this section, we give an overview of the coordination numbers and geometries found within rf-block metal compounds. It is impossible to give a comprehensive account, and several points should be borne in mind ... 

In this section, we give an overview of the coordination numbers and geometries found within d-block metal compounds. 

Where the lanthanide ionic radius and the macrocyclic cavity are incompatible, though in hydrous conditions the crown ether is likely to be displaced by water ligands, the crown may still be present in the structure of the crystal as a hydrogen-bonded adduct. This behaviour is seen in [Gd(N03)3(H20)3]-(18-crown-6).445 This type of compound is quite well known in the case of s block metals also, e.g. [Mg(H20)6]Cl2 (12-crown-4)454 and, a more subtle case, [Ca(nitrobenzoate)2(benzo-15-crown-5)]-3H20(benzo-15-crown-5)455 in which an apparent 2 1 complex has only half its crown ligand coordinated to Ca2+. 

There are many other shapes possible for complexes. The simplest are linear, with coordination number 2. An example is dimethylmercury(O), [Hg(CH3)2] (5), which is a toxic compound formed by bacterial action on aqueous solutions of Hg2+ ions. Coordination numbers of up to 12 are found for members of the f block, but they are rare in the d block. One interesting type of d-metal compound in which the coordination number is formally only 2 but there are 10 links between the ligands and the central metal ion is ferrocene, dicyclopentadienyliron(O), [Fe(C5H5)2] (6). Ferrocene is an aptly named sandwich compound, with the two cyclopentadienyl ligands the bread and the metal atom the filling. The formal name for a sandwich compound is a metallocene. 

The use of bulky monodentate m-terphenyl ligands in the stabilisation of d-block organometallic compounds is surveyed. Importantly, these ligands have facilitated the isolation of hitherto unknown species containing low-coordinate centres and metal-metal multiple bonds. This review reports on these advances with emphasis on the synthesis, structural characterisation and, where possible, reactivity studies of complexes featuring metal-carbon bonds between m-terphenyl ligands and the transition metals. 

Cyclopentadienyl (and substituted variants thereof) has been the most versatile ligand used in organolanthanide chemistry. Unlike the situation with the d-block metals, where a maximum of two pentahapto- () -)cyclopentadienyls can coordinate, up to three -cyclopentadienyls can be found for the lanthanides, in keeping with the higher coordination numbers found for the f-block elements. In terms of the space occupied, a -cyclopentadienyl takes up three sites in the coordination sphere. Three types of compound can be obtained, depending upon the stoichiometry of the reaction mixture ... 

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