Piano stools

The structure of these complexes is not regular and has been described as 4 3 (C ) piano stool , which is obtained by slight distortion of a capped trigonal prism (C2d). 

BMIM]C1/A1C13 have been used for the synthesis of the piano stool complexes [Mn(CO)3(r -arene)] (Scheme 6.1-7) [12]. 

Scheme 6.1-7 The synthesis of [Mn(CO)3(ri -arene)]-F piano stool complexes in [BMIMJCI/AICIj.

Of course, commercially available transition metal complexes are stable at room temperature because they have achieved an 18-electron noble gas-like electronic configuration. Thus, molecules like iron pentacarbonyl [Fe(CO)s], ferrocene [Fe(C5H5)2], as well as piano-stool complexes such as C5H5Co(CO)2 are chemically quite inert. In order to study bimolecular reactions, it is necessary to first prepare unsaturated complexes. For studies using molecular beams, one approach is through photolysis of a stable volatile precursor in a supersonic nozzle. 

The synthesis of (80) from [Ircp Cl2]2 and 2,2 -biimidazole has been detailed.119 The X-ray structure of (80) shows the expected three-legged piano-stool arrangement round the Ir center. 

The synthesis of [Ircp Cl(bpy-cd)]Cl, where bpy-cd is a /3-cyclo-dextrin attached at the 6 position to a bpy ligand, is detailed.138 The complexes [Ircp (diimine)X]+, X = C1, H, diimine = bpy, phen, are active catalysts for the light-driven water-gas-shift reaction.139 The hydride complexes luminesce at 77 K and room temperature, whereas the chloride complexes do not.140 The three-legged piano-stool arrangement of the ligands in [Ircp (bpy)Cl]+ and [Ircp (4,4 -COOFl-bpy)Cl]+ is confirmed by X-ray crystallography.141,142 Further mechanistic studies on the catalytic cycle shown in reaction Scheme 11 indicate that Cl- is substituted by CO and the rate-determining step involves loss of C02 and H+ to leave the Ir1 species, which readily binds Fl+ to yield the lrIH hydride species.143... 

The complex [(cp )IrCl(2,2 -bpy-4,4 -COO(CH2)3-pyrrole)]C104 has a typical three-legged piano-stool structure, with short Ir-cp distances due to the strong 7r-acceptor properties of the bpy ligands.540... 

General Structural Features. The general structure of halfsandwich ruthenium(II)-arene complexes is shown in Fig. 12. The structural, stereochemical and electronic features of metal-arene complexes have been discussed (63). A typical piano-stool geometry consists of an rj6-arene occupying three coordination sites of the pseudo-octahedral complex, leaving the three legs X, Y, and Z available for coordination. The sites X and Y can be taken up by two monodentate ligands, but are more commonly... 

Fig. 12. (a) General structure of the half-sandwich, piano-stool ruthenium—arene complexes (b) X and Y are commonly occupied by a bidentate ligand L giving a monofunctional complex (c) tethering of a monodentate ligand to the arene results in a bifunctional complex. 

In this case, it is highly educational to look at the voltammetric behaviour of the three-legged piano stool [(i76-C6H6)Cr(CO)3] in dichloromethane solution,10 see Figure 10. 

The dinitrogen molecule bridges the two tungsten atoms, each of which assumes a four-legged piano stool geometry. 

As has been deseribed, the parent monoearbollide-metal earbonyl piano-stool species [2-(CO) -c/oi o-2,l-MCBioHii] are now known for all of the metals M = Mo (12), W (13), Re (14), Fe (11), Ru (6), Os (8), and Ni (18). Evidence also exists for a diearbonyl-platinum analogue of eompound 18, ° and as mentioned earlier, the manganese analogue of 14 has also briefly been reported. A notable absenee from this list, however, is any representative of the Group 9 metals. The earbonyl nitrosyl-cobalt complex 21 is very elosely related to the hitherto unknown dicarbonyl-cobalt dianion [2,2-(CO)2-c/oi o-2,l-CoCBioHii] and this species remains an attractive synthetie target. 

A number of cyclopentadienyl-ffee amidoAn -cyclooctatetraene complexes have been reported. Sattelberger s group found that [An(cot)Cl2(thf)2] with 2Na[N(SiMe3)2] in toluene yielded the crystalline [An(cot) N(SiMe3)2 2], which for An = Th was X-ray-characterised as a four-legged piano stool , with the Th- -Centroid distance of 2.04 A, each amido ligand having one short Th- - -C contact (3.15 or 3.04 A). The compounds [U(cot)-... 

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