Extended metal atom chains

Pig. 1.7 Ball and stick representation of a Cu3(OH)3 complex (left) and an extended metal atom chain (EMAC) made of three Cr + ions hold together by four tridentate organic ligands... 

Extended Metal Atom Chains, 669 - 706, Chapter 15 in Multiple Bonds Between Metal Atoms, Cotton, F.A., Murillo, C. and Walton,... 

Extended metal-atom chains (EMACs), or metal string complexes consist of a linear chain of three or more metal atoms bridged by an organic ligand [15]. The ligand is typically a polypyridylamide derivative, and examples of these types of ligands that wiU be discussed in this section are shown in Figure 9.6. 

Keywords Electronic structure Extended metal atom chains Heterometallic compounds Metal-metal bonds Molecular wires Single-molecule conductance... 

In this chapter, we describe linear metal chain compounds, consisting of at least three metal ions, supported by ligand systems. Extended metal atom chains (EMACs) of Cr, Co, Ni, and Cu, supported by polypyridylamide ligands, are discussed. This is followed by linear and bent Pd chains sandwiched by unsaturated hydrocarbons. Unsupported linear metal chain compounds are discussed in subsequent sections. Because synthetic protocols and the bonding schemes are different for different metals, our discussion is classified according to metal ions. 

Let us start our examination with the dpa-metal complexes. A series of trimetallic complexes of general formula [M3(dpa)4Cl2] (M = Cr,30 Ni,31 Co,32 Cu,33 Ru,34 Rh,34) have been structurally characterized. As illustrated in Scheme 2, such trinuclear complexes in the solid state can display either a symmetrical or an unsymmetrical M3 chain, depending upon if the metal-metal bond is extended over the three metal atoms, or is localized only between two metal atoms.35... 

There has been considerable interest in the synthesis of polymers such as structure (6) in which aromatic rings and metal atoms alternate, due to the possibility of building extended electron-delocalized organometallic polymer chains. However, such a multiple decker polymer has not yet been synthesized, and only a few triple decker-sandwich compounds such as tris( 5-cyclopentadienyl)dinickel cation and tris-(cyclooctatetraene)dititanium dianion have been reported10. ... 

Figure 7.7a shows the extended-zone electronic band structure for a one-dimensional crystal - an atom chain with a real-space unit cell parameter a and reciprocal lattice vector Tr/n - containing a half-filled (metallic) band. In this diagram, both values of the wave vector, +k, are shown. The wave vector is the reciprocal unit cell dimension. The Fermi surface is a pair of points in the first BZ (Fig. 7.7c). When areas on the Fermi surface can be made to coincide by mere translation of a wave vector, q, the Fermi surface is said to be nested. The instability of the material towards the Peierls distortion is due to this nesting. In one dimension, nesting is complete and a one-dimensional metal is converted to an insulator because of a Peierls distortion. This is shown in Figure 7.7b, where the real-space unit cell parameter of the distorted lattice is 2a and a band gap opens at values of the wave vector equal to half the original values, 7r/2a.

We have described the structures of some boron-rich borides in which there are extensive 3D systems of B—B bonds. At the other extreme there are crystalline borides with low boron content in which there are isolated B atoms, that is, B atoms surrounded entirely by metal atoms as nearest neighbours. With increasing boron content the B atoms link together to form first B2 units, then chains, layers, or 3D frameworks extending throughout the whole crystal (Table 24.3). 

Cahn-Ingold-Prelog rules as extended by Stanley and Baird.168,171 For instance, the diastereoisomer with intrinsic R configuration at the central metal atom is shown in Figure 9(c), for the case of a metallocene with a H2C(Cp)(9-Flu) ligand. It is important to note that chirality of type (c) requires that the two coordination positions available for the growing chain and the monomer are occupied by different ligands. This implies that compounds such as the dichloride pre-catalysts are not chiral. 

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