Metallic bonds molecular-orbital model

Complexes containing anions of the above formulation have attracted a large number of studies because of their alleged simplicity. This is illustrated by the central position such complexes have played in the evolution of crystal field, ligand field and molecular orbital models of bonding in transition metal complexes. 

Bond [294] used comparisons between homogeneously and heterogeneously catalysed interconversions of unsaturated hydrocarbons to deduce that the reactive state of an adsorbed hydrocarbon may reasonably be assumed to be a jr-complex (see Sect. 3.2, p. 22). On this assumption, a molecular orbital model appropriate to a face-centred cubic metal was developed. By considering the direction of emergence and degree of occupation of the metal atomic orbitals at the (100), (110) and (111) faces, assuming that the atomic orbitals on the surface keep the same orientation as in the bulk metal, which may not be valid [295], he concluded that the (111) planes were least suited to the adsorption requirements of... 

Before leaving this brief introduction to molecular orbital theory, it is worth stressing one point. This model constructs a series of new molecular orbitals by the combination of metal and ligand orbitals, and it is fundamental to the scheme that the ligand energy levels and bonding are, and must be, altered upon co-ordination. Whilst the crystal field model probably over-emphasises the ionic contribution to the metal-ligand interaction, the molecular orbital models probably over-emphasise the covalent nature. 

The steady, gradual variation of the P-P distance would seem to be as inconsistent with the molecular orbital model shown here as it was with the Zintl concept. This is not so. If we turn on the interaction between the P atoms and the metal layer (and we have seen before that this interaction is substantial), we will get a mixing of P and Mn orbitals. The discontinuity of the above picture (either single-bond or no bond) will be replaced by a continuous variation of P o and o orbitals occupation between 2 and 0. 

The structural strengths of the hybridization model were combined with the electronic strengths of the crystal-field model in a molecular-orbital model albeit with the loss of the simplicity of the earlier models. The essential aspects of this MO model will be discussed in Chapter 1. The key point here is that, if one wishes to understand the electronic structure of metal-coordination compounds, one need go beyond the Lewis model of two-center-two-electron bonds. It should be obvious, then, that this is also a requirement for organometallic complexes, metal clusters and extended solid-state systems containing metal atoms. 

Unlike [CpMo(CO)2]2 in which unsaturation generates localized metal-metal multiple bonding, here the unsaturation is spread out over the entire six-atom cluster system a conclusion supported by the difference in geometric parameters between the Cr and Re compounds as well as by molecular orbital modeling. Because of the invariant stoichiometry, these observed geometry changes can only be attributed to the change in metal. 

Recent valence bond studies of multiple bonds in molecules with only s, /7-orbitals indicate that bent bonds are preferred to the usual <7 and tt bonds. This has potentially important implications for the description of multiple metal-metal bonds. However, the description of E, IT and A ion states in photoemission from a ground state of bent bonds is not so obvious as in the <7, tt, -molecular orbital model. We examine these issues in the present contribution. 

In the course of investigating multiple bonds in molecules and complexes by the valence bond approach, we have recently found that such multiple bonds are more accurately described as bent bonds rather than as a and tt bonds (7-5). In order to understand the potential implications of these results for multiple metal-metal bonds, it is important to brieffy review the basic assumptions of the valence bond model and compare them to those of the more familiar molecular orbital model of bonding. 

In this section, we consider a third approach to the bonding in metal complexes the use of molecular orbital theory. In contrast to crystal field theory, the molecular orbital model considers covalent interactions between the metal centre and ligands. 

Whether a complex is high- or low-spin depends upon the energy separation of the t2g and eg levels. Nationally, in a fj-bonded octahedral complex, the 12 electrons supplied by the ligands are considered to occupy the aig, and eg orbitals. Occupancy of the and eg levels corresponds to the number of valence electrons of the metal ion, just as in crystal field theory. The molecular orbital model of bonding in octahedral complexes gives much the same results as crystal field theory. It is when we move to complexes with M—L TT-bonding that distinctions between the models emerge. 

The existence of empty molecular orbitals close in energy to filled molecular orbitals explains the thermal and electrical conductivity of metal crystals. Metals conduct electricity and heat very efficiently because of the availability of highly mobile electrons. For example, when an electric potential is placed across a strip of metal, for current to flow, electrons must be free to move. In the band model for metals, the electrons in partially filled bonds are mobile. These conduction electrons are free to travel throughout the metal crystal as dictated by the potential imposed on the metal. The molecular orbitals occupied by these conducting electrons are called conduction bands. These mobile electrons also account for the efficiency of the conduction of heat through metals. When one end of a metal rod is heated, the mobile electrons can rapidly transmit the thermal energy to the other end. 

In 1962, Sugano showed that the Seitz model (115) could be interpreted as a molecular orbital model (123), an interpretation that clarifies analysis of these systems. In this interpretation, the absorption bands observed in the TI(I) doped alkali halide system come from the electronic transition aigf a g) hu), but the excited states are still calculated assuming an ionic interaction between the metal and the hgand. Since the thallium-chlorine bond is actually largely covalent, Bramanti et al. (118) modified the approach and used a semiempirical molecular orbital (MO) calculation to describe the energy levels of T1(I) doped KCl. Molecular orbitals were constructed by the linear combination of atomic orbitals (LCAO) method from the 6s and 6p metal orbitals and the 3p chlorine orbitals. Initial calculations were conducted with the one-electron approximation the method was then expanded to include Coulomb and spin-orbit interactions. The results of Bramanti et al. were consistent with experimental... 

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