Molecular orbital model metals

The molecular orbital model can also be applied to complexes of the d-block elements. In octahedral complexes the d-orbitals of the metal are not degenerate, as they are in the free metal, because of the interaction between the ligand and metal orbitals. The five d-orbitals are split into three t2g (nonbonding) and two e (antibonding) MOs that is ... 

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. 

Max Wolfsberg and L. Helmholz Molecular orbital model applied to transition metal complexes... 

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. 

The molecular orbital model as a linear combination of atomic orbitals introduced in Chapter 4 was extended in Chapter 6 to diatomic molecules and in Chapter 7 to small polyatomic molecules where advantage was taken of symmetry considerations. At the end of Chapter 7, a brief outline was presented of how to proceed quantitatively to apply the theory to any molecule, based on the variational principle and the solution of a secular determinant. In Chapter 9, this basic procedure was applied to molecules whose geometries allow their classification as conjugated tt systems. We now proceed to three additional types of systems, briefly developing firm qualitative or semiquantitative conclusions, once more strongly related to geometric considerations. They are the recently discovered spheroidal carbon cluster molecule, Cgo (ref. 137), the octahedral complexes of transition metals, and the broad class of metals and semi-metals. 

Fig. 1 B G molecular orbital model of the electronic structures of tetragonal oxo-metal complexes...

Jorgensen, C. K. (1964) A simple molecular orbital model for transition metal complexes. J. Chem. Soc. 6226. 

There are two models by means of which the loss of degeneracy of the d orbitals can be explained the electrostatic model, and the molecular orbital model. In the first model, the differentiation of the d orbitals is attributed to the electric field produced by the symmetrical disposition of the attached groups, which may be anions like Cl"" or CN or dipole molecules like H20 or NH3. In a crystal also, the metal ion finds itself in a similar environment and hence the original name Crystal Field Theory. The five d orbitals which may be denoted as dxy, dyz, dxz, dx2-y2 and d7 have the shapes as represented in Figure 12.L The dxy, dxz and dYZ orbitals have their maximum in a diagonal direction between the co-ordinate axes in each of the three planes. The dx2-y2 and d72 orbitals are directed along the co-ordinate axes. Although... 

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. 

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