Molecular orbitals transition metal coordination

Figure 2.5 Schematic molecular orbital energy level diagram for a transition metal coordination cluster, [ML6]. (a) Energy levels of atomic orbitals of the free cation, M (b) energy levels for the six ligands, L, before bonding (c) molecular orbital energy levels for the octahedral [ML6] cluster.

Carbonyl Complexes of the Transition Metals Coordination Numbers Geometries Coordination Organometalhc Chemistry Principles Diffraction Methods in Inorganic Chemistry Molecular Orbital Theory. 

We have seen that the crystal-field model provides a basis for explaining many features of transition-metal complexes. In fact, it can be used to explain many observations in addition to those we have discussed. Many lines of evidence show, however, that the bonding between transition-metal ions and ligands must have some covalent character. Molecular-orbital theory (Sections 9.7 and 9.8) can also be used to describe the bonding in complexes, although the application of molecular-orbital theory to coordination compounds is beyond the scope of our discussion. The crystal-field model, although not entirely accurate in all details, provides an adequate and useful first description of the electronic structure of complexes. 

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

These findings led to the concept of the Metal-oxo Wall or Ru-oxo Wall , namely terminal metal-oxo units are well known for nearly all early and mid-transition metal elements but simply unknown for the late transition metal elements (Fig. 1). The generic explanation for this phenomenon is that as one moves to the right in the d block, the metal center necessarily has more d electrons. This in turn requires an increasing population of orbitals that are antibonding with respect to the terminal metal-oxo unit. A simplified molecular orbital diagram for a six-coordinate C41 transition metal-oxo unit shown in Fig. 2 explains... 

Fig. 2. Significant molecular orbitals of terminal transition metal-oxo units in a six-coordinate 4 ligand environment. The d° configuration is a formal triple bond. The highest occupied molecular orbital in the d configuration is formally nonbonding (8 symmetry) so the metal-oxo bond order remains 3.0. However, d-electron counts above d populate orbitals that are antibonding between the metal and the terminal multiply bonded ligand (0x0 in this case, but alternatively, alkyl-imido, nitrido, sulfido, etc.). Note that all the equatorial ligand orbitals and the metal dx2 y2 orbital (hi in 4 symmetry) are ignored for simplicity.

The coordination of oxygen to transition metal ions which occurs mostly in the side-on fashion on surfaces (Section III,A,2 and Appendix B) can be described following the model of acetylene-metal complexes (467). Both 7tu and 7tg orbitals of molecular oxygen have proper symmetry to interact with the bonding set of s, p, and d orbitals on the metal. The bonding orbitals are shown in Fig. 29. 

Figure 7.5 A molecular orbital diagram for octahedrally coordinated transition metal ions...

---