Orbitals of Reactive Metal Complexes

Intrinsic Interaction Matrix Element, ab, Between d Orbitals and Orbitals of C 

In addition to the effects of ligands, Aq (and A) depends on the type of metal and its oxidation state [280]  

Aq increases with increasing formal oxidation state 

The dependence on formal oxidation state can be attributed to electrostatic lowering of the metal d orbitals thereby narrowing the gap with ligand orbitals. The effect of principal quantum number may be due to better overlap of the larger 4d and 5d orbitals with ligand orbitals, compared to the more compact 3d orbitals. Thus Aq increases in the 

As the above discussion shows, we will adopt the simple view that an octahedrally hexa-coordinated metal complex, ML6, is coordinatively saturated, just as is a tetracoordinated carbon atom. Metal complexes with fewer than six ligands will be treated on the same footing as organic reactive intermediates. The distinction we are making is that direct involvement of the metal center in a reactive process (bond making or breaking) is easy only at coordinatively unsaturated metal centers, just as it is at carbon. Of course, just as at saturated carbon, ligand substitution by association or dissociation can take place, and the presence of the center (C or M) with its array of substituents can influence 

Ao increases with increasing formal oxidation state Ao increases down a group 

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Side reactions specific to one component play an important role in the reforming of a mixture. For example, aromatics are more prone to coking upon reforming, so their presence in a mixture can lower syngas yields over time due to catalyst deactivation. Also, the catalyst surface-component interactions may play an important role in the reforming of a mixture. For example, aromatics have an abundance of 71-electrons, so they may occupy active sites for a longer duration, due to 71-complexation between d-orbitals of the metal and 7i-elec-trons. Hence there will not be enough reactive sites available for the desired reaction to occur. 

In view of the large difference in the reactivity of Cr+2 and Mn+2 aquo complexes it seems that the reactivity of the metal complex toward eaq is not related to the energy required to remove a second 4s electron. The similar reactivity of aquo and amino complexes makes the correlation between reactivity and redox potentials rather unlikely. As an alternative mechanism, we suggest that the availability of a vacant d orbital on the central atom and the energy gain on adding an electron are the major factors which determine the reactivity of transition metal ions. 

The partial confusion arising after Dewar s and Chatt s reviews were published, was resolved after Chatt and Duncanson reported in 1953 in the Journal of the Chemical Society the results of infrared spectroscopic studies on a range of olefin platinum(II) complexes [38]. In this highly cited paper they proposed, with particular reference to Dewar s model, that in the olefin platinum(II) complexes the cr-type bond would be formed by overlap of the filled re-orbital of the olefin with a vacant 5d6s6p2 hybrid orbital of the platinum atom, and the re-type bond by overlap of a filled 5d6p hybrid orbital of the metal with the empty antibonding re-orbital of the olefin (Fig. 7.8). In addition, Chatt and Duncanson illustrated how the model could be used to interpret not only the physical properties of the olefin platinum compounds, such as the spectroscopic data and dipole moments, but also their reactivity and their greater stability compared to the olefin silver salts. 

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