Ligand character

Like palladium(II) and platinum(II), gold(III) has the d8 electronic configuration and is, therefore, expected to form square planar complexes. The d-orbital sequence for complexes like AuC14 is dx2 yi dxy > dvz, dxz > dzi in practice in a complex, most of these will have some ligand character. 

The electron transfer Au(R2voltametric measurements 163). The half-wave potentials of the quasi-reversible process depends on the substituent R according to the Taft relation, as was described for Mo, W and Mn 37). The value of p decreases in the series Au > Mn > Mo = W, which indicates that in this sequence the mixing of ligand orbitals into the redox orbital decreases. The dominant ligand character of the unpaired electron MO in Au(R2dtc)2 relative to those in copper and silver compounds is found from Extended Hiickel MO calculations, as will be discussed later on. 

As a further illustration of the dependence of n i 7t pi-backbonding interactions on metal and ligand character, we may compare simple NiL complexes of nickel with carbonyl (CO), cyanide (CN-), and isocyanide (NC-) ligands, as shown in Fig. 4.41. This figure shows that the nNi 7rL pi-backbonding interaction decreases appreciably (from 28.5 kcal mol-1 in NiCO to 6.3 kcalmol-1 in NiNC-, estimated by second-order perturbation theory) as the polarity of the 7Tl acceptor shifts unfavorably away from the metal donor orbital. The interaction in NiCO is stronger than that in NiCN- partially due to the shorter Ni—C distance in the... 

Complexes of 1,2-dithiolate dianions (225) are of interest, although because they undergo successive one-electron transfer processes which involve orbitals of ligand character so unlike those of the dithiocarbamate type of ligand, it is unprofitable to attempt to assign oxidation states.995,1005 The chromium complexes are shown in Table 89. 

A ligand field model has been used for the interpretation of the magnetic behavior of the Mo(R2Dtc)4 complexes (485). Extended Hiickel calculations compare the electron-donating characteristics of the dithiocarbamate ligand to other 1,1-dithio chelates. This calculation concluded that (1) there is more metal character in the lowest unoccupied MO in the dithiocarbamate complexes, and (2) there is more ligand character in the lowest unoccupied MO in the dithioacid complexes (485). This seems to be in agreement with the electronic spectra. 

Unambiguous assignments of the band maxima of the allowed electronic transitions are only possible for the longest-wavelength bands of ions with Td and C3p symmetry. However, all bands of Table 1 of Chapter 16.1 can be assigned qualitatively to metal reduction transitions (from MOs with predominantly ligand character to ones essentially localized on the central atom). Empirically, this... 

Spectroscopies are also used to experimentally probe transient species along a reaction coordinate, where often the sample has been rapidly freeze quenched to trap intermediates. An important theme in bioinorganic chemistry is that active sites often exhibit unique spectroscopic features, compared to small model complexes with the same metal ion.8 These unusual spectroscopic features reflect novel geometric and electronic structures available to the metal ion in the protein environment. These unique spectral features are low-energy intense absorption bands and unusual spin Hamiltonian parameters. We have shown that these reflect highly covalent sites (i.e., where the metal d-orbitals have significant ligand character) that can activate the metal site for reactivity.9... 

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