Metal—ligand bonds bond valence

Changes in the charge of the central atom also strongly affect the metal-ligand bond length and the ionic-covalent share in fluoride complexes, which in turn impact the vibration spectra. Fig. 46 shows the dependence of asymmetric valence vibrations on the charge of the central atom. The spectral data for Mo, W, Zr, Hf fluoride compounds were taken from [71,115,137]. 

Removing electrons from a metal atom always generates vacant valence orbitals. As described in Chapter 20, many transition metal cations form complexes with ligands in aqueous solution, hi these complexes, the ligands act as Lewis bases, donating pairs of electrons to form metal-ligand bonds. The metal cation accepts these electrons, so it acts as a Lewis acid. Metal cations from the p block also act as Lewis acids. For example, Pb ((2 g) forms a Lewis acid-base adduct with four CN anions, each of which donates a pair of electrons Pb ((2 ( ) + 4 CN ((2 q) -> [Pb (CN)4] (a g)... 

The valence bond picture for six-coordinate octahedral complexes involves dispi hybridization of the metal (Fig. I i.lc. d). The specific d orbitals that meet the symmetry requirements for the metal-ligand o bonds are the four-coordinate d complexes discussed above, the presence of unpaired electrons in some octahedral compounds renders the valence level ( — l)J orbitals unavailable for bonding. This is true, for instance, for paramagnetic [CoFJ3- (Fig. I I.lc). In these cases, the VR model invokes participation of -level dorbitals in the hybridization. 

Two additional points that are self-evident are discussed below for the sake of completeness. The coordination number of a metal ion counting all ligands other than the adjacent metal involved in multiple metal bonding is less than the maximum coordination number possible for that metal center. Furthermore, the number of d-electrons on the metal is nearly equal to the number of metal valence orbitals not involved in metal-ligand bonding to optimize metal-metal bond formation by filled bonding MOs. 

Photoelectron spectroscopy (PES) has been shown to provide a convenient probe of metal ion effective nuclear charge and the nature of the metal-ligand bond via the energy of valence-electron photoionizations (16, 20, 22, 284, 285, 312, 332-334). Recently, PES spectroscopy has been employed in the study of oxo-molybdenum compounds of the type (L-A5)MoE(X,Y) [E = O, S, NO X, Y = halide, alkoxide, or thiolate] in order to evaluate the synergy between the axial (E) and equatorial (X,Y) donors in affecting the ionization energy of the HOMO localized on the Mo center (16, 284, 334). These studies have conclusively shown that equatorial dithiolene coordination electronically bulfers the Mo center in (L-A pMoEttdt) (Fig. 13) from the severe electronic perturbations associated with the enormous variation in the Ji-donor/acceptor properties... 

The electronic structure and spectroscopy of metallo-bis(dithiolenes) are considerably more complicated than that of the metallo-mono(dithiolenes) discussed in Section II.C because there are now two dithiolene donors, which result in twice as many sulfur-based MOs that contribute to the overall metal-ligand bonding scheme. The result is an increase in the density of states in the valence region, with a concomitant increase in the number of Sdithioiene — M CT excitations. Nevertheless, numerous spectroscopic studies and bonding calculations have been undertaken in order to explain the unique electronic properties of these molecules. The fact that two dithiolene ligands are now coordinated to... 

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