Complexes ionic bonding models

Reactions of UCI4 with [Li RC(NCy)2 (THF)]2 (R = Me, Bu ) in THF gave the tris(amidinate) compounds [RC(NCy)2]3UCl that could be reduced with lithium powder in THF to the dark-green homoleptic uranium(lll) complexes [RC(NCy)2]3U. Comparison of the crystal structure of [MeC(NCy)2]3U with those of the lanthanide analog showed that the average U-N distance is shorter than expected from a purely ionic bonding model. ... 

The nature and extent of /-orbital participation in the bonding of uranocene and other bis(cyclooctatetraenyl) actinides has never been satisfactorily established, although a good deal of effort has been expended on it. The X-ray structures do not resolve the issue because an ionically bonded model would also lead to a sandwich-type structure (for example, MgCp2 has essentially the same structure as ferrocene). Other physical techniques have been used, but the complexity of the electronic structures often leads to ambiguous interpretations. 

A simple ionic bonding model accounts for many properties of transition metal complexes, including variations in the hydration and lattice enthalpies and the ionic radii of the metal ions. The observation of high- and low-spin states for complexes of some metal ions can also be explained. 

The bonding in tetrahedral and septare planar complexes can also be described using an ionic bonding model. The distortions found in the geometries of some metal ions can be explained by Jahn -Teller effects within the ionic model. 

The magnetic properties of d- or f-bloek metal ion complexes can usually be predicted from ionic bonding models. Orbital contributions to magnetism can often be neglected for first row d-block metal ions but must be included when considering f-block metal ions. 

Fig. 51. Ionic bonding model of PVA-boiax complex according to Shibayama et al [193]...

The oxidation state is assigned and justified on the basis of the relative electronegativities of the central metal atom and the coordinating atoms. The important point to note is that an ionic bonding model is assumed for the purpose of assigning the oxidation state and to that extent the bonding in the metal complex may not correspond to the real situation. 

Surface complexation models attempt to represent on a molecular level realistic surface complexes e.g., models attempt to distinguish between inner- or outer-sphere surface complexes, i.e., those that lose portions of or retain their primary hydration sheath, respectively, in forming surface complexes. The type of bonding is also used to characterize different types of surface complexes e.g., a distinction between coordinative (sharing of electrons) or ionic bonding is often made. While surface coordination complexes are always inner-sphere, ion-pair complexes can be either inner- or outer-sphere. Representing model analogues to surface complexes has two parts stoichiometry and closeness of approach of metal ion to... 

The concept of bond valence, which, as will be shown below, is the same as the bond flux derived in Chapter 2, grew out of attempts to refine Pauling s principles determining the structures of complex ionic crystals (Section 1.7). In this empirical evolution of Pauling s model, both the electrostatic and short-range components were developed simultaneously. Only later did it become apparent that it was also possible to derive the properties of the electrostatic component independently using the ionic theory. 

Historically, crystal field theory was the first theoretical model (11, 86, 101, 123) used to explain d-d transition energies in metal complexes. Its usefulness is restricted to those complexes whose bonding is largely ionic, and its mqjor deficiency arises from its inability to account for charge transfer transitions. The iterative extended Hiickel and the ab initio, limited basis set, Hartree-Fock calculations are capable of de-... 

The reaction between poly-4-vinylpyridine and PAA in water-ethanol (1 1 by volume) solutions has been investigated by calorimetry,2). This reaction proceeds without the release of H+ or OH- ions. As the heat of dissociation of the polyacid and the heat of formation of ionic bonds between macromolecular components are near zero, the protonation heats of PVPy at different pH both in the presence or absence of PAA have been measured. It has been found that in neutral solutions the heats of polyvinylpyridine protonation in the presence of PAA considerably exceeds the corresponding values in the absence of PAA, i.e. a considerable portion of pyridine rings is protonated in the polyelectrolyte complexes (Fig. 12). This may be caused only by the cooperative trasfer of the proton from the PAA carboxy group to the pyridine ring. Similar reactions cannot occur between low molecular model substances and neither when only one component is a polymer. 

A more complex situation was found for Na Pb. The unusual stability of this cluster was explained by comparison with the analogous Mg compound (Albert et al. 1995) and was found to originate in a larger charge transfer to the more electronegative lead and a larger polarizability of the Pb atom. In a combined experimental and theoretical study it was shown for the clusters Na Au and Cs Au that certain properties of the bulk are qualitatively present at the level of small clusters (Heiz et al. 1995). While the Na compounds show metallic behaviour, and the electronic structure can be described by means of the jellium model, in the Cs-Au clusters an ionic bond is most prominent. 

As well as providing a description of the transition between purely covalent and purely ionic bonding, the model described above has a consequence that is important in more complex cases. AOs of very different energy do not mix significantly the resulting MOs are hardly different from the AOs themselves, and it is a good approximation to neglect their interaction. 

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