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. ... 

Chapter 1 discusses classical models up to and including Lewis s covalent bond model and Kossell s ionic bond model. It reviews ideas that are generally well known and are an important background for understanding later models and theories. Some of these models, particularly the Lewis model, are still in use today, and to appreciate later developments, their limitations need to be clearly and fully understood. 

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. 

Oxidic surfaces in particular develop acid or basic properties which are important in catalysis. We will approach this subject first by taking as a starting point the ionic bond model [2]. The lattice is considered to consist of cations and anions held together by electrostatic interactions. Later we will discuss a more balanced theory that also accounts for covalent bonding aspects. 

The first ionic bonding model was suggested by the German chemist Richard Abegg (1869-1910) shortly after the British physicist Joseph John Thomson (1856-1940) discovered the electron. Abegg was smdy ing the inert gases and noticed that their electron... 

For the heavier elements As, Sb, and Bi, further diversity in structure and stoichiometry is found. The ionic bond model becomes less useful as these species may be thought of as intermetallics, possessing metallic luster, and conduction or semiconduction properties. Typical examples include Na3Bi and NaBi, which becomes superconducting at low temperatures (<2.5 K). Further details will be found in the relevant article for each element, As, Sb, and Bi. Zintl anions of these elements are also known. ... 

The plot for MgO (Figure 8.5a) is typical of a main group metal oxide which fits with the classical ionic bonding model of oxide structures. The lower valence band (LVB) consists almost exclusively of 0(2s) states and the UVB of 0(2p) states. In the conduction band (CB) both Mg and O basis functions contribute to the crystal orbitals. The valence bands are completely filled and, since they have mainly O character, this corresponds to complete transfer of valence electrons from Mg to O to give the ionic species Mg and... 

The directionality in the bonding between a d-block metal ion and attached groups such as ammonia or chloride can now be understood in terms of the directional quality of the d orbitals. In 1929, Bethe described the crystal field theory (CFT) model to account for the spectroscopic properties of transition metal ions in crystals. Later, in the 1950s, this theory formed the basis of a widely used bonding model for molecular transition metal compounds. The CFT ionic bonding model has since been superseded by ligand field theory (LFT) and the molecular orbital (MO) theory, which make allowance for covalency in the bonding to the metal ion. However, CFT is still widely used as it provides a simple conceptual model which explains many of the properties of transition metal ions. 

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. 

The validity of the simple ionic bond model is supported by the fact that not only the observed static structures of the clusters are in rather good agreement with the computed stable configurations, but that the dynamic behavior of the clusters is also described by the model. 46)... 

Knowledge of the interatomic distances allows the experimental valence of an atom to be calculated using tables of bond-valence parameters. If the apparent valence of an atom is higher than that expected for an ionic bonding model, say... 

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