Bonds Madelung energy

The detachment of the Fe(II) into solution. This detachment is facilitated by the increase in surface protonation that is accompanied by the ligand binding. The fact that iron(II) is more readily detached than an iron(III) site from the mineral surface is due to the lower Madelung energy of the Fe(II)-oxygen bond than the Fe(III)-oxygen bond... 

The experimental trends in bonding and structure which we have discussed in the previous chapter cannot be understood within a classical framework. None of the elements and only very few of the thousand or more binary AB compounds are ionic in the sense that the electrostatic Madelung energy controls their bonding. And even for ionic systems, it is a quantum mechanical concept that stops the lattice from collapsing under the resultant attractive electrostatic forces the strong repulsion that arises as the ion cores start to overlap is direct evidence that Pauli s exclusion principle is alive and well and hard at work ... 

Two points of view are applicable to these species, as they also are to the isoelectronic noble gas fluorides (1) a valence bond approach with promotion of electrons to d orbitals and (2) three-center, four-electron bonds. The same arguments, pro and con. apply as given previously, so they will not be repeated here. Independent of die alternative approaches via VB or MO theory, all are agreed that Madelung energy ( ionic character ) is very important in stabilizing both the polyhalide tons and the polyhalogens.27... 

Further, it is observed experimentally that electron-pair bonds are frequently associated with anisotropic, i.e. directed, atomic orbitals. This gives rise to open structures. However, the electrostatic (Madelung) energy associated with ionic crystals favors close packing Therefore largely ionic crystals favor more close-packed, two-sublattice structures such as rock salt versus zinc blende. In the case of two-sublattice structures induced by d electrons, electron-pair bonds are generally prohibited by the metallic or ionic outer s and p electrons that favor close packing. Nevertheless, it will be found in Chapter III, Section II that, if transition element cations are small relative to the anion interstice and simultaneously have Rti RCf electron-pair bonds may be formed below a critical temperature. 

The estimation of lattice energies is based on the calculation of the coulombic (Madelung) energy, which comprises most of the lattice energy, to which an additional bonding energy due to metal-metal attractive interaction—for example, as in some rutile type oxides (26)— is added. The latter may be obtained empirically or by use of ligand field theory (11). 

An ab initio molecular-orbital calculation for the NH3-HC1 dimer was performed and the existence in it of a hydrogen bond was predicted [98-700]. This result was confirmed experimentally when it was found that three N-H bonds are shorter than the fourth one [707], NH4C1 crystallizes in the CsCl-type structure, where NH4 has tetrahedral symmetry, because of the stabilization effect of the Madelung energy... 

Rutile, and many other transition-metal compound structures, arc characterized by dense packing and high coordination numbers (numbers of nearest neighbors). Their bonding properties arc those of ionic solids, and many of the structures have been rationalized in terms of ionic radii and Madelung energies. 

This energy is termed the Madelung energy since it represent a (attice energy" internal to the molecule with a Madelu ng constant, course, ecjual to I.OO. It is a maximum in a purely ionic bond (5 -> Z ) and decreases to whatever extent the charges on X and Y decrease. 

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