Metal cations electronegativity

In principle, any molecule or anion with an unshared pair of electrons can act as a Lewis base. In other words, it can donate a lone pair to a metal cation to form a coordinate covalent bond. In practice, a ligand usually contains an atom of one of die more electronegative elements (C, N, O, S, F, Cl, Br, I). Several hundred different ligands are known. Those most commonly encountered in general chemistry are NH3 and HzO molecules and CN , Cl-, and OH- ions. 

The designation of hard acids is not restricted to metal cations. For example, in BF3 the small boron atom in its +3 oxidation state is bonded to three highly electronegative fluorine atoms. All the B—F bonds are polarized away from a boron center that is already electron-deficient. Boron trifiuoride is a hard Lewis acid. 

Because of their low intrinsic electronegativities, neutral late transition metals (bearing an abundance of lone pairs) can serve as good donors in nM— ctah interactions of the form (5.69a). Furthermore, transition-metal-hydride bonds (Section 4.4.1) often display sufficient covalency or polar-covalency (particularly in transition-metal cations) to serve as good acceptors in ns— ctmh interactions of the form (5.69b). In the present section we shall briefly examine the simple example of platinum dihydride (PtH2) as a water-mimic in binary H-bonded complexes with H20,... 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Y. Nakamori, K. Miwa, A. Ninomiya, H. Li, N. Ohba, S.-l. Towata, A. Ziittel, S.-l. Orimo, Correlation between thermodynamical stabilities of metal borohydrides and cation electronegatives First principles calculations and experiments , Phys. Rev. B 74 (2006) 45126. 

Many nonionic organic contaminants require extreme acid conditions to accept H+ ions. In clays, the extent of protonation is related to the electronegativity and polarizing power of structural metal cations, in the order H+ > Al + > Fe > Mg +... 

Later on, Pearson [75] introduced the concept of hard and soft acid and bases (HSABs) hard acids (defined as small-sized, highly positively charged, and not easily polarizable electron acceptor) prefer to associate with hard bases (i.e., substances that hold their electrons tightly as a consequence of large electronegativities, low polarizabilities, and difficnlty of oxidation of their donor atoms) and soft acids prefer to associate with soft bases, giving thermodynamically more stable complexes. According to this theory, the proton is a hard acid, whereas metal cations may have different hardnesses. 

The reactions of chlorobenzene and benzaldehyde with ammonia over metal Y zeolites have been studied by a pulse technique. For aniline formation from the reaction of chlorobenzene and ammonia, the transition metal forms of Y zeolites show good activity, but alkali and alkaline earth metal forms do not. For CuY, the main products are aniline and benzene. The order of catalytic activity of the metal ions isCu> Ni > Zn> Cr> Co > Cd > Mn > Mg, Ca, Na 0. This order has no relation to the order of electrostatic potential or ionic radius, but is closely related to the order of electronegativity or ammine complex formation constant of metal cations. For benzonitrile formation from benzaldehyde and ammonia, every cation form of Y zeolite shows high activity. 

The ionic radius or electrostatic potential represents the physical property of metal cations and does not reflect the bonding character. The electronegativity of metal cations may be the more direct measure of the polarizing power than the ionic radius or electrostatic field when chemical bonding is expected between metal cations and the reactants. 

As described above, the catalytic activity of metal ion-exchanged zeolites for aniline formation has a good correlation with electronegativity and with the formation constant of ammine complexes of metal cations. The order of the activity agrees with the Irving-Williams order. These facts give irrefutable evidence that the transition metal cations are the active centers of the reaction. 

The good correlation of catalytic activity and the formation constant of the ammine complex or the electronegativity of the metal cation could... 

As for type (1), Niiyama et al. (42) proposed that protons are generated by dissociation of water and that the equilibrium of the dissociation is a function of the electronegativity of the metal cations. Formation of Bransted acid sites in the aluminum salt of H3PW12O40 as a result of exposure to water vapor at 573 K was confirmed by IR spectra of sorbed pyridine (138). 

In carbanions, for instance enolates, in which delocalization of the negative charge to a more electronegative atom (N, O, S) is possible, the cation is usually bound to this atom if the cation is hard (e.g. alkali metal cations, Mg2+ [530], Zn2+) in this case the nucleophilic carbon atom becomes a planar sp2 hybrid. If the cation is soft (e.g. the cations of late transition metals) it will often be bound to the depro-tonated carbon atom these organometallic compounds are, however, usually only weak nucleophiles and will not be treated here. 

This treatment of coordination polymerization with Ziegler type catalysts is independent of whether the growing polymer end is attached to a Group I—III metal or to a transition metal. Both types can be obtained depending upon the alkylating ability of the alkyl metal with respect to the transition metal component. In both cases the ionic character of the M-R bond can vary appreciably with the cation electronegativity, i. e., with the metal, its oxidation state and ligands. The... 

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