Cations main-group

Essentially all transition metal ions behave like Zn2+, forming a weakly acidic solution. Among the main-group cations, Al3+ and, to a lesser extent, Mg2+, act as weak acids. In contrast the cations in Group 1 show little or no tendency to react with water. 

Four of the main-group cations are essential in human nutrition (Table A). Of these, the most important is Ca2+. About 90% of the calcium in the body is found in bones and teeth, largely in the form of hydroxyapatite, CatOH)2 - SCa PO. Calcium ions in bones and teeth exchange readily with those in the blood about 0.6 g of Ca2+ enters and leaves your bones every day. In a normal adult this exchange is in balance, but in elderly people, particularly women, there is sometimes a net loss of bone calcium, leading to the disease known as osteoporosis. 

Polysulfides have been prepared with many different types of cations, both monoatomic Hke alkah metal ions and polyatomic Hke ammonium or substituted ammonium or phosphonium ions. In this chapter only those salts will be discussed in detail which contain univalent main-group cations although a large number of transition metal polysulfido complexes have been prepared [7-9]. 

In these examples, it can be seen that the carbon and chlorine atoms can achieve octets of electrons by sharing pairs of electrons with other atoms. Hydrogen atoms attain duets of electrons because the first shell is complete when it contains two electrons. We note from Sec. 5.4 that main group cations generally lose all their valence electrons, and then have none left in their valence shell. 

Why are transition metals well suited for catalysis of this process Certainly the electrophilicity of cationic metal centers is important, as is the relative weakness of transition-metal-carbon bonds. However, similar electrophilicities and bond strengths could be found among main-group cations as well. A key to the effectiveness of Ti catalysts is the presence of two metal-based acceptor orbitals. In effect, two such orbitals are needed to choreograph the reversal of net charge flow at the two alkene carbons as the intermediate alkene complex moves through the transition state toward the final product. 

The generation of main-group cations by use of xenon cations as oxidants has been limited to salts of the XeF+ 49,56 and C6F5Xe+ 58 cations and has focused on the oxidation of the central element rather than on the oxidation of a ligand. 

Do the bonding principles found for dihydrogen-transition metal complexes apply to main group cations ... 

One of the best known examples of electronically induced distortion is the steric effect of the non-bonding valence-shell electrons found in main group cations in low oxidation states, cations such as S" " ", and Sn, in which one or more pairs of valence electrons are not involved in chemical bonding. Such nonbonding electrons are popularly known as lone pairs because they occur as localized spin-paired electrons. 

Table 2.3 contains the standard Gibbs energies of formation and the standard enthalpies of formation of a selection of main group cations at 25 °C. They refer to the formation of 1 mol dm- solutions of the cations from their elements and are relative to the values for the hydrated proton taken as zero. 

Table 2.3 Standard molar Gibbs energies ol formalion and standard molar enthalpies of formation of some main group cations at 25 "C (in kj mol-1)...

Table 2.9 Ionic radii, and ihose given by assuming equalion (2.43) to be correct, for some main group cations...

The conventional standard entropies of some main group cations are given in Table 2.13, which also includes the standard entropies of the elements. 

FIGURE 2.11 Main-group cations and anions. A cation bears the same name as the element it is derived from an anion name has an -ide ending. 

Similar calculations have yet to be completed for molecules with the main group X-cations for rows in the periodic table beyond the second. Nonetheless, it was found6 that the observed bond length data for the main group cations for all six rows of the periodic table correlate with s in six separate but essentially parallel trends similar to those displayed in Figure 2. In a search for a parameter that would rank all of the bond length data in a single trend, a bond order parameter p = s/r was defined where r =... 

FIGURE 4. Scatter diagrams of bond length data (a) calculated for H2n-m X+mO molecules and (b) observed for main group cations vs the bond order parameter p. The expression R(X—O) = 1.39p-2/9 serves to model both sets of data equally well... 

A. H. Cowley, C. L. B. Macdonald, J. S. Silverman, J. D. Gorden, and A. Voigt, Triple-Decker Main Group Cations, Chem. Comm. 2001, 175-176. 

Most main group cations and all monatomic anions have characteristic charges that are easy to learn, so in their compounds with other ions, the charges on the other ions are easy to deduce. 

The tetrahedrally coordinated complex [Fe HB(Bu Im)3 Br] is isostructural to the main group compound [Mg HB(Bu Im)3 Br] [403] and the transition metal compound [Co HB(Bu Im)3 Cl] [404], The smaller main group cation Li+ combined with the smaller tris-NHC ligand HB(EtIm)3 surprises us with a dinuclear [Li3 HB(EtIm)3 3] complex... 

Note that the main group cation needs no valence electrons. No covalent bonds exist in MgCl2 there is no electron sharing. 

Coordination to metals follows the usual trends. The transition metals try to achieve octahedral coordination (with a few exceptions), but the cations of the electropositive group 1-3 elements exhibit a rich variety. The coordination polyhedra are determined by radius ratios more than by topological preferences. For polyphosphides in general, all P atoms are involved in M-P interactions according to the number of lone pairs present. The anionic (lb)P and (2b)P as well as the neutral (3b)P° species adopt quasi-tetrahedral coordination, especially if main-group cations are involved. Only a few exceptions are known, for example Li3P7. With more covalent M-P bonds, the number (m + n) of available lone pairs of a polyanion P " is strongly related to the metal coordination number that is, CN(M) < m + n). If CN(M) > m + n), ion-ion and ion-dipole interactions dominate. The relation <7[M-(2b)P] > <7[M-(3b)P] is true in most cases. 

Acknowledgement This chapter is dedicated to Jack Passmore with whom I had the honor to spent my postdoctoral fellowship and who introduced me to the fascinating world of small but electronically dehcate main group cations such as the homopolyatomic sulfur cations. Thank you This would have been impossible without the fine and bureaucracy-free sponsorship of the Alexander von Humboldt Stiftung in Bonn, Germany. 

While the network equations hold for the majority of inorganic compounds there are occasions when they fail. The Equal Valence Rule is not obeyed when the atomic environment is distorted by electronic effects. Such effects are found in two classes of cation main group cations in which the valence shell contains electron pairs that are not involved in chemical bonding, e.g. Sn and S , and transition elements in which the partially filled d shell can influence the coordination environment, e.g. the Jahn-Teller distortion typically found around the Cu " cations. The influence of electronic effects on the bond valences are discussed in Section 10.6.1. 

Neither gold(i) nor gold(lll) forms a stable aquated ion ([Au(OH2)2 4 ] or [Au(OH2)4 ], respectively) analogous to those found for many transition metal and main group cations. Both are thermodynamically unstable with respect to elemental gold and can be readily reduced ... 

SiH3(H2)2 > and analogous B and A1 series starting with BHg, AIH4, and AlHe are rationalized theoretically as highly dynamic H2 complexes of main group cations. 

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