Main-group metals

Kappes M M 1988 Experimental studies of gas-phase main-group metal olusters Chem. Rev. 88 369... 

It is immediately obvious that the transition metals are more dense, harder, and have higher melting points and boiling points than the main group metals (for example, the metals of Group II,... 

Organic compounds M—R and hydrides M—H of main group metals such as Mg, Zn, B, Al, Sn, SI, and Hg react with A—Pd—X complexes formed by oxidative addition, and an organic group or hydride is transferred to Pd by exchange reaction of X with R or H. In other words, the alkylation of Pd takes place (eq. 9). A driving force of the reaction, which is called transmetallation, is ascribed to the difference in the electronegativities of two metals. A typical example is the phenylation of phenylpalladium iodide with phenyltributyltin to form diphenylpalladium (16). 

Palladation of aromatic compounds with Pd(OAc)2 gives the arylpalladium acetate 25 as an unstable intermediate (see Chapter 3, Section 5). A similar complex 26 is formed by the transmetallation of PdX2 with arylmetal compounds of main group metals such as Hg Those intermediates which have the Pd—C cr-bonds react with nucleophiles or undergo alkene insertion to give oxidized products and Pd(0) as shown below. Hence, these reactions proceed by consuming stoichiometric amounts of Pd(II) compounds, which are reduced to the Pd(0) state. Sometimes, but not always, the reduced Pd(0) is reoxidized in situ to the Pd(II) state. In such a case, the whole oxidation process becomes a catalytic cycle with regard to the Pd(II) compounds. This catalytic reaction is different mechanistically, however, from the Pd(0)-catalyzed reactions described in the next section. These stoichiometric and catalytic reactions are treated in Chapter 3. 

Reactions with Orgunometullic Compounds of the Main Group Metals via Transmetallation... 

In addition, a catalytic version of Tt-allylpalladium chemistry has been devel-oped[6,7]. Formation of the Tr-allylpalladium complexes by the oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of the complex with soft carbon nucleophiles are the basis of catalytic allylation. After the reaction, Pd(0) is reformed, and undergoes oxidative addition to the allylic compounds again, making the reaction catalytic.-In addition to the soft carbon nucleophiles, hard carbon nucleophiles of organometallic compounds of main group metals are allylated with 7r-allylpalladium complexes. The reaction proceeds via transmetallation. These catalytic reactions are treated in this chapter. 

Dienes and allylarcncs can be prepared by the Pd-catalyzcd coupling of allylic compounds with hard carbon nucleophiles derived from alkenyl and aryl compounds of main group metals. Allylic compounds with various leaving groups can be used. Some of them are unreactive with soft nucleophiles, but... 

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

Main Group Element Metallaboranes. A variety of metaHaborane clusters, which incorporate main group metals in vertex positions of polyhedral metaHaborane clusters, have been reported. Examples are (BH BeB H Q (165), MgB2QH22 20(C2H )2 (166), [(CH2)HgB2QH22]A (167),... 

The volatile, air-sensitive Hquid species (CH3)2AlB3Hg and (CH3)2GaB3Hg are prepared by the direct reaction of the corresponding main group metal hahde and salts of the [B3Hg] ion, in the absence of solvent (178). The reaction of (CH3)2AlB3Hg and A1(BH 3 results in the species (BH 2AlB3Hg. These small metallaboranes are fluxional in solution and have limited thermal stability at room temperature. 

Metals (main-group) Metals (transition) Metals (inner transition) Metalloids... 

Design of self-adapting N-heteroaromatic-substituted claw ligands as E /M" " (E = p-block element, M = main group metal)chargedspacers 97CB1365. 

Dibenzotetraaza[14]annulenes as versatile ligands for transition and main group metal chemistry 98CSR105. 

Sh. Garje and V. K. Jain, Main Group Metal Chem. 22,45 (1999). 

In previous chapters we have referred from time to time to compounds of the transition metals. Many of these have relatively simple formulas such as CuSO CrCI3, and FetNO These compounds are ionic The transition metal is present as a simple cation (Cu2+, Cr3+, Fe3+). In that sense, they resemble the ionic compounds formed by the main-group metals, such as CaS04 and AKNOJ ... 

EDTA forms 1 1 complexes with a large number of cations, including those of some of the main-group metals. The complex formed by calcium with EDTA is used to treat lead poisoning. When a... 

The diazonio group in an arenediazonium salt can be replaced by one of several transition metal ions in subgroups lb (Cu), Illb (Tl), IVb (Ge, Sn, Pb), or Vb (P, As, Sb, Bi) or by certain compounds of the transition elements. There is only one report of a substitution by a main group metal, magnesium, but the primary product has not been clearly identified (Nesmeyanov and Makarova, 1959). 

FIGURE 2.2 When a main-group metal atom forms a cation, it loses its valence s-and p-electrons and acquires the electron configuration of the preceding noble-gas atom. The heavier atoms in Croups 1 S/lll and 14/IV retain their complete subshells of d-electrons. 

The elements in Groups 3 through 11 are called the transition metals because they represent a transition from the highly reactive metals of the s block to the much less reactive metals of Group 12 and the p block (Fig. 16.1). Note that the transition metals do not extend all the way across the d block the Group 12 elements (zinc, cadmium, and mercury) are not normally considered to be transition elements. Because their d-orbitals are full, the Group 12 elements have properties that are more like those of main-group metals than those of transition metals. Just after... 

The exhibition of variable valency is indeed a characteristic of transition metals. Main group metal ions such as those of groups 1 or 2 exhibit a single valence state. Other main group metals may show a number of valencies (usually two) which are related by a change in oxidation state of two units. This is typified by the occurrence of lead(iv) and lead(ii) or thallium(iii) and thallium(i). However, all the transition metals exhibit a range of valencies that is generally not limited in this manner. 

Colour - A striking feature of transition-metal compounds is their colour. Whether it is the pale blue or pink hues of copper(ii) sulfate and cobalt(ii) chloride, or the intense purple of potassium permanganate, these colours tend to be associated most commonly with transition-metal compounds. It is rare for compounds of main group metals to be highly coloured. 

The lobes of electron density outside the C-O vector thus offer cr-donor lone-pair character. Surprisingly, carbon monoxide does not form particularly stable complexes with BF3 or with main group metals such as potassium or magnesium. Yet transition-metal complexes with carbon monoxide are known by the thousand. In all cases, the CO ligands are bound to the metal through the carbon atom and the complexes are called carbonyls. Furthermore, the metals occur most usually in low formal oxidation states. Dewar, Chatt and Duncanson have described a bonding scheme for the metal - CO interaction that successfully accounts for the formation and properties of these transition-metal carbonyls. 

In each case, both the entropy and enthalpy terms favour the formation of the chelated complex, regardless of the t/-electron configuration. Note, however, that outside the d block, i.e. with alkaline earths and other main group metals, it is often found that the entropy term is dominant. 

A. Amidinate and guanidinate complexes of main group metals 188... 

Table 8. Selected bond lengths (pm) and angles (°) of tetraorgaonodipnictines and their main group metal and transition metal complexes...

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