Compounds with Group IV Elements

Chemical Shifts and Coupling Constants of Germanium and Lead Compounds (8 in ppm, J in Hz) 

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Transition Metal Pnictides and Compounds with Group IV Elements. 

The chemistry of compounds containing phosphorus and Group IV elements other than carbon, has been much less explored than that of phosphorus with the latter element. In a distinction parallel to that made between organophosphorus and organic phosphorus compounds in Chapter 6.1, it is convenient to divide the compounds with Group IV elements into (a) those which contain a P-X bond (where X = Si, Ge, Sn or Pb), and (b) those which do not. In recent years, a special interest in compounds with P-Si linkages has developed [8]. 

Many molecules and crystals containing equal numbers of B and N (or P) atoms form structures similar to those of the corresponding compounds of Group IV elements. Examples include BN and BP with the diamond structure, the silica-like structures of BPO4 already mentioned, and analogues of substituted cyclohexanes such as [H2B. N(CH3)2] 3 with the same chair configuration as CgHi2-... 

Several examples of oxidative addition reactions of compounds involving group (iv) elements, silicon and germanium, with iridium(i) complexes have been described and reviewed. In the reaction of silicon hydrides (CH3) (C2H50)3 SiH with bis(bis-l, 2-diphenylphosphinoethane)iridium(i), [Ir (diphos)2],... 

General.—A particularly extensive study has been made of the kinetics of reaction with iodine of compounds containing Group IV element to Group IV element bonds, RJM —M R, where R may or may not be the same as R and likewise M and M ... 

All Group IV elements form both a monoxide, MO, and a dioxide, MO2. The stability of the monoxide increases with atomic weight of the Group IV elements from silicon to lead, and lead(II) oxide, PbO, is the most stable oxide of lead. The monoxide becomes more basic as the atomic mass of the Group IV elements increases, but no oxide in this Group is truly basic and even lead(II) oxide is amphoteric. Carbon monoxide has unusual properties and emphasises the different properties of the group head element and its compounds. 

Two-component systems are obtained by the interaction of transition metal compounds of groups IV-VIII of the periodic system with or-ganometallic compounds of groups I-III elements (Ziegler-Natta catalysts). An essential feature of the formation of the propagation centers in these catalysts is the alkylation of the transition metal ions by an organo-metallic cocatalyst. 

Figure 5.2 Correlation of the hardnesses of the Group IV elements, and the associated isoelectronic III-V compounds, with their bond moduli. Room temperature data. For the elements, the molecular volumes refer to the diatoms C-C, Si-Si, Ge-Ge, and Sn-Sn.

Since chemical hardness is related to the gaps in the bonding energy spectra of covalent molecules and solids, the band gap density (Eg/Vm) may be substituted for it. When the shear moduli of the III-V compound crystals (isoelec-tronic with the Group IV elements) are plotted versus the gap density there is again a simple linear correlation. 

It is shown that the stabilities of solids can be related to Parr s physical hardness parameter for solids, and that this is proportional to Pearson s chemical hardness parameter for molecules. For sp-bonded metals, the bulk moduli correlate with the chemical hardness density (CffD), and for covalently bonded crystals, the octahedral shear moduli correlate with CHD. By analogy with molecules, the chemical hardness is related to the gap in the spectrum of bonding energies. This is verified for the Group IV elements and the isoelec-tronic III-V compounds. Since polarization requires excitation of the valence electrons, polarizability is related to band-gaps, and thence to chemical hardness and elastic moduli. Another measure of stability is indentation hardness, and it is shown that this correlates linearly with reciprocal polarizability. Finally, it is shown that theoretical values of critical transformation pressures correlate linearly with indentation hardness numbers, so the latter are a good measure of phase stability. 

Conversely, the stability of compounds of oxidation state +11 increases dramatically as the atomic number of the element increases. Carbenes, CX2, and silylenes, SiX2, are well established as transient reaction intermediates, and structural data have been obtained in several cases either at high temperatures or by their generation in low-temperature matrices. However, only for germanium, tin and lead are compounds in this oxidation state stable under ordinary conditions. Compounds with the Group IV element in oxidation state +111, the formal oxidation state of the radical species, R3M, are also usually considered as unstable transients. However, when R is very bulky, these metal-centred radicals, such as for example Sn[CH(SiMe3)2]3, have extremely long, perhaps indefinite, lifetimes in solution. 

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