Divalent silicon

According to ab initio calculations49 the hydrogen isosilanitrile HN=Si contains a divalent silicon atom and an Si=N double bond and is far more stable than the isomeric hydrogen silanitrile HSi=NI. Similar ab... 

It has become common to classify all molecular compounds, which fulfill the above characteristics, as carbene analogs 9,13>. As a consequence, compounds of divalent silicon, germanium, tin, and lead may be regarded as carbene-like and are therefore called silylenes, germylenes, stannylenes, and plumbylenes. In contrast to carbenes they have one property in common the energetically most favorable electronic state is the singlet 1a2 found by experiments and calculations 9). 

To a large extent the chemical shifts of carbon and silicon run parallel, but the chemistry of the two elements is somewhat different. Thus silicon can have extend its valence shell beyond the coordination number of 4. A few stable or-ganosilicon compounds in which silicon is divalent are known (the silylenes), and compounds with a silicon-silicon double bond also exist (the disilenes). 

Some years ago we were successful in our attempts to synthesize a Jt-complex with divalent silicon as the central atom. Starting from dihalogeno(pentamethylcyclopentadienyl)silanes, we have been able to prepare decamethylsilicocene (1) by reductive elimination processes [1]. Characteristic data concerning the synthesis, structure, and bonding of 1 have been published elsewhere together with preliminary results concerning the chemistry of this compound [2]. Here we describe some further progress in this field. 

We have recently prepared some new and very thermolabile CO- and N2 comPlexes derived from titanocene [1] or decamethyltitanocene [2], and have characterized them by their vibrational spectra. As well as "classical" matrix spectroscopy, we have used spectroscopy in liquid xenon (LXe). The application of chemistry and methodology indicates the decamethylsilicocene structure, which represents the first example of a stable jt-complex of divalent silicon [3]. Reaction with CO or N2 leads to the two title complexes [4] ... 

In contrast to silicon, germanium has a well-established though limited chemistry of inorganic compounds in the +11 state which are of reasonable thermal stability though usually air-sensitive. Divalent organogermanium(II) species known at present fall into three groups ... 

The initial photochemical step in almost all of the reactions described in this chapter is formation of either trivalent radicals of the type R3E-, or else the divalent analogues of carbenes, R2E . Such species are obviously very reactive, and are only observed as intermediates or in experiments in the presence of trapping agents. The relative stability of the intermediates depends greatly on the nature of the substituents R, and this can influence the type of reaction products ultimately formed. Where appropriate, comparisons with the behaviour of the analogous silicon species are made. 

Divalent silylenes, silicon congeners of carbenes, are well-established reactive intermediates, whose chemistry has been... 

This is the regime of anodic current densities below JPS. A hole approaching the interface initiates the divalent electrochemical dissolution of a silicon surface atom at the emitter. The dissolution proceeds under formation of H2 and electron injection, as shown in Fig. 4.3. The formation of PS structures is confined to this region. 

Fig. 4.3 Reaction scheme proposed for the anodic, divalent dissolution of silicon electrodes in HF.

Divalent dissolution is initiated by a hole from the bulk approaching the silicon-electrolyte interface which allows for nucleophilic attack of the Si atom (step 1 in Fig. 4.3). This is the rate-limiting step of the reaction and thereby the origin of pore formation, as discussed in Chapter 6. The active species in the electrolyte is HF, its dimer (HF)2, or bifluoride (HF2), which dissociates into HF monomers and l ions near the surface [Okl]. The F ions in the solution seem to be inactive in the dissolution kinetics [Se2], Because holes are only available at a certain anodic bias, the Si dissolution rate becomes virtually zero at OCP and the surface remains Si-H covered in this case, which produces a hydrophobic silicon surface. 

For the electrochemical dissolution of Si in electrolytes composed of anhydrous HF and an organic solvent a reaction is proposed that is similar to the divalent dissolution in aqueous HF. However, molecular hydrogen is not observed and four charge carriers are consumed per dissolved silicon atom, as in the tetravalent case [Pr7, Ril]. 

The origin of the electron injection peak at the end of the dissolution of an oxide film is not understood in detail. Silicon interface atoms with three Si-O bonds and a single Si-Si bond are proposed to be responsible for the effect [Mai]. On the other hand, during the dissolution process silicon interface atoms with one Si-O bond and three Si-Si bonds lead to a configuration identical to the one for which electron injection is observed during divalent dissolution (Fig. 4.3, step 2). In any case, the injected charge exceeds by a factor of 3 to 5 the charge expected... 

For homogeneously doped silicon samples free of metals the identification of cathodic and anodic sites is difficult. In the frame of the quantum size formation model for micro PS, as discussed in Section 7.1, it can be speculated that hole injection by an oxidizing species, according to Eq. (2.2), predominantly occurs into the bulk silicon, because a quantum-confined feature shows an increased VB energy. As a result, hole injection is expected to occur predominantly at the bulk-porous interface and into the bulk Si. The divalent dissolution reaction according to Eq. (4.4) then consumes these holes under formation of micro PS. In this model the limited thickness of stain films can be explained by a reduced rate of hole injection caused by a diffusional limitation for the oxidizing species with increasing film thickness. 

Quinary and senary framework compositions have been synthesized containing aluminum, phosphorus and silicon, with additional combinations of divalent (Me) metals. In the ElAPO and ElAPSO compositions the additional elements Li, Be, B, Ga, Ge, As and Ti have been incorporated into the AIPO4 framework [27]. 

Table 5.4 lists analyses of some natural specimens. In some instances, there is a silicon deficiency with respect to the stoichiometric formula and diadochy of Ai3+ jj4+ pg3+ Qjj tetrahedral sites (cases 1, 2, 3, and 4). In other cases, the amount of sihcon is near the theoretical value and only divalent cations are virtually present in the M and Ml positions (cases 5 and 6). 

Agreement is also poor concerning entropy and volume excess terms. Because divalent cations (Mg, Ca, Fe, Mn) occupy only dodecahedral sites whereas octahedral sites are reserved for trivalent cations (Cr, Fe, Al), each cation has only one site at its disposal and permutability is fixed by stoichiometry (cf. section 3.8.1). As regards the occupancy on tetrahedral positions, we have already seen that analyses of natural specimens show silicon deficiencies, compensated by AF ... 

The chemical properties of germanium fall between those of silicon and tin. It forms both the divalent and tetravalent compounds, the oxidation state +4 being more stable than the +2 oxidation state. The metal is stable in air and water at ambient temperatures. However, it reacts with oxygen at elevated temperatures forming divalent and tetravalent oxides, GeO and Ge02. 

Silicon forms two oxides, silica or the dioxide, Si02, and a divalent monoxide, SiO. The latter is obtained by heating silica with elementary silicon at 1,450°C in vacuum. Silicon sublimes as its monoxide, which on rapid cooling forms light brown amorphous SiO ... 

The unit cell of garnet contains eight formula units. It s silicon-oxygen tetra-hedra exist as independent groups linked to octahedral of the trivalent ions, while the divalent metal ions are situated in the interstices within the Si-Al network, each divalent ion being surrounded by eight oxygen atoms. 

The soluble divalent SiF2 compound is in turn transformed by disproportionation into SiF6 and elemental amorphous silicon. This mechanism is responsible for the effective dissolution valence of Si, which was found to be equal to 2 in the range of potential between 0 and +0.4V/SCE. 

Divalent silicon compounds (silylenes), one of the most interesting class of low-coordinated silicon compounds, had been known as highly reactive, short-lived transient species that resembled the carbon analogues (carbenes), until the recent... 

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