Si-N dissociation-recombination

Summary The preparation of new pentacoordinate silicon complexes 4-6 from polychloro- or fluoro-silanes and 0-silylated-iV -dimethylhydrazides is described, along with a typical trigonal-bipyramidal (TBP) crystal structure. Evidence is presented that 4-6 undergo rapid N—>Si dissociation-recombination. Coupling constants are reported extending across the dative bond, even at temperatures in which N-Si dissociation is rapid. A Karplus-type correlation between dihedral angles and vicinal coupling constants is shown. 

Scheme 2. Exchange of the A -methyl groups via N->Si dissociation-recombination.

Reactions 10 and 11 will be sketched in Section 4.3. In Reaction 12, Si+ is supposed to react on NH3 a hydrogen atom is ejected leaving a complex with (H, H, Si, N)+ stoechiometry. This last compound then gives both HSiN and HNSi upon dissociative recombinations. This approach assumes that the intermediate complex has an H-Si-N-H connectivity, as found for the (H, H, C, N)+ cation. 

Coupling Constants through a Rapidly Dissociating-Recombining N—>Si Dative Bond in Pentacoordinate Silicon Chelates... 

We now report the first measurements of coupling constants extending over one, two and three bonds across the dative bond in pentacoordinate silicon chelates. Coupling via the N— Si bond was found to persist even at temperatures at which rapid dissociation and recombination of this bond take place [3,4]. 

It is unnecessary to require two neighbouring activated sites for the dissociative adsorption of N2. The effect of the promotor is not a strictly localized one but influences also the neighbourhood. At the typical concentrations of K+ it is rather unlikely that two neighbour sites are both activated. Thus our rales take into account the fact that an activated site also influences the energetic behaviour of the neighbouring sites. The dissociative adsorption of H2 can take place on every pair of free sites Si-Si, S1-S2 or S2-S2. The concentration of S] is a measure for the concentration of K+ on the surface. If N atom is the nearest neighbour of a H atom reaction occurs to HN — Si,2. Via further reaction steps the product molecule NH3 is formed which desorbs immediately after formation. We neglect recombination reactions. Therefore the basic reaction steps are... 

The extensive surface reconstruction in the presence of N has implications for our discussion of the recombination process, since we must consider whether N2 forms from recombination on the unreconstructed Cu(l 1 1) surface or is formed by decomposition of copper nitride islands. In the latter case N recombination may either leave the local Cu atoms in a metastable (100) arrangement or else recombination might be associated with substantial motion of the Cu atoms as they relax from the nitride adsorption geometry. If N recombination occurs at nitride islands then the dynamics of recombinative desorption will sample a phase space which is completely different to that for dissociation on clean flat Cu terraces, making it impossible to relate these two processes by detailed balance. This is the behaviour of H recombination on Si where the large change in the Si equilibrium geometry induced by H adsorption ensures that the adsorption and desorption processes sample very different channels [13]. 

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