Silicon electron donor-acceptor

The versatility of silicon in organic chemistry has been attributed to its mildness as a metal5 to this should be added its dichotomous electron donor-acceptor properties. Under appropriate conditions proximate silicon groups can stabilize negative or positive charge and can strongly perturb the rc-system in a variety of molecules. 

Unlike carbon, the silicon atom may utilise vacant orbitals to expand its valence beyond four, to five or six, forming additional bonds with electron donors. This is shown by isolated amine complexes. The stabiUty of the organosHane amine complexes varies over a wide range and depends on the nature of the donor and acceptor (2). 

Dihydro-lH-l,5,2-azasilaboroles derive from the 2,5-dihydro-lH-l,2-aza-boroles ( 6.5.3.3) by substitution of the carbon neighboring N by a silicon atom. They may act as four-electron donors using electron density from the C=C double bond and the N atom. The B atom behaves as an acceptor center. Two pathways are known for complex synthesis reaction with a generated transition-metal complex fragment and reaction with metal atoms by the metal-vapor synthesis method. 

It is assumed that this behaviour can be explained by an additional interaction between the undivided p-electrons of the oxygen atom and the vacant 3d-orbitals of the silicon atom (the so called (p -dj-interaction).96 The ideas about the nature of the siloxane bond were developed by Voronkov.97 He stated that (pT-dT)-bonds of a donor-acceptor type are formed in a Si-O-Si system owing to the undivided electron pairs of oxygen and the vacant d-orbitals of the silicon atoms. In this case, the oxygen atom is in a state of hybridization, intermediate between sp2 and sp. 

Since an interaction between two molecules generally involves charge transfer, it may be considered as a donor-acceptor interaction in the widest sense of the word The formation of coordination donor-acceptor bonds between acceptors with low-lying vacant AOs and donors with accessible electron pairs is typical of the majority of elemnts of the periodic system, e.g., the silicon atom can form more bonds than appears to be allowed by the octet rule An eminent problem... 

While complete X-ray analysis will establish the structure in the solid state, it is useful to have NMR data on the solution state that illustrate the increase of the coordination number of silicon. It would seem that NMR spectroscopy of nuclei participating directly in donor-acceptor interaction is especially important in investigating silicon compounds with an expanded coordination sphere. This requires the use of Si NMR spectroscopy since the electron shell of the silicon atom, the bond angles and lenghts are strongly affected upon complexation. Valuable information could also be obtained with by " N, N, 0, F NMR data since these elements act as donors. Chemical shifts of nuclei other than hydrogen are determined by various factors and not yet understood well anough to provide easily applied correlations of other physical properties of the molecules. 

Silicon-nitrogen coupling constants, J( Si— N), measuring the s electron density in the Si<-N bond, form a regular trend in reasonable agreement with the known order of the donor-acceptor bond strengths (Table 20). 

Summary Isolobal replacement of the NRj-unit in silatranes 1 by a d ML4-unit leads to metallosilatranes 2. The cage compounds 2 show weak, attractive donor-acceptor interactions. As observed by X-ray difihaction there exist sub-van-der-Waals distances between silicon and nickel. The proposed 3-center 4-electron interaction is in good agreement with some unusual NMR data of 2. 

Silatranes 1 are meanwhile classical cage compounds with donor-acceptor interactions and represent examples of hypercoordinated silicon [2], The donor-acceptor contact in 1 is formed by an interaction of the Lewis-basic amino group with the Lewis-acidic silicon center favored by the chelate effect. Numerous examples show that electron-rich transition metal complexes also possess Lewis-basic properties [3, 4]. Isolobal replacement of the NR3-unit in 1 by a d ML4-unit [5] leads to compounds of type 2 [1,6, 7]. These Si/Ni-cages 2 can be regarded as metallosilatranes. Here we report on the s5mthesis, structure and bonding of 2. 

Extrinsic semiconductors are materials that contain donor or acceptor species (called doping substances) that provide electrons to the conduction band or holes to the valence band. If donor impurities (donating electrons) are present in minerals, the conduction is mainly by way of electrons, and the material is called an n-type semiconductor. If acceptors are the major impurities present, conduction is mainly by way of holes and the material is called a p-type semiconductor. For instance, in a silicon semiconductor elements from the vertical row to the right of Si of the Periodic Table (e.g.. As) behave as electron donors (As - As + e ) while elements from the vertical row to the left of Si (e.g., Ga) behave as hole donors (Ga + e Ga ) that is, in the latter case, electrons are excited from the vb into the acceptor sites, leaving behind mobile holes in the vb with the formation of isolated negatively charged acceptor sites. 

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