6. Comparision of Carbon and Silicon

• Catenation: formation of E-E bonded compounds

 For the reaction:

 E₂H₆  EH₄ + E + H₂

 DH = B(E-E) + 2B(E-H) -  DH_atom(E) - B(H-H)
 

 Long chain C-C stable. due to strong C-C and C-H bonds.
 

• Long chain silanes unstable with respect to SiH₄ due to weak Si-Si and Si-H bonds.

• Down group IV(14), E-E bonds increasingly inlikely.

Single vs. Multiple Bonds

• C-C/C=C versus Si-Si/Si=Si

 
• Small core for C, good overlap => strong p-bonds

 
• Large Si, Si-Si longer => weak p-bonds

So could expect C=C but not Si=Si   => no silicon analogues to graphite.

2B(E-E) > B(E=E) for both Si and C
 

=>  C=C, Si=Si unstable with respect to E-E

 => Kinetic stability in C again: So ethene is unstable with respect to polyethene but needs a catalyst.

 
Compounds containing Si=Si double bonds are now known: They are stabilised by using sterically bulky ligands eg. Also CO₂ molecular, CO₃^2- isolated ions => Intramolecular p-bonding

 SiO₂, silicates are "polymerised => Internals-bonds strong
 

B(C=O) < 2B(C-O) but B(Si=O) > 2B(Si-O)

Copyright Dr. D. O'Hare 1996.