Outline

The aim of this project is to identify the effect of ligands in the double bonding of group 14 elements. Several examples of heavier alkene analogues are studied with different ligands. Unlike in organic chemistry where double bonds are almost always planar (to maximise the carbon p orbital overlap which results in a stronger pi bond) heavier alkene analogues have been show to distort from planar geometry. The geometries produced vary but all examples contain large sterically clashing ligands which are required to kinetically stabilise the compounds from polymerisation. This project will try and look at the geometry produced by different sized ligands and ligands with different electronic properties.

Introduction\ Chemistry Bit

Aims and Objectives

Optimise the geometry of molecules L₂E=EL₂ where L = Cl, C₆Cl₆, NH₂, C₆H₃(NH₂)₃ and E = C, Si, Ge, Sn, Pb
Compare the E=E stretching frequency and bond down the group and for different ligands and compare to literature values if possible.
Compute and observe the MOs going down the group for Cl ligands.

Procedure

After initial optimisation the molecules were subject to geometry optimisation by Gaussian. The method used was B3LYP with the LANL2DZ basis set. As some of the molecules initially belonged to point groups other than C1 (and final geometry not expected to be in same point group) and gaussain doesn't change between symmetries the key word "nosymm" was added to the input file. All the calculations were checked to ensure that the RMS gradient was low enough to indicate sucesful completion.
After geometry optimisation a frequency analysis was preformed to ensure that a minimum has been found in the optimisation. Due to restrains on computing time molecules containing C₆Cl₆ ligands did not have a frequency analysis preformed so geometries are unreliable (but are included here as I'd already computed them)

Geometry Results

**Cl Ligand**
Element	Stretch Frequency\cm^-1	Bond Length\nm	Geometry	D-Space Link
Si	303	2.13	Planar	[1]
Ge	256	2.25	Planar	[2]
Sn	201	2.58	Planar	[3]
Pb	n/a	n/a	Apparently not stable in any geometry explodes out into two fragments	[4]

The clear trend here that is that as the group descends and the atomic radius of the atom increases then the central double bond strength decreases resulting in lower energy vibrational frequency of the bond and a longer bond length. This is a general bonding trend seen going down groups rationalised as by saying that more diffuse valence orbitals of a heavier have a smaller overlap than a lighter element. As discussed above it is expected that small strongly electron withdrawing groups should favour the singlet state of the fragment and thus favour trans bending to overcome any steric clashes. This behaviour is not observed as the molecule has no steric interactions as the chlorine atoms are very small.

**C₆H₃(NH₂)₃ Ligand**
Element	Stretch Frequency\cm^-1	Bond Length\nm	Geometry
C	1.37	1632	Planar with large groups twisting to avoid clash	[5]
Si	2.15	712	Twisted by 3.6 degrees	[6]
Ge	2.33	630	Trans Bending of 2.16 degrees	[7]
Sn	3.02	545	Trans Bending of about 10 degrees	[8]
Pb	3.23	none reported	significant distortion (cis bending) suggests that calculation did not work correctly.	[9]

For the amine ligand no elements (other than carbon) formed stable compounds. Lead even formed a four membered ring with two nitrogen atoms. [10][11][12][13][14]

**C₆Cl₆ Ligand**
Element	Bond Length\nm	Geometry
C	1.38	A 4 degree twist was observed.	[15]
Si	2.18	Trans bending of 16 degrees.	[16]
Ge	2.43	Trans bending of 25 degrees.	[17]
Sn	2.99	Trans bending of 35 degrees.	[18]
Pb	40nm<	Bond over 4nm and unusual geometry suggests that two compounds formed.	[19]

Conclusion and Further Work

This project was not explored fully (eg all analysis). Molecular orbitals were calculated for all the chlorine substituted compounds shown above. A more full analysis would include MO analysis for one element with different ligands, a check of the frequencies of the larger ligands and more comparison with the literature.