Predict the geometries of (CH 3 ) 2 P(CF 3 ) 3 and (CH 3 ) 3 P(CF 3 ) 2 . Do you

Question

Predict the geometries of (CH3)2P(CF3)3 and (CH3)3P(CF3)2. Do you expect these molecules to undergo pseudorotation?

My understanding is that (CH3)3P(CF3)2 would be trigonal bypramidal with the CF3 ligands being the axial ligands, whiel the methyl groups would be the equatorial ligands. CF3 is the more electronegative ligand. According to bent's rule, they prefer orbitals with less s character. the tringal plane character is sp2, p dominant but consists of s character. The axial ligands would be d dominant.

(CH3)2P(CF3)3 would be the same but would undergo pseudorotation into square pyramidal? I'm just unsure why this is a favorable conformation, even though the energy diagrams are the same.

Explanation / Answer

X-Ray diffraction studies show that bond angles are closer to 90 degrees. No bond angle is closer to 120 degrees. This confirm that square pyramidal structrure is more favaorable. Because in square pyramidal structure all the bond angles are 90 degrees. In triginal bipyramidal strcture some bond angles are 90 degrees and some are 120 degrees, bond angles between equitorial bonds are 120 degree and bond angles between axial bond and equitorial bond are 90 degrees.

F-19 NMR data also supports square pyramidal structure rather than trigonal pyramidal structure.

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