Dative bonds And the VSEPR model

Since the C atom marked by an asterisk must form single, covalent bonds to one C atom in each of the two phenyl rings, it follows that the C 0 bond must be double the donor molecule in this complex is Ph2C=0 and the Lewis structure of the free donor would place two non-bonding electron pairs on the O atom. It is thus able to form a dative bond to the A1 atom. The dative bond 0 - Al bond distance is 18 pm longer than the covalent Al-O bond. 

Problem 16.2 Carry out a similar analysis of the bonding in II and show that the Al-N bond must be covalent while the Al-N bond must be dative. Compare the Al-N bond distance with the dative bond distance in (CH3 )3 N- A1(CH3 )3. Suggest a reason why the dative bond distance in II is shorter. 

Problem 16.3 Suggest Lewis structures for the following compounds (CH3)3GeGe(CH3 3, (CH3)3GaAs(CH3)3, (CH3)3GeAs(CH3)2, (CH3)3GeGa(CH3)2, (CH3)2GaAs(CH3 2. 

In Fig. 16.4 we compare the structures of the two complexes (CH3)3N A1(CH3)3 and (CH3)3P- -A1(CH3)3 with the structure of the free acceptor A1(CH3)3. It is seen that the Al-C bonds are elongated and pushed away from the electron donor as the complex is formed. These changes may be interpreted as due to repulsion between the dative D— -Al bond electron pair and the three covalent Al-C bond electron pairs, but may be augmented by steric repulsion between the entire donor molecule and the methyl groups bonded to the A1 atom. It should be noted, however, that in both complexes the angle ZDAIC is smaller than tetrahedral, indicating that the accepted electron pair requires less space on the acceptor atom than the three covalent bond pairs. 

While valence shell electron pair repulsion and steric repulsion between ligands may act in a synergetic manner on the acceptor molecule, they are expected to be opposed on the donor molecule the VSEPR model predicts that partial removal of a lone pair on the donor atom should lead to a reduction of D-C bond distances and an increase of the CDC valence angles. Steric repulsion between the methyl groups bonded to D and the entire acceptor 

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