Chem 426/527 problem set 2
1. Consider the following two anions and accompanying data in light of the possibility of BN and PN p bonding.
The BN and PN bond lengths are 1.51 and 1.968 Å, respectively.
The sum of the three bond angles at N is 360º and the sum of angles at P is 310º
(a) What is learned by comparing the BN and PN bond lengths? What (else) would you want to compare the bond length data to?
(b) What is the significance of the bond angle sum data?
(c) Sketch the p bonding interaction between the heteroatoms for both compounds. Use the data and your answers to explain any differences in behaviour between A and B and offer reasons why they are different.
2. Consider the following three compounds. The first of these is the phosphazonium cation we discussed in class which contains a PN triple bond. The specific bonding in the latter two compounds is not drawn in. Mes* = 2,4,6-tri(t-butyl)phenyl. Use the data provided and describe the bonding in B and C. Draw reasonable Lewis structures for these molecules.
Compound A: d(PN) = 1.467 Å
Bond angle at N = 176º
Compound B: d(PN) = 1.486 Å
d(PP) = 2.625Å
Bond angle at N = 170º
Bond angle at (two-coordinate) P = 110º
Compound C: d(central PP) = 2.025 Å
d(outer PP) = 2.206Å
Bond angle at leftmost P = 99º
Bond angle at central P = 97º
3. (a) Why doesn’t the following reaction occur to form a PO p bond?
(b) Is there p bonding in phosphine oxides, R3PO? If so, describe it. If not, describe the PO bond in these molecules.
4. Use isolobal relationships with well-known organic p bonded systems to design a hypothetical (but chemically reasonable; include appropriate substituents top provide any necessary stability) species with
(a) a boron-boron double bond;
(b) a gallium-gallium triple bond.