lecture 7

Question 1

the 2 different types of pi bonding in phospazenes

Answer 1

in plane pi bonding
out of plane pi bonding

these 2 can be described bby using the d-orbitals or the sigma* orbitals on P!!

Question 2

why is the use of orbitals controversial in phosphazenes

Answer 2

bc the use of 3d orbitals on P would help create a MO description of the bonding situation in phosphazines

HOWEVER

d orbitals are probs too high in energy to use.

without using the high energy d orbitals - we need to us the electrostatic effects // the lower energy sigma* orbitals on P.

Question 3

with the use of the d orbitals on PF5,, what shape do we get

Answer 3

trigonal bipyramidal

sp3d

the p would couple to the 2 axial F’s (triplet) and the 3 equitorial F’s (quartet)

to give a triplet of quartets.

Question 4

if we arent using the d orbitals,, what hybridisation will PF5 have

Answer 4

it will be sp2
1 unhybrid p orbital which can bond to axial F’s

and 3 hybrids to bond with the equitorial F’s.

3c-4e bond: 3 atoms 4 e- bond.

Question 5

using d orbitals on P i phosphates: pi acceptor ability of Phosphines as ligands to transition metals

Answer 5

the lone pair on P can be donated to the d orbitals on the metals

backbonding can also occur where the d orbitals n the metal can donate to the d orbitals on the P.

Question 6

without using d orbitals on P i phosphates: pi acceptor ability of Phosphines as ligands to transition metals

Answer 6

the d orbital on the metal donates to the empty p orbital on the P.

theres a sigma* between the P and the R group its attached to.

Question 7

sigma framework of phosphazine: hybridisation of N and P

Answer 7

N = sp2 (lone pair)
P is approx sp3

P uses 4 e- in the sigma framework

Question 8

d orbitals on p: out of plane pi bodning

Answer 8

P has one e- in the out if plane dxz orbital :can be used for pi bonding with the N pz orbital

both N and P give 1e- for pi bonding

gives 6MO:
3/6 = bonding
2/3= strongly bonding
1/3 = weakly bonding

neither bonding MO has full delocalisation around the ring: so phosphazes are not true aromatics.

Question 9

what causes a slight PP bond shortening in out of plane pi bonding

Answer 9

the P has a second out of plane (dyz) orbital which can provide a minor bonding contribution inside the heterocyclic ring to the out of plane pi bonding.

so theres a dyz bonding contribution to the pi bond - and this adds a small cross ring PP bond interaction which leads to a slight PP bond shortening distance.