CHM 401 Fall 2000 Final Exam Question 10 answer

Final Exam

December 21, 2000

10. A. D. Richardson, K. Hedberg, and G. M. Lucier (p. 2787-2793) measured the structures of gas-phase WF₆, ReF₆, OsF₆, IrF₆, and PtF₆. The average M-F bond distances were found to be 1.829 Å, 1.829 Å, 1.828 Å, 1.839 Å, and 1.852 Å, respectively. Are these values consistent with what you would expect based on periodic trends? Why or why not? Find the LFSE for each of these complexes. How would you expect the LFSE to affect the bond lengths? Which of these complexes would you expect to be Jahn-Teller active? For those that are Jahn-Teller active, predict the mode of distortion.

Answer:

Based on the increasing Z* from W to Pt, the bond lengths should be expected to decrease from WF₆ to PtF₆. This prediction is not consistent with the observation.

F^– is a weak ligand, so all the complexes will be high spin in an octahedral environment.

Compound

Bond Length

Electron Configuration

LFSE

Jahn-Teller Active?

Distortion

WF₆

1.829 Å

W^{6+, d⁰}

0 Dq

t_2g^{0e_g^{0, No}}

–

ReF₆

1.829 Å

Re^{6+, d¹}

–4Dq

t_2g^{1e_g^{0, Yes}}

axial compression

OsF₆

1.828 Å

Os^{6+, d²}

–8Dq

t_2g^{2e_g^{0, Yes}}

axial elongation

IrF₆

1.839 Å

Ir^{6+, d³}

–12Dq

t_2g^{3e_g^{0, No}}

–

PtF₆

1.852 Å

Pt^{6+, d⁴}

–6Dq

t_2g^{3e_g^{1, Yes}}

either

With increasing LFSE, it might be expected that the bond lengths would decrease from WF₆ to IrF₆ and then become longer for PtF₆. This is not consistent with observation, either.

The best simple explanation is to use MO theory, where the t_2g orbitals are mostly nonbonding and the e_g orbitals are antibonding. Then, from WF₆ to IrF₆ there should be little change in the M-F bond length (pretty much as observed) and then a significant increase in the Pt-F since the antibonding e_g orbital becomes occupied.

Compound	Bond Length	Electron Configuration	LFSE	Jahn-Teller Active?	Distortion
WF₆	1.829 Å	W^{6+, d⁰}	0 Dq	t_2g^{0e_g^{0, No}}	–
ReF₆	1.829 Å	Re^{6+, d¹}	–4Dq	t_2g^{1e_g^{0, Yes}}	axial compression
OsF₆	1.828 Å	Os^{6+, d²}	–8Dq	t_2g^{2e_g^{0, Yes}}	axial elongation
IrF₆	1.839 Å	Ir^{6+, d³}	–12Dq	t_2g^{3e_g^{0, No}}	–
PtF₆	1.852 Å	Pt^{6+, d⁴}	–6Dq	t_2g^{3e_g^{1, Yes}}	either