Deeth, Robert J., Anastasi, Anna E. and Randell, Kris. (2009) Ligand field torque : a pi-type electronic driving force for determining ligand rotational preferences. Dalton Transactions (No.30). pp. 6007-6012. ISSN 1477-9226

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Abstract

Transition metal complexes with triply-degenerate T ground states are formally Jahn-Teller active but do not usually display the significant bond length distortions familiar from their E ground state counterparts like d(9) Cu(II). The electronic 'asymmetry' for T-state systems lies in the d(pi) orbitals, which interact with the ligands relatively weakly compared to the stronger sigma-type interactions for E-state systems. However, in combination with asymmetric M-L pi bonding, T-type systems have an additional mechanism for relieving the electronic strain. Density functional theory, ligand field theory and ligand field molecular mechanics calculations are used to show how rotations around the M-L bonds can affect their pi-pi (d(pi)-L-pi) interactions and lead to significant energy lowering. For example, d(6) [ Fe(OH2)(6)](2+), which has a T-5(g) state in cubic T-h symmetry, 'distorts' to an S-6 structure 4.4 kcal mol(-1) lower in energy (by DFT) but with six equal Fe-O distances via Fe-O rotations of similar to 20 degrees. and thus masquerades as an apparently regular geometry. Using model systems, we show that this effect is not restricted to formally Jahn-Teller active complexes. The combination of asymmetric p bonding and asymmetric d(pi) orbital occupations can generate an M-L 'torque' worth up to 6 kcal mol(-1) per bond which can 'lock' the ligand in a particular orientation relative to the partially-occupied d orbital(s). The effect is particularly marked for imidazole, the donor group of histidine, which, in a model low-spin d(5) Fe(III) system, shows almost no orientational preference in its neutral form but a very strong (similar to 6 kcal mol(-1)) orientational preference in its deprotonated form.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Divisions:	Faculty of Science > Chemistry
Library of Congress Subject Headings (LCSH):	Ligand field theory, Transition metal complexes -- Structure, Jahn-Teller effect
Journal or Publication Title:	Dalton Transactions
Publisher:	Royal Society of Chemistry
ISSN:	1477-9226
Date:	2009
Number:	No.30
Number of Pages:	6
Page Range:	pp. 6007-6012
Identification Number:	10.1039/b905154f
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
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URI:	http://wrap.warwick.ac.uk/id/eprint/27611

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