In the strictest sense, no molecule is absolutely rigid even at absolute temperature……

FLUXIONAL MOLECULES

What is Fluxional Molecule?There are some molecules in which molecular

vibration can convert a molecule from one configuration into another..

Such sort of molecules are called Stereo chemically no rigid molecules…

Whenever the different configurations of a stereo chemically no rigid molecules are equivalent, they are called Fluxional molecules …..

The rate of such type of interconversion is in the range of 102 to 105 S-1 at temperature between +1500 to -1500 c

Distinction Between Polytopal Rearrangement and Fluxionality A number of stereo chemically non rigid molecules execute a special type of

rearrangement by which they can be interconverted to nonequivalent polyhedral topology. Such sort of rearrangement is called Polytopal Rearrangement…That means it is different from Fluxional molecules where different configurations are equivalent. It is normally seen in 4,5 and 8 coordinated system where energy difference between different topologies is comparable ….

A classic example is given below- (Allogons)

PEtPh2 Br PEtPh2

CS2,-780 C Ni Ni Br PEtPh2 Heat Ph2EtP Br

Br Tetrahedral(dark green)paramagnetic Square planar (dark brown)diamagnetic

FLUXIONALITY OF PF5PF5 has TBP(trigonal bipyramidal structure) with three F

atoms are in equatorial position and two F atoms are in axial position…

But in reality these equatorial F atoms are interchanged with two axial F atoms leading to the generation of Fluxionality in the molecule…

F1 F4 F4

F1 F3 P F3 P

F5

F2 F2 F5

Mechanistic Representation Of Fluxionality In PF5

The Fluxionality of PF5

molecule can bemechanistically explored depicting Berry Pseudo rotation process in which two axial and two equatorial F atoms are interchanged with each other..But one equatorial F atom remains intact in its original position, this group is called pivot group..

Berry’s Pseudo Rotation

Spectroscopic Evidence Of Fluxionality In PF5

By Means of 19F NMR Spectroscopy Fluxionality Of PF5 can be monitored..

Qualitative NMR Spectra Of PF5 can be -

( At room temperature) Only single intense peak of five equivalent F atoms is

observed. This is because of the rapid exchange of position among axial and equatorial F atoms ,they remain practically in equivalent environment even in short time-span of NMR experiment…

( At very low temperature) Two peaks with relative intensity of 3:2 are observed. This

is due to the fact that at very low temperature the above mentioned rapid exchange of F atoms is seized down resulting two distinct type of environment of F atoms i.e. one axial type(two F atoms) and another equatorial type (three F atoms)…

Structure of XeF6

XeF6 has total 14 electrons in its Xe centre having six bond pairs and one stereochemically active lonepair.So, from VSEPR theory we expect a pentagonal bipyramidal structure with a lonepair.

But in reality it exist as distorted octahedral structure in solid state, which is illustrated as [XeF5]+F- with five bond pairs and one lone pair.

In the vapour phase it exist in three different structures.They are –

1.Capped trigonal prism2.Pentagonal bipyramid(lone pair is projected along one corner of basal

plane)3.Capped Octahedron(lone pair is projected along the centre of a trigonal

face )

Fluxionality Of XeF6

Xe Xe

Xe

Capped Trigonal prism Pentagonal bipyramid Distorted octahedron

FF

F

F F

F

..

F

FF

F

..

F

F

F F

F F

F

F

..

Fluxionality Of Organometallic Compounds There are variety of organometallic compounds

which shows fluxional behavior..Some of the examples are provided below….

1.Metal –Allyl Complexes2.Fe(CO)2(η1-Cp)(η5-Cp)

3.Ti(η1-Cp)2(η5-Cp)2

4.Co(η3-C7H7)(η5-C5H5)5.Mo(CO)2(η3-C7H7)(η5-C5H5)

Fluxional Behavior Of Metal-Allyl Complex

Ha Ha Ha

Hb Hb Hb

H M H M H

Hb M Hb

Ha Ha Hb Ha

η3-Allyl Complex η1-Allyl Complex η3- Allyl Complex

σ-π Rearrangement Of Allyl Ligand

Spectroscopic Characterization Of Fluxional Metal-Allyl Complex

Fluxionality Of Metal-Allyl complex can be judged by 1H-NMR Studies….

(At Room Temperature)

one peak Two Doublet total three signals

(At 1400 C Temperature)

One peak One doublet Total two signals

Fluxionality Of Fe(CO)2(η1-Cp)(η5-Cp)

CO H Fe CO

Its Fluxionality can be easily monitored by 1H-NMR

Study…. At room temperature two peaks are observed one at 4.4

ppm and another at 5.7 ppm At low temperature a number of signals are obtained

between the value of 6.5 to 5.9 ppm…

A Mechanistic Overview Of 1,2 Shift in Metal(η1-C5H5) system

A detail of 1,2 shift mechanism is depicted below-

M

H

M M H H

Fluxionality Of Ti(η1-Cp)2(η5-Cp)2

1H- NMR Study reveals H At -320c temperature it shows two peaks Ti H

Where as at 62o c temperature it on

gives only one peak

A Special type of Fluxionality-Metal Carbonyl Scrambling

Di and poly nuclear metal carbonyl complex shows a special kind of fluxional behavior in which CO group is scrambled over a skeleton of two or more metal atoms, such kind of Fluxionality is termed Metal carbonyl Scrambling… a Classic example is given below

CO CO CO

Fe Fe Fe Fe

CO CO CO CO CO Cis-Form Trans-

Form

Spectroscopic Detection Of Metal Carbonyl ScramblingBy 1H-NMR Studies it can be quite

convincingly detected…..

At room temperature (280 C)- We get one peak

At -700 C temperature- we get two distinct peaks

Special Thank s to Dr. Subrata Bhattacharya

Thanks for listening

Viswanath

Temple,BHU