When two or more Nuclear Configurations are Chemically equivalent then this type of stereochemical non-Rigidity is called Fluxionality and molecules are called Fluxional.
●A fluxional molecule is one that undergoes a dynamic molecular process that interchanges two or more chemically and/or magnetically different groups in a molecule.
●two Fluxional molecules have comparable stability.
●such molecules change between or among these structure continuously even at room temperature .thus these molecule are called stereochemically non rigid.
●Fluxional molecules have small Homo-Lumo gap and hence one configuration can be transformed into another configuration with low energy barrier to rotation.
How fluxionality is detected
1.all molecules are fluxional in some respects, by bond rotations in most organic compounds , the term fluxional depends on the context and the method used to assess the dynamics.
2.Specroscopic study:-Often, a molecule is considered fluxional if its spectroscopic signature exhibits line-broadening due to chemical exchange.
3.Isotope labeling:-In some cases, where the rates are slow, fluxionality is not detected spectroscopically, but by isotopic labeling.
Rate of Fluxionality
•Sometime the exchange takes places at a rate that is comparable with NMR time scale .
•when this happens ,we can usually show the exchange down by cooling the sample until we see the static spectrum this is called low temperature limit.
•On the other hand if we warm the sample, the rate of exchange may rise to the extent that the fully averaged spectra is observed i.e. high temp-limit.
•in between these two extremes , broader resonance are usually seen.
Let us consider simple dynamic process involving only two molecular configurations A & B having equal probability.
their interconversion is shown in fig. As
•Case1:-When temp. is lowered,rate of interconversion of configuration become slower then NMR time scale.
In this situation two separate set of equal intensity resonance ,one for each configuration will be observed .it is called static NMR Spectrum in the lower temp limit.
•Case 2:-at higher temp limit, two configuration of molecule interconvert so rapidly tha NMR cannot distinguish between two .i.e.is called dynamic NMR spectrum.
•In between above two extremes,i.e.at intermediate interconversion rate , broadened resonance are usually observed.
The maximum broadening of spectral lines occurs when the life time (δt) of a configuration gives rise to a line-alt=”abc”width that is comparable to difference of resonance frequencies ,δv and both broadened lines blend together in a very broad line as in fig. Thus greatest broadening occurs when.
Where,δt=lifetime of configuration A & B
the position of the averaged signal at the higher temp limit is simply the weighted average of resonance position at low temperature limits
ans n2 at δ2, then at high-temperature limit , the resonance position will be the weight average δav given by
Mechanism of Fluxionality
To Find a chemically reasonable pathway for the interconversion. Possibilities to look for include:
a. Dissociation and recoordination of a ligand . To probe for such behavior, add some of the free ligand to the solution. If the fluxional process involves dissociation of that species, the chemical shift of the free ligand and bound species will come at the weighted average of the individual species. One can also try adding an isotopically-labeled version of the free ligand and see if the label is incorporated into the complex.
b. Rotation about a hindered bond . We can typically ignore rotation about simple bonds such as a metal- alkyl, metal- Cp and metal-alkoxide because these are so facile that they are almost impossible to freeze out. However, large groups, or phenyl rings with ortho substituents can display hindered rotation. Alkenes and other pi-bonding ligands sometimes have a preferred orientation for coordination.
c. Opening/closing of bridges . In dinuclear systems, it is not uncommon for a carbonyl or alkoxide ligand to switch between a bridging and terminal position.
d. Monomer-dimer or dimer-tetramer equilibrium . Dimers (or dimers of dimers) held together only by weakly bridging ligands often undergo dissociation. Note: This is unlikely in a case where a metal-metal bond exists. To probe such equilibria, try decreasing the concentration, which should give more of the lower nuclearity species at a given temperature, as well as increasing the concentration, which should favour the higher nuclearity species. Likewise, high T should favour the dissociated form and low T the more associated form.
e. Structural or skeletal rearrangements . 5-coordinate systems are quite notorious for fluxional behavior as the energy barrier between trigonal bipyramid (TBP) and square pyramidal (SP) geometries is often quite low. Such interconversions can occur through a Berry pseudorotation or turnstile mechanism.
Examples of Fluxional molecule:
A common type of fluxional behavior is the inversion of pyramidal molecules.
In NH3, & NRR’R” atom crosses from one side of equilibrium position to another equilibrium position through state of higher pot. Energy .this pot. Energy barrier to inversion is only ~25-40 KJ mol-1. And rate of inversion is very high (~1010s-1)
NRR’R” is chiral and should have optical activity because the lack in centre of symmetry
plane of symmetry and axis of improper rotaion.
but its not true No optical isomer of NRR’R” is been isolated.
This is due to pyramidal inversion in which N atom oscilates through the plane of the three R Groups like an Umbrella which can turn inside out.
For PH3, AsH3, NRR’R” ,the activation barrier is higher.(>100kj mol-1) & inversions are slow and optical isomers can be seperated.
Fluxionality in TBP Complexes
The trigonal bipyramidal organometallics compounds Fe(CO)5, exhibits fluxionality.
its 13-C-NMR spectrum exhibits only one resonance signal instead of two in ratio 3:2.this is explained by saying that the axial and equatorial carbonyl group interchange their positions more rapidly than NMR time scale .this process is termed as pseudorotation.
●The TBP and square pyramidal (SP) configuration of an AB5 molecule differ only little in energy .At the same time ,they can be interconverted by relatively small and simple angle deformation motions.
●As a result of this TBP-SP-TBP interconversion, the axial and equatorial vertices of TBP are interchanged rapidly and all five CO group of Fe(CO)5 become equivalent. this is why Fe(CO)5 exhibit only one signal in its 13C-Nmr spectrum.
FLUXIONALITY IN η3 ALLYL COMPLEXES
The η3-allyl complexes generally exhibit ﬂuxionality at room temperature 0r
slightly above it.
The ﬂuxionality of η3 -allyl complexes has been studied extensively
by 1H NMR spectroscopy. The η3-ally1 group (C3H5) has three types of H atoms as
(i) One Hc: It is present on the central
carbon atom, i. e., at C2.
(ii) Two Hs : These syn H atoms are
present on terminal carbon atoms (C1 and C 3).
These are towards the metal atom.
(iii) Two Ha : These anti H atoms are
present on terminal carbon atoms. These are away from the metal atom.
Therefore, the coordinated η3-ally1 group exhibit three signals in 1H Nmr Spectrum at very low temperature as follows :
δHa = 5 to 2.5 (doublet)
δHs = 3 to 1 (doublet)
δHc = 6.5 to 4 (multiplet)
JHaHc~ 7 Hz
JHsHc ~ 11 HZ
JHaHc= 0 Hz
The NMR signals due to Ha and Hs atoms split up into doublet due to spin-spin
interaction with Hc. It is called static NMR spectrum.
[Mn(CO)4(η3 -C3H5)] exhibits three signals in the ratio 2 : 2 :1 in its 1H NMR ,
spectrum at low temperatures at
δHs = 1.8 (doublet) (2H)
δHa= 2.8 (doublet) (2H)
δHc= 4.7 (multiplet)(1H)
But at room temperature or elevated temperatures, [Mn(CO)4 (η3 —C3H5)]
exhibits only two signalsin the ratio 4 : 1 in its H NMR spectrum at
δHs,δHa = 2.3 (doublet)(4H)
δHc= 4.7 (multiplet) (1H)
. It is called dynamic NMR spectrum. This is due to the fact that at higher
temperatures, the syn and anti H atoms interchange their positions rapidly probably
via a short-lived η1 — allyl-metal intermediate as shown here:-
Interchange via η3-η1-η3 process
•The rate of this interchange is faster than the NMR time scale As a result, the NMR spectrum
becomes unable to distinguish between syn and anti H atoms and all the four H
atoms present on terminal carbon atoms (C1 and C3) give rise to a single NMR signal
(doublet) at δ: 2.3 which is the weighted mean of (δHa = 2.8 and δHs=1.8 in
low-temperature limit NMR spectrum.
•First of all the coordinated η3-allyl group is transformed into a short lived η1-allyl species which undergoes rotation about C-C bond producing a rearranged eta1-allyl species.
Then η1-allyl species transformed into a η3-allyl group .
I.e. the anti H atom becomes syn and syn H becomes anti H atom.
This process is repeated and thus all the syn and anti H atoms interchange their position rapidly and they become indistinguishable by NMR spectrum.
Fluxionality in cyclopentadienyl complexes
(ɳ1-C5H5)M should have three environments for H atoms in the stereochemically rigid form
It should exhibit three signals in its 1H NMR spectrum in the ratio 2:2:1as follows:
1Hc:tiplet up field)
But at room temperature ,nearly all the compounds containing moiety exhibit one sharp singlet in the 1H and 13 C NMR spectrum for the entire C5H5 group.
As at high temperature ,the place of attachment of metal atom to the C5H5 ring is shifting rapidly over all the carbon atoms of C5H5 ring
Hence all H become time average equivalent and all the C atoms also become time average equivalent(due to 1,2-shift & 1,3-shift) and they give rise to singlet signals. Such systems are called ring whizzers due to motion of the ring relative to the metal atom.
The crystal structure of [(ŋ1-C5H5)2Ti(ŋ5-C5H5)2] exhibits the presence of the two mono hapto and two penta hapto cyclopentadienyl rings.
At 62℃,1H NMR spectrums of the organometallic compound exhibits only one singlet signal. This is due to dynamic process in which the monohapto C5H5 and pentahapto C5H5 rings interchange their roles rapidly.
At the same time, the point of attachment of each of the monohapto C5H5 rings to the Ti atom changes continuously between 5 carbon atoms of the ring .
As a result of these time averaging processes , all the four C5H5 ligands become equivalent and all the 20 H becomes indistinguishable and they give single singlet signal in the 1H NMR spectrum at ᵹ=4.45 at 62 °C
As the temperature is lowered, thus signal is broadened and gradually splits up into two lines which sharpen into two equally intensity singlets at -27°C .
At this point, the interconversion of monohapto and pentahapto rings is slower than NMR time scale and thus both are observed in 1H NMR spectrum.
But even at -27°C , the monohapto rings are in dynamic process which averages the signals for the three types of ring protons.Therefore , a single fairly sharp line is obtained instead of separate resonances in a 2:2:1 ratio the ɳ1-C5H5 ring .
Fluxionality in ŋ2 Olefin Cordination complexes
ŋ2 Olefin Cordination tendencies:-
Olefins Tends to be perpendicular to plane of SquarePlaner Complex and In plane of Trigonal & TBP compounds.
In solutio,Olefins are not in fixed orientation(Olefins Rotate)
CpRh(C2H4) at -20℃ Cp:-outer
5: 4 : 4
Two C2H4 peaks stronly broadened & non-equivalent H’s exchange at Rate intermediate on NMR time Scale.
At +57℃ two C2H4 peaks coalescence to one.
due to fast exchange Nmr can not distinguish between non-equivalent H’s.
•Cp remains Singlet throughout whole Temp. range.
Modes of rotation Consistent with Nmr Spectroscopic data
Ha‘s are equivalent to each other and Hb‘s are equivalent to each other but Ha & Hb differ
•Rotation about C-C axis would not change situation
Propeller movement would exchange non equivalent hydrogens
Nmr Spectroscopy shows two separate peaks at -90℃ and the coalescence at -65℃.