Because the mag,susceptibility or magnetic moment of mono nuclear copper in complexes seems to be 1.73 that means in dinuclear it should be 3.46 but its not what may be the reason any suggestions
Yes; It is possible for Cu(II) complexes to have mag. Moment=2.26 BM. Before explaing this value, I cite two references[ in fact there are many] in support of this value as:
[A]Fill/ Click:
magnetic moment of Cu(II) dinuclear complexes
Reach/ Click:
Copper(II) Complexes with Subnormal Magnetic Moments
To find the value 1.9-2.2BM for most copper complexes
[B] Fill/ Click;
The magnetic moments of mononuclear copper(I1) complexes
Reach/Click:
1. [PDF]and Ni(II) complexes with Schiff base ligands - doiSerbia
www.doiserbia.nb.rs/ft.aspx?id=0352-51391100140C
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by B Cristóvão - 2011 - Cited by 9 - Related articles
Abstract: Mononuclear copper(II) and nickel(II) complexes of the formulae. [Cu(L1)] ...magnetic susceptibility of the Cu(II) complexes changed with temperature.
Pls. look in abstract of the paper to find the values of Cu(II) Sciff base as high as that of 2.29BM.
How does the moment change?
Cu(II) complexes, in general, but with Sch iff’s bases in particular show an anomalous behaviour.When the temperature is below 40-60K, they show antiferromagnetic character to give magnetic moment less than 1.732BM[ may be as low as as 1.4-1.6BM[ called subnormal ]. As the temperature is raised , magnetic moment rises to say 1.732BM[ as is expected for a single unpaired electron]. If the temperature is still raised the magnetic moment becomes 1.90-.26BM.
[i] At very low temperature, there comes a AntiFerroMagnetic character due to direct exchange to show subnormal moment.
[ii] As the temperature rises, it shows the normal paramagnetic behaviour to give 1.732 to 2.26BM values.
.
Now I start answering your query-
(a) Why should the moment be not equal to 2.1.73=3.46BM. Pls bear in mind the magnetic balances are so designed that they give readings for Molar magnetic susceptibility/ electron and so the corresponding moment should also / unpaired electron.
(b) Then comes the doubt- why 2.26 and why not 1.732- the one corresponding to one electron? From where does this figure 2.26 come? This is again a diffucult question but we have reason for it.
Again note that the value 1.732BM is from spin only[μso] but magnetic moment can also arise from orbital motion which is called [μl]. But whole of the spin motion does not contribute . Some gets quenched and we apply the following formula:
μ eff= μso[1-α.λ /10Dq]; α =4, λ = -- 830cm^-1; Dq is calculated from reflectance specta and μso =1.732. So μ eff will be more than 1.732 BM and this value comes to about 2.2-2.26Bm.
I briefly add few more points:
[a] The magnetic moment can arise both from the spin motion and the orbital motion with magnetic moments represented by μ( s.o) and μ( l) respectively . Of course, the contribution from from orbital motion is much less than that from spin motion.
[b] If the magnetic memen were to arise from both these motions then then the total value of the moment in a free metal ion should have been:
μ( L+S)= {4S(S+1}+L(L+!)}^0.5 B.M.
[c] But when a metal ion becomes a part of the complex,then the orbital motion may get restricted because of electrostatic field of ligands[ called quenching of orbital motion] while the spin motion remains unaffected.
[d] Now three cases may arise wrt to orbital motion- either it is not quenched at all[ only a remote possibility[ apply above formula] or completely quenched[ put L=0 and S=n/2] to obtain:
μ( s.o)= [ n(n+2]^0.5 B.M.
Then comes a third case[tedious one] when orbital motion is partially quenched. In such cases we first derive term[ A, E,T] and depending upon the term, we apply different formulas. The formulas for T terms[ T1or T2] are different for each T term with different multiplicities.
But there is a saving grace for A and E terms for which tha same formula can be applied as:
μ eff= μso[1-α.λ /10Dq]; α =4[for A terms] or =2 [ for E terms]; λ has different values for different metal ions and for Cu, it is = -- 830cm^-1; Dq is calculated from reflectance spectra.
[e] Now another simple point about experimental determination of magnetic moment. The experiments give parmagnetic susceptibility; multiply it by the molecular mass and from this, substract diamagnetic susceptibility to obtain correct molar paramagnetic susceptibility and apply the following formula to obtain:
μ( eff)= [μ( eff)] = 2.828 [χ(Mol.eff. T ]^0.5 at temperature T K.
[f] AS EXPERIMENT GIVES MOLAR PARAMAGNETIC SUSCEPTIBILITY , SO WILL BE THE VALUE OF μ( eff).
[g] As regards to the presence of three possible values of magnetic moments- subnormal[less than 1.732BM; corresponding to its antiferromagnetic character- direct exchange at 40-60K]; 1.732[ when orbital motion does not contribute; only spin cotributes] and 2.26BM when there is a complete contribution from spin motion but a partial contribution from orbital motion. Calculate this value by putting value around 12000-14000cm^-1 in the following formula, [ obtain the exact value from the reflectance spectra of the complex you have prepared] and you find value around 2.0 to 2,25BM
μ eff= μso[1-α.λ /10Dq]
[h] Still another point, One BM is a very very small quanity [9.2732.10^-21 erg/ gauss or 9.2732.10^-24 J/ T. Or stll if you want more , it is 1.3994 MHz/Gauss .
[i] And the last point- whenever , there is bridging sort of structure/s[ called μ- bridging with oxo, hydroxo, halo], there should be further reduction of the magnetic moment/s.
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