Download Magnetic Properties of TMs So far we have seen that some

Magnetic Properties of TMs
So far we have seen that some important properties of TM complexes
such as the magnitude of Δ are a function of the electronic configuration
of the d orbitals Î number of unpaired electrons.
One method of determining the number of unpaired electrons is by
looking at the magnetic susceptibility of a complex Î measure of the
force exerted by magnetic field on a unit mass of complex is related to the
population of unpaired electrons/per unit weight Î per mole.
The Gouy method is a simple technique to determine the magnetic dipole
moment μ.
This involves weighing a sample of the TM complex in the presence and
absence of a strong magnetic field.
By careful calibration using a known standard, such as HgCo(SCN)4 the
number of unpaired electrons can be determined.
Magnetic Properties of TMs
To predict the magnetic moment, we can use the simple spin-only
formula derived from Curie’s law:
μ = √[4S(S+l)] Bohr Magneton (BM) where S is the spin quantum
number (½ for each unpaired electron).
An alternative representation is:
μ = √[n(n+2)] Bohr Magneton (BM) where n is the number of unpaired
electrons.
These simple formulae give good results for most first row transition
metal compounds, although it can be refined to include orbital
contributions.
For many of these complexes the spin contribution is so predominant
that measured values of μ = no. of unpaired electrons.
Hence we can simplify to say that: paramagnetism of the complexes of
the first transition series corresponds to a ‘spin-only’ value.
Note: for d electrons = 1-3 or 8-9 in an octahedral complex, the value
of μ is not affected by weak field-strong field considerations.
Magnetic Properties
To calculate the spin-only magnetic moment of
compound Î
• the number of d-electrons in the central metal ion,
• the stereochemistry
• weak field-strong field.
E.g. Î
K3[Fe(oxalate)3] 3H2O
metal ion
Fe3+
number of d
5
electrons
stereochemistry
octahedral
High Spin/Low Spin High Spin
# of unpaired
5
electrons
magnetic moment
√(35)
any coordination
K2CuCl4
Cu2+
9
tetrahedral
Not relevant
1
√(3) B.M
High-spin / Low-spin crossovers
Octahedral complexes with between 4 and 7 d electrons can be either
high-spin or low-spin depending on the size of Δ.
When the ligand field splitting has an intermediate value such that the
two states have similar energies, then the two states can coexist in
measurable amounts at equilibrium.
Many "crossover" systems of this type have been studied, particularly
for iron complexes.
Also departures from spin-only values are generally recorded for lowspin d5 and high-spin d6 and d7 complexes.
Q. Account for the temperature dependence of the effective
magnetic dipole moment for the iron (II) complex Fe(phen)2(NCS)2,
shown below.
Answer:
This is a case that involves a spin crossover for the d6 Fe(II) ion.
The crossover involves going from high spin S = 2 (t2g4eg2) to low spin S
= 0 (t2g6eg0).
μ = √[4(4+2)] = 4.9 B.M.
μ = √[0(0+2)] = 0 B.M.
Î orbital contributions
To account for orbital contribution the spin only magnetic moment
is modified as Î μS+L= √{4S(S+1)+L(L+1);
where L = sum of orbital QN
(e.g. for n = 3 Î L = 0+1+2 =3)
For the d6 high spin case S = 2 and L = 3.
μeff= √{4S(S+1)+L(L+1) = √ 24 + 12 = 6.0 B. M.
For the d6 low spin case S = 0 and L = 3
μeff= √{4S(S+1)+L(L+1) = √ 0 + 12 = 3.5 B. M.
For the high spin complex, the spin only contribution predominates,
even at high temperatures, while for the low spin complex the value
of μS+L = 1.5 B. M. observed at the cross over point is probably due
to partial quenching of the contribution due to orbital angular
momentum at low temperatures.