This seminar deals only with paramagnetic coordination … · 2015. 7. 13. · General remarks This...
Transcript of This seminar deals only with paramagnetic coordination … · 2015. 7. 13. · General remarks This...
General remarks
This seminar deals only with paramagnetic coordination compounds.
Complicated mathematics will be avoided, where possible!
TMn+ ions have pure d-electron configurations (recall: s electrons are lost first,
as the diffuse s-orbitals are destablized in complexes)
Cr2+: d4
Fe3+: d5
Ni2+: d8
metal organic compounds have also dn
Fe2+, Cr(CO)6, Cr(h6-C6H6)2 d6
Fe3+, V(CO)6, V(h6-C6H6)2 d5
General remarks
Constants and units
cm molar magnetic suceptibility [cm3·mol–1] (cgs/emu)
[emu·mol–1] „
[m3·mol–1] (SI)
Conversion factor: cm(cgs) × 4p10–6 = cm (SI)
K3[Fe(CN)6] cm = -122.7×10–6 emu/mol
cm = -1.542×10–9 m3/mol
----------------------------------------------------------------------------------------------------------
N = 6.023×1023 mol–1
mB = 0.92731×10–20 erg/Gauss;
mB = 9.27×10–24 J/T
kB = 1.38×10–23 J/K
NmB2/(3kB) = 0.125 cm3/(K·mol)
)(828.23
32 Kcm
mol
N
k
B
B m
cgs units, N = 6.023×1023 mol–1
mB = 0.92731×10–20 erg/Gauss; 9.27×10–24 J/T
TBeff cmm 828.2/
1. Spin-Only-Paramagnetism
Effective magnetic moment, meff, of 3d metal complexes can be estimated
to a first approximation with the spin-only formula
Beeff SSg mm )1(
)1( SSgn e
B
eff
effm
m
mB = Bohr Magneton = eħ/(2me) =9.27408×10–24 J/T
meff = effective magnetic moment
neff = effective magnetic moment in units of mB
ge = 2.00232
S = Ssi (Total spin quantum number)
si = spin quantum number (+1/2 or -1/2)
i S neff
1 ½ 1.73
2 1 2.83
3 3/2 3.88
4 2 4.90
5 5/2 5.92
Note: in the OCP text book meff is represented as meff
Spin-Only-Formula spin-state of complex and
number of unpaired electrons can be determined
d3: CrIII, MoIII, MnIV, VII: 3.88 mB
d5: MnII, FeIII: 5.92 mB
neff data (~ 300 K) for selected compounds of d3 and d5 ions
d3
CrCl3 3.90 K3[Cr(ox)3].3H20 3.62
[Cr(NH3)6]Br3 3.77 KCr(SO4)2.12H2O 3.84
[Cr(en)3]Br3 3.82 K3[MoCl6] 3.79
[Cr(bpy)3]Cl3 3.81 K2[MnCl6] 3.84
K3[Cr(CN)6] 3.87 [V(en)3]Br2 3.81
K3[Cr(NCS)6].4H2O 3.79 [V(bpy)3]Cl2 3.67
K3[Mo(NCS)6].4H2O 3.70 [Mo(bpy)3]Cl3 3.66
(NnBu4)3[Cr(N3)6] 3.76 K4[V(CN)6] 3.78
d5
MnCl2 5.79 FeCl3 5.73
MnBr2 5.82 (Et4N)[FeCl4] 5.88
(NH4)2Mn(SO4)2.6H2O 5.88 (NH4)Fe(SO4)2.12H2O 5.89
[Mn(NH3)6]Cl2 5.92 K3[Fe(ox)3].3H2O 5.90
(Et4N)2[MnCl4] 5.94
neff(theor.)
Spin-Only-Formula spin-state and
number of unpaired electrons can be determined
d3: CrIII, MoIII, MnIV, VII: 3.88 mB
d5: MnII, FeIII: 5.92 mB
[nBu4]2[Mn(CH3)6] 3.90 mB MnIV, d3
V(Cp)2, Vanadocene 3.78 mB VII, d3
Mn(Cp)2, Manganocene 5.86 mB MnII, d5
this is true also for more exotic compounds
Spin-Only Formula only valid for the following conditions:
room temperature (295 K)
for 3d TM ions (i.e. K2[ReIVCl6] = 3.25 mB (expected = 3.88 mB)
for mononuclear complexes (polynulcear complexes may show
cooperative phenomena (antiferro- or ferromagnetic interactions))
for totally quenched orbital momentum (= TM ions with E or A ground terms)
ferrocene
Fe
vandocene
V
manganocene
Mn Mn CH3H3C
CH3
H3C
CH3
CH3
2
I- < Br- < S2- < SCN- < Cl- < N3- < F- < OH- < O2
- < OH2 <
NCS- < NH3 ~ py < en < bpy < NO2- < CH3
- < CN- < CO
spectrochemical series:
Orbital contribution to the magnetic moment
Beeff SSg mm )1(
Spin-only formula
the orbital angular momentum L has also
a magnetic moment associated with it, for free ions with L and S,
Beeff SSgLL mm )1()1(2
spinorbit
Orbital contributions to the magnetic moment do explain the deviations from the spin-only values
the orbital contribution to the magnetic moment is not totally quenched
Two prominent examples:
CoCl2 5.47 mB
CoCl42─ 4.67 mB
expected 3.88 mB h.s.-CoII has d7 (3 unp. electrons)
general trends:
d6 to d9: larger values than calculated
d1 to d4: smaller values than calculated
only d5 is well behaved
This is readily explained
a) by the fact that
> 0 for d1-d4 and
< 0 for d6-d9
= spin-orbit coupling constant
b) Fe3+ (S=5/2), L = ML = Sml = 0
L
S
L
S
Spin-orbit coupling can cause temperature dependent magnetic moments (Ti3+, d1)
Orbital momentum in transition metal ions and complexes
In coordination compounds orbital momentum means:
electron can move from one d orbital to another degenerate
d orbital. However, dxy, dxz, dyz, and dzz, dx2-y2 are no longer
degenerate in a complex.
In an octahedral complex, e– can only move within an
open t2g shell (first order orbital momentum => of importance in magnetochemistry)
d1, d2, (l.s.)-d4, (l.s.)-d5, etc have first order orbital momentum (T ground terms),
d3, d4 have no first order orbital momentum (A, E ground terms)
Terms with T symmetry
exhibit orbital angular momentum
can show spin-orbit coupling
This rule is only applicable in Oh
Symmetry.
Terms with T symmetry
exhibit L = 1,
HSO = -ALS
EJ = -1/2A[J(J+1)-L(L+1)-S(S+1)
For (t2g)n less than half occupied: positive
more than half occupied: negative
dx2-y2
dxy
(leer)
Orbital contribution to the magnetic moment
Quenching of the orbital contribution, T-term and A, E-term ions
Quenching of the orbital contribution, to the magnetic moment, due to ligand field
n ground t2gneg
m ligand field quenching
term term
1 2D t2g1 2T2g No
2 3F t2g2 3T1g No
3 4F t2g3 4A2g Yes
4 5D t2g3eg
1 5Eg Yes
t2g4 3T1g No
5 6S t2g3eg
2 6A1g Yes
t2g5 2T2g No
6 5D t2g4eg
2 5T2g No
t2g6 1A1g Yes
7 4F t2g5eg
2 4T1g No
t2g6eg
1 2Eg Yes
8 3F t2g6eg
2 3A2g Yes
9 2D t2g6eg
3 2Eg Yes
These ions
actually
have L = 1
and thus a
„residual“
contribution
(not full
contribution)
to the
spin moment
Octahedral symmetry
Typical Ions: Ti3+ (d1), V3+ (d2), l.s-Mn3+ (d4), l.s.-Fe3+ (d5, i.e. K3[Fe(CN)6])
h.s-Fe2+ (d6), h.s.-Co(2+)
Magnetic moment depends also on C.N.
Nickel(II), d8
octahedral (3A2g) 2.9 – 3.4 mB
tetrahedral (3T1) 3.2 – 4.0 mB
trigonal bipyramidal 3.2 – 3.8 mB or 0
square pyramidal 3.2 – 3.4 mB or 0
square planar 0
Ni CNNC
CN
NC
2
Ni
Cl
ClCl
Cl
2
N
NiH2N
NH
NH2
NH2
Ni
N
NH2
NH2
H2N
Cl
2
Orbital momentum
quenched
not quenched
CoII, tetr. 4.4-4.8 4A2
CoII, oct., 4.8-5.3 4T1g
tetr. [NiX4]2- (X = Cl, Br, I)
tetr. [Ni(SPh)4]2─
[Ni(PPh3)2Br2] 3.27 mB
Spin equilibria
NiII(tetr.) NiII(sq.pl) (in solution)
High-spin and low-spin complexes
possible for d4-d7 electronic configurations (in octahedral complexes)
possible for d3-d6 electronic configurations (in tetrahedral complexes)
Examples (all are low-spin):
d4 [Cr(bpy)3]2+ , [Cr(CN)6]
4–, [Mn(CN)6]3– t2g
4 S = 1
3.20 mB
d5 [Fe(CN)6]3–, [Fe(en)3]
3+, [Mn(CN)6]4– t2g
5 S = 1/2
2.25 mB 2.40 mB 2.18 mB
d6 [Fe(CN)6]4–, [Co(NH3)6]
3+, [Cr(CO)6] t2g6 S = 0
d7 [Co(diars)3]2+, [Co(NO2)6]
4–, [NiF6]3– t2g
6eg1 S = ½
1.84 mB
the deviations from the ideal values are again attributable to orbital
contributions to the magnetic moment
AsPh2
AsPh2
diars
High-spin → low-spin transitions, spincrossover
Become feasible for d4 to d7 in octahedral case, if Do(h.s.) ~ Do(l.s.)
h.s.->l.s transitions can be affected by
variation of temperature or pressure
At lower temperature the l.s-form
always dominates
l.s. and h.s. form can be present in
an equilibrium (in solution as well as
in solid state)
Prominet examples:
Fe, d5: [Fe(S2CNR)3]
High-spin → low-spin transitions
Fe, d5: [Fe(S2CNR)3]
High T meff → 4.7 mB (h.s., S = 5/2)
Low T meff → 2.25 mB (l.s., S = ½)
S S-
N Fe SS
S
S
S
S
Spin-equilibria are rare.
Abrupt spincrossover more often
encounteredc(50%L.S./50%H.S.) = c(L.S.) + c(H.S.)
High-spin and low-spin tetrahedral complexes
d3: K3FeVO4 3.71 mB S = 3/2 (e)2(t2)1 high-spin
ReIV(o-tolyl)4 1.31 mB S = ½ (e)3(t2)0 low-spin
MnIV(1-nor)4 3.78 mB S = 3/2 (e)2(t2)1 high-spin
h.s. l.s.
n 3 1
h.s. l.s.
4 0
h.s. l.s.
5 1
d3 d4 d5 d6
h.s. l.s.
4 2
M1-nor
d4: [CoV(1-nor)4]+ S = 0 (e)4(t2)
0 low-spin
[FeIV(1-nor)4]+ S = 0 (e)4(t2)
0 low-spin
[MnIII(1-nor)4]- S = 2 (e)2(t2)
2 high-spin
MR4-complexes with 4d and 5d elements and sterically demanding ligands
are low-spin
High-spin → low-spin transitions
Fe, d6: [FeII(bpy)2(NCS)2]
High T meff → 5.2 mB (h.s., S = 4/2)
Low T meff → 2.25 mB (l.s., S = 0)
Fe NCSN
NCS
N
N
N
N
N
cis-[FeII(NCS)2(phen)2]
Phenanthrolin (phen)
High-spin and low-spin tetrahedral complexes
d5: [NEt4][FeCl4] 5.88 mB S = 5/2 (e)2(t2)3 high-spin
[NEt4][Fe(SPh)4] 5.73 mB S = 5/2 (e)2(t2)3 high-spin
CoIV(1-nor)4 1.89 mB S = 1/2 (e)4(t2)1 low-spin
h.s. l.s.
n 3 1
h.s. l.s.
4 0
h.s. l.s.
5 1
d3 d4 d5 d6
h.s. l.s.
4 2
M1-nor
d6: [CoIII(1-nor)4]– 3.18 mB S = 1 (e)4(t2)
2 low-spin
General observations:
low-spin tetrahedral complexes are rare (Dt = –4/9 Do)
a tetrahedral complex with low-spin configuration requires:
strong ligand field, a high-metal oxidation state, sterically demanding ligands
(particularly for bigger 4d 5d elements) to prevent the formation of M-M bonds
or adoption of coordination number 6
High-spin and low-spin complexes
l.s.-d4, l.s.-d5, and l.s.-d7 display positive and commonly large deviations
from the spin only expectation (for the first transition series)
Explanation: (t2g)n configurations behave magnetically like (p)n configs;
when more than half-filled subshell (as is the case in d4-d7); S and L
are parallel; and any orbital contribution increases meff beyond the spin-only
value
All octahedral complexes of 4d and 5d elements are low-spin
Gleichungen zur Berechnung magnetischer Momente
a) Spin-Only-Formel (Näherung für 3d-Ionen mit A- und E-Ligandenfeldgrundtermen,
vgl. Tab. 1)
)1(.. SSgB
os
g = 2.0023 ~ 2, S = Gesamtspinquantenzahl, S = n/2, n = Anzahl ungepaarte Elektronen
b) Berechnung des magnetischen Momentes aus der experimentell bestimmten molaren
magnetischen Suszeptibilität (m)
TN
km
Ba
B
B
2
.exp 3 = Tm828.2
kB = Boltzmann-Konstante, Na = Avogadro-Zahl, B = Bohrsches Magneton
m = molare magnetische Suszeptibilität (Dimension: cm3∙mol–1)
im cgs-System: 1 J = 10–7 erg, 1 T(esla) = 104 Gauss, 1 erg = Gauss2∙cm3
kB = 1.38∙10–16 erg/K, B = 9.274∙10–21 erg/Gauss
Na∙B2/(3kB) = 0.125 cm3∙K∙mol–1
c) bei nicht-unterdrücktem Bahn-Moment wäre der Erwartungswert für :
)1()1(4 LLSSB
S = Gesamtspinquantenzahl, S = n/2, n = Anzahl ungepaarte Elektronen
L = Gesamtbahndrehimpulsquantenzahl = Summe der ml-Werte, L = ml
d) für Lanthanoid-Ionen ( ist groß) gilt
)1(2
)1()1()1(1
)1(
JJ
JJLLSSg
JJg
J
J
B
, gJ = Lande-Faktor
f) Russel-Saunders-Symbole
Termsymbol für Grundzustand eines Atoms oder Ions: 2S+1LJ
S = Gesamtspinquantenzahl, S = n/2, n = Anzahl ungepaarte Elektronen
L = Gesamtbahndrehimpulsquantenzahl = Summe der ml-Werte, L = ml
J = Quantenzahl des Gesamtdrehimpulses,
bei weniger als halbgefüllten Schalen gilt für den Grundterm: J = (L-S)
bei mehr als halbgefüllten Schalen gilt für den Grundterm: J = (L+S)
Tab.1 Berechnete und gemessene effektive magnetische Momente (in B) oktaedrischer
Komplexe der 3d-Metalle.
Zahl der d-
Elektronen
Grundterm EK Ligandenfeld-
grundterm
eff(ber.) eff(gem.) Bahnmoment
erwartet?
1 (Ti3+) 2D t2g1 2T2g Ja
2 (V3+) 3F t2g2 3T1g Ja
3 (Cr3+) 4F t2g3 4A2g Nein
4 (Mn3+) 5D t2g3eg
1 5Eg Nein
t2g4eg
0 3T1g Ja
5 (Fe3+) 6S t2g3eg
2 6A1g Nein
t2g5eg
0 2T2g Ja
6 (Fe2+) 5D t2g4eg
2 5T2g Ja
t2g6eg
0 1A1g Nein
7 (Co2+) 4F t2g5eg
2 4T1g Ja
t2g6eg
1 2Eg Nein
8 (Ni2+) 3F t2g6eg
2 3A2g Nein
9 (Cu2+) 2D t2g6eg
3 2Eg Nein
Aufgaben:
1) Bei Raumtemperatur ist der gemessene Wert von eff für [Cr(en)3]Br2 4.75 B. Ist der
Komplex low-spin oder high-spin?
[Cr(en)3]Br2 enthält ein Cr2+-Ion (d4). In der low-spin-Konfiguration hätte der Komplex 2
ungepaarte Elektronen. In der high-spin-Konfiguration 4. Mit der Spin-Only-Formel ergeben
sich effektive magnetische Momente von 2.83 bzw. 4.90. Der letztere Wert liegt deutlich
näher am experimentell bestimmten als der erstere. Der Komplex hat daher eine high-spin-
Konfiguration.
2) Das magnetische Moment eines oktaedrischen Co(II)-Komplexes beträgt eff = 4.0. Welche
Elektronenkonfiguration hat dieser Komplex?
Oktaedrische Co(II)-Komplexe (d7-Konfiguration) können low- (1 ungepaartes Elektron) oder
high-spin (3 ungepaarte Elektronen) konfiguriert sein. Nimmt man an, dass die Spin-Only-
Formel in erster Näherung Gültigkeit besitzt, ist:
eff (high-spin) = 3.90
eff (low-spin) = 1.73
Der Komplex ist daher high-spin Komplex.
3) Das magnetische Momente der Komplexe [Mn(H2O)6]2+, [Fe(H2O)6]
3+, [MnCl4]2− und
[FeCl4]− beträgt jeweils ungefähr 5.92. Was sagt Ihnen das über die geometrische und
elektronische Struktur der Komplexe? Warum ist die Spin-Only-Formel in diesen Fällen so
genau?
[Mn(H2O)6]2+: d5, t2g
3eg2, high-spin
[Fe(H2O)6]3+: d5, t2g
3eg2, high-spin
[MnCl4]2–: d5, e2t3, high-spin, tetraedrisch gebaut
[FeCl4]–: d5, e2t3, high-spin, tetraedrisch gebaut
Die Komplexe besitzen einen A-Ligandenfeldgrundterm (L = 0).
4) Bestimmen Sie das Russel-Saunders-Termsymbol 2S+1LJ für den jeweiligen Grundzustand
der folgenden Ionen:
a) Ti3+, b) Ni2+, c) Cr3+, d) Ce3+, e) Ho3+, f) Er3+, g) La3+, h) Lu3+
Lösungsweg:
Schritt 1) n, Anzahl ungepaarter Elektronen bestimmen
=> S = n/2
Schritt 2) L bestimmen:
Summe der ml-Werte für die Anordnung mit niedrigster Energie (Hundsche
Regeln), Zuordnung des bestimmten Wertes für L nach folgendem Schema:
L 0 1 2 3 4 5 6
S P D F G H I
Schritt 3) J bestimmen:
J = (L–S) bei weniger als halbgefüllten Schalen
J = (L+S) bei mehr als halbgefüllten Schalen
Ti3+, d1 , n = 1, S = ½, Ms = 2∙1/2 + 1 = 2
ml 2 1 0 -1 -2
L = 2, => 2D
J = L – S = 2 – 1/2 = 3/2 => 2D3/2
Ni2+, d8, n = 2, S = 1, Ms = 2∙1 + 1 = 3
ml 2 1 0 -1 -2
L = (2 · 2) + (2 · 1) +(2 · 0) – 1 – 2 = 3, => F
J = L + S = 3 + 1 = 4 => 3F4
Cr3+, d3, n = 3, S = 3/2, Ms = 2∙3/2 + 1 = 4
ml 2 1 0 -1 -2
L = (1 · 2) + (1 · 1) +(1 · 0) = 3, => F
J = L – S = 3 – 3/2 = 3/2 => 4F3/2
Ce3+, f1 n = 1, S = 1/2, Ms = 2∙1/2 + 1 = 2
ml 2 1 0 -1 -23 -3
L = (1 · 3) = 3, => 2F
J = L – S = 3 – 1/2 = 5/2 => 2F5/2
Ho3+, f10 n = 4, S = 2, Ms = 2∙2 + 1 = 5
ml 2 1 0 -1 -23 -3
L = (2 · 3) + (2 · 2) + (2 · 1) +(1 · 0) – (1 · 1) – (1 · 2) – (1 · 3) = 6, => I
J = L + S = 6 + 2 = 8 => 5I8
La3+: f0, n=0, S=0, MS=1, L = 0, J = 0, => 1S0
Lu3+: f14, n=0, S=0, MS=1, L = 0, J = 0, => 1S0
5) Für welche der folgenden Ionen erwarten Sie einen Bahnbeitrag zum magnetischen
Moment:
a) V3+, b) Cr3+, c) Ti4+, d) Fe3+ (low-spin), e) Fe3+ (high-spin)
Für Ionen mit T1g- oder T2g-Ligandenfeldgrundtermen. Das sind: V3+ (3T1g) und Fe3+, low-
spin (2T2g), vgl. Tabelle 1.
6) Berechnen Sie den Wert des effektiven magnetischen Moments der folgenden Ionen:
a) Ti3+, Ce3+, Yb3+, Ho3+, Eu3+.
Ti3+: Spin-Only-Formel: eff = 1.73
Für Lanthanoide (Ce3+-Eu3+) gilt:
)1(2
)1()1()1(1
)1(
JJ
JJLLSSg
JJg
J
J
B
,
wobei S, L und J aus den Termsymbolen bestimmt werden können
Ce3+: 2F5/2, S = 1/2, L = 3, J = 5/2, gJ = 0.857, = 2.535
Yb3+: 2F7/2, S = ½, L = 3, J = 7/2, gJ = 1.143, = 4.536
Ho3+: 5I8, S = 2, L = 6, J = 8, gJ = 1.25, = 10.607
Eu3+: = 0
7) Berechnen das magnetische Moment von Dy2(SO4)3∙8H2O.
8) Berechnen Sie den Wert der Curie-Constanten C für die Verbindung
(NH4)2Mn(SO4)2∙6H2O in cgs-Einheiten.