Bonding in complexes of d-block metal ions – Crystal Field Theory.

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Bonding in complexes of d- block metal ions – Crystal Field Theory. energy e g t 2g Co 3+ ion in gas- phase (d 6 ) Δ Co(III) in complex 3d sub-shell d-shell split by presence of ligand donor-atoms

description

Bonding in complexes of d-block metal ions – Crystal Field Theory. e g. energy. d-shell split by presence of ligand donor-atoms. Δ. 3d sub-shell. Co 3+ ion in gas-phase (d 6 ). t 2g. Co(III) in complex. The d-orbitals:. the t 2g set. z. z. z. y. y. y. x. x. x. d yz. - PowerPoint PPT Presentation

Transcript of Bonding in complexes of d-block metal ions – Crystal Field Theory.

Page 1: Bonding in complexes of d-block metal ions – Crystal Field Theory.

Bonding in complexes of d-block metal ions – Crystal Field Theory.

energy eg

t2g

Co3+ ionin gas-phase

(d6)

Δ

Co(III) in complex

3d sub-shell

d-shellsplit bypresenceof liganddonor-atoms

Page 2: Bonding in complexes of d-block metal ions – Crystal Field Theory.

The d-orbitals: the t2g

set

the eg

set

dyz dxy dxz

dz2 dx2-y2

x x x

x x

zzz

zz

y y y

y y

Page 3: Bonding in complexes of d-block metal ions – Crystal Field Theory.

Splitting of the d sub-shell in octahedral coordination

dyz dz2 dx2-y2

the three orbitals of

the t2g set lie between

the ligand donor-atoms

(only dyz shown)

the two orbitals of the eg set lie along the

Cartesian coordinates, and so are adjacentto the donor atoms of the ligands, which

raises the eg set in energy

z z z

blue = ligand donor atom orbitals the egsetthe t2g set

y y y

x x x

Page 4: Bonding in complexes of d-block metal ions – Crystal Field Theory.

energy

eg

t2gCo3+ ion

in gas-phase(d6)

Δ

Co(III) in octahedral

complex

3d sub-shell

d-shellsplit bypresenceof liganddonor-atoms

Splitting of the d sub-shell in an octahedral complex

Page 5: Bonding in complexes of d-block metal ions – Crystal Field Theory.

The crystal field splitting parameter (Δ)

Different ligands produce different extents of splitting between

the eg and the t2g levels. This energy difference is the crystal field splitting parameter Δ, also known as 10Dq, and has units of cm-1. Typically, CN- produces very large values of Δ, while F- produces very small values.

[Cr(CN)6]3- [CrF6]3-

eg eg

t2g

t2g

energy

Δ = 26,600 cm-1 Δ = 15,000 cm-1

Page 6: Bonding in complexes of d-block metal ions – Crystal Field Theory.

High and low-spin complexes:

energy

eg eg

t2gt2g

low-spin d6

electrons fill the t2g level first. In this case the complex is diamagnetic

high-spin d6

electrons fill the whole d sub-shell according to Hund’s rule

The d-electrons in d4 to d8 configurations can be high-spin, where they

spread out and occupy the whole d sub-shell, or low-spin, where the t2g

level is filled first. This is controlled by whether Δ is larger than the spin-pairing energy, P, which is the energy required to take pairs of electrons with the same spin orientation, and pair them up with the opposite spin.

Δ > P Δ < P

Paramagnetic4 unpaired e’s

diamagneticno unpaired e’s

Page 7: Bonding in complexes of d-block metal ions – Crystal Field Theory.

energy

eg eg

t2gt2g

low-spin d5 ([Fe(CN)6]3-)electrons fill the t2g level first. In this case the complex is paramagnetic

high-spin d5 ([Fe(H2O)6]3+)electrons fill the whole d sub-shell

according to Hund’s rule

For d5 ions P is usually very large, so these are mostly high-spin. Thus, Fe(III) complexes are usually high-spin, although with CN- Δ is large enoughthat [Fe(CN)6]3- is low spin: (CN- always produces the largest Δ values)

Δ > P Δ < P

Paramagnetic5 unpaired e’s

paramagneticone unpaired e

High and low-spin complexes of d5 ions:

[Fe(CN)6]3- Δ = 35,000 cm-1

P = 19,000 cm-1

[Fe(H2O)6]3+ Δ = 13,700 cm-1

P = 22,000 cm-1

Page 8: Bonding in complexes of d-block metal ions – Crystal Field Theory.

energy

eg eg

t2gt2g

low-spin d7 ([Ni(bipy)3]3+)The d-electrons fill the t2g level first,

and only then does an electronoccupy the eg level.

high-spin d7 ([Co(H2O)6]3+)electrons fill the whole d sub-shell

according to Hund’s rule

The d7 metal ion that one commonly encounters is the Co(II) ion. For metalions of the same electronic configuration, Δ tends to increase M(II) < M(III) < M(IV), so that Co(II) complexes have a small Δ and are usually high spin. The (III) ion Ni(III) has higher values of Δ, and is usually low-spin.

Δ > P Δ < P

Paramagnetic3 unpaired e’s

paramagneticone unpaired e

High and low-spin complexes of d7 ions:

[Ni(bipy)3]3+ [Co(H2O)6]2+ Δ = 9,300 cm-1

Page 9: Bonding in complexes of d-block metal ions – Crystal Field Theory.

energy

eg eg

t2gt2g

low-spin d6 ([Co(CN)6]4-)electrons fill the t2g level first. In this

case the complex is diamagnetic

high-spin d5 ([CoF6]3-)electrons fill the whole d sub-shell

according to Hund’s rule

For d6 ions Δ is very large for an M(III) ion such as Co(III), so all Co(III) complexes are low-spin except for [CoF6]3-.high-spin. Thus, Fe(III) complexes are usually high-spin, although with CN- Δ is large enoughthat [Fe(CN)6]3- is low spin: (CN- always produces the largest Δ values)

Δ >> P Δ < P

Paramagnetic4 unpaired e’s

diamagneticno unpaired e’s

High and low-spin complexes of some d6 ions:

[Co(CN)6]3- Δ = 34,800 cm-1

P = 19,000 cm-1

[CoF6]3- Δ = 13,100 cm-1

P = 22,000 cm-1

Page 10: Bonding in complexes of d-block metal ions – Crystal Field Theory.

The spectrochemical series:

One notices that with different metal ions the order of increasing Δ with different ligands is always the same. Thus, all metal ions produce the highest value of Δ in their hexacyano complex, while the hexafluoro complex always produces a low value of Δ. One has seen how in this course the theme is always a search for patterns. Thus, the increase in Δ with changing ligand can be placed in an order known as the spectrochemical series, which in abbreviated form is:

I- < Br- < Cl- < F- < OH- ≈ H2O < NH3 < CN-

Page 11: Bonding in complexes of d-block metal ions – Crystal Field Theory.

The place of a ligand in the spectrochemical series is determined largely by its donor atoms. Thus, all N-donor ligands are close to ammonia in the spectrochemical series, while all O-donor ligands are close to water. The spectrochemical series follows the positions of the donor atoms in the periodic table as:

C N O F

P S Cl

Br

I

The spectrochemical series:

S-donors ≈between Brand Cl

very littledata onP-donors –may be higherthan N-donors

?

spectrochemicalseries followsarrows aroundstarting at I andending at C

Page 12: Bonding in complexes of d-block metal ions – Crystal Field Theory.

Thus, we can predict that O-donor ligands such as oxalate or acetylacetonate will be close to water in the spectrochemical series. It should be noted that while en and dien are close to ammonia in the spectrochemical series, 2,2’bipyridyl and 1,10-phenanthroline are considerably higher than ammonia because their sp2 hybridized N-donors are more covalent in their bonding than the sp3 hybridized donors of ammonia.

The spectrochemical series:

O

O-

O

-O O O-

H3C CH3

H2N NH2

H2N NH

NH2N N N N

oxalate acetylacetonate en

dien bipyridyl 1,10-phen

Page 13: Bonding in complexes of d-block metal ions – Crystal Field Theory.

For the first row of donor atoms in the periodic table, namely C, N, O, and F, it is clear that what we are seeing in the variation of Δ is covalence. Thus, C-donor ligands such as CN- and CO produce the highest values of Δ because the overlap between the orbitals of the C-atom and those of the metal are largest. For the highly electronegative F- ion the bonding is very ionic, and overlap is much smaller. For the heavier donor atoms, one might expect from their low electronegativity, more covalent bonding, and hence larger values of Δ. It appears that Δ is reduced in size because of π–overlap from the lone pairs on the donor atom, and the t2g set orbitals, which raises the energy of the t2g set, and so lowers Δ.

The bonding interpretation of the spectrochemical series:

Page 14: Bonding in complexes of d-block metal ions – Crystal Field Theory.

When splitting of the d sub-shell occurs, the occupation of the lower energy t2g level by electrons causes a

stabilization of the complex, whereas occupation of the eg level causes a rise in energy. Calculations show that the t2g level drops by 0.4Δ, whereas the eg level is raised by 0.6Δ. This means that the overall change in energy, the CFSE, will be given by:

CFSE = Δ(0.4n(t2g) - 0.6n(eg))

where n(t2g) and n(eg) are the numbers of electrons in

the t2g and eg levels respectively.

Crystal Field Stabilization Energy (CFSE):

Page 15: Bonding in complexes of d-block metal ions – Crystal Field Theory.

The CFSE for some complexes is calculated to be:

[Co(NH3)6]3+: [Cr(en)3]3+

egeg

t2gt2g

Δ = 22,900 cm-1 Δ = 21,900 cm-1

CFSE = 22,900(0.4 x 6 – 0.6 x 0) CFSE = 21,900(0.4 x 3 – 0.6 x 0)

= 54,960 cm-1 = 26,280 cm-1

Calculation of Crystal Field Stabilization Energy (CFSE):

energy

Page 16: Bonding in complexes of d-block metal ions – Crystal Field Theory.

The CFSE for high-spin d5 and for d10 complexes is calculated to be zero:

[Mn(NH3)6]2+: [Zn(en)3]3+

egeg

t2gt2g

Δ = 22,900 cm-1 Δ = not known

CFSE = 10,000(0.4 x 3 – 0.6 x 2) CFSE = Δ(0.4 x 6 – 0.6 x 4)

= 0 cm-1 = 0 cm-1

Crystal Field Stabilization Energy (CFSE) of d5 and d10 ions:

energy

Page 17: Bonding in complexes of d-block metal ions – Crystal Field Theory.

For M(II) ions with the same set of ligands, the variation of Δ is not large. One can therefore use the equation for CFSE to calculate CFSE in terms of Δ for d0 through d10 M(II) ions (all metal ions high-spin):

Ca(II) Sc(II) Ti(II) V(II) Cr(II) Mn(II) Fe(II) Co(II) Ni(II) Cu(II) Zn(II)

d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 d10

CFSE: 0 0.4Δ 0.8Δ 1.2Δ 0.6Δ 0 0.4Δ 0.8Δ 1.2Δ 0.6Δ 0

This pattern of variation CFSE leads to greater stabilization in the complexes of metal ions with high CFSE, such as Ni(II), and lower stabilization for the complexes of M(II) ions with no CFSE, e.g. Ca(II), Mn(II), and Zn(II). The

variation in CFSE can be compared with the log K1 values for EDTA

complexes on the next slide:

Crystal Field Stabilization Energy (CFSE) of d0 to d10 M(II) ions:

Page 18: Bonding in complexes of d-block metal ions – Crystal Field Theory.

CFSE as a function of no of d-electrons

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5 6 7 8 9 10 11

no of d-electrons

CF

SE

in m

ult

iple

s o

f Δ

.

Crystal Field Stabilization Energy (CFSE) of d0 to d10 M(II) ions:

Ca2+ Mn2+ Zn2+

double-humpedcurve

Ni2+

Page 19: Bonding in complexes of d-block metal ions – Crystal Field Theory.

log K1(EDTA) as a function of no of d-electrons

10

12

14

16

18

20

0 1 2 3 4 5 6 7 8 9 10 11

no of d-electrons

log

K1(E

DT

A) .

Log K1(EDTA) of d0 to d10 M(II) ions:

Ca2+

Mn2+

Zn2+

double-humpedcurve

= CFSE

rising baselinedue to ioniccontraction

Page 20: Bonding in complexes of d-block metal ions – Crystal Field Theory.

log K1(en) as a function of no of d-electrons

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9 10 11

no of d-electrons

log

K1(e

n) .

Log K1(en) of d0 to d10 M(II) ions:

double-humpedcurve

Ca2+Mn2+

Zn2+

rising baselinedue to ioniccontraction

= CFSE

Page 21: Bonding in complexes of d-block metal ions – Crystal Field Theory.

log K1(tpen) as a function of no of d-electrons

0

5

10

15

20

0 1 2 3 4 5 6 7 8 9 10 11

no of d-electrons

log

K1(

tpen

).

Log K1(tpen) of d0 to d10 M(II) ions:

Ca2+

Mn2+

Zn2+

double-humpedcurve

N N NN

N Ntpen

Page 22: Bonding in complexes of d-block metal ions – Crystal Field Theory.

Irving and Williams noted that because of CFSE, the log K1 values for virtually all complexes of first row d-block metal ions followed the order:

Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II)

We see that this order holds for the ligand EDTA, en, and TPEN on the previous slides. One notes that Cu(II) does not follow the order predicted by CFSE, which would have Ni(II) > Cu(II). This will be discussed under Jahn-Teller distortion of Cu(II) complexes, which leads to additional stabilization for Cu(II) complexes over what would be expected from the variation in CFSE.

The Irving-Williams Stability Order: