CHEM 511 Chapter 21 page 1 of 15

Chapter 21

d-Block metal chemistry: coordination complexes

Recall the shape of the d-orbitals...

(as usual, skip the valence bond discussion)

Electronic structure

Crystal Field Theory: an electrostatic approach to understand d-orbital complexes. It provides

an approximate description of the electronic energy levels that determine the UV and visible

spectra, but does not describe the bonding. It predicts that the d-orbitals will not be degenerate.

d-orbitals split. Why?

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Electronic spectra and split orbitals

Imagine a d1 electron species: Ti(H2O)6

3+

o is different for every complex, but there are patterns.

The Spectrochemical Series

I-<Br

-<S

2-<SCN

-<Cl

-<NO2

-<N3

-<F

-<OH

-<C2O4

2-<H2O<NCS

-<CH3CN<py<NH3<en<bipy<phen

<NO2-<PPh3<CN

-<CO

o also depends on the metal

o increases with higher oxidation number

o increases going down a group

Spectrochemical Series for metals (partial list)

Mn2+

<Ni2+

<Co2+

<Fe2+

<V2+

<Fe3+

<Co3+

<Mn4+

<Mo3+

<Rh3+

<Ru3+

<Pd4+

<Ir3+

<Pt4+

CHEM 511 Chapter 21 page 3 of 15

Crystal Field Stabilization Energy (CFSE)

The split d-orbitals results in a lowering of the energy for three orbitals (dxy, dxz, dyz) and an

increase in energy for two orbitals (dx2-y2, dz2)—relative to the orbitals in a spherical field

For a d0 metal, Ca

2+, Sc

3+, Ti

4+, CFSE = 0

For a d1 metal (e.g., Ti

3+)

For a d2 metal (e.g., V

3+)

For a d3 metal (e.g., Cr

3+)

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For a d4 metal (e.g., Cr

2+)

Which configuration for d4? Depends on placement in the spectrochemical series!

If you have ligands low on the series and metals low on the series, you will generally get

a maximum for unpaired electrons

This is called a high spin complex or a weak field complex

EX. [CrCl6]4-

For ligands high in the series and metals high in the series, you get a low spin complex

(strong field case)

EX. [Ru(NO2)6]3-

(what is NO2- called in this case?)

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What about the in-between cases?

Best to have experimental evidence on unpaired electrons

Generalizations:

o Ligands high on the series and 3d metals = strong field cases

o 4d and 5d metals with just about any ligand = strong field cases

Measuring electron spin

Use a Guoy balance to measure paramagnetism.

Paramagnetism: attraction to a magnetic field due to unpaired electrons

Diamagnetism: repulsion of a magnetic field by paired electrons (weaker than paramagnetism)

Can measure the magnetic moment ()

= 2×(S(S+1))½ × B S = spin quantum number (total spin), B = Bohr magneton

Also, = (N(N+2))½ × B N = number of unpaired electrons

EX. The magnetic moment of an octahedral complex for Co2+

is 4.0B. What is the electron

configuration?

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The Jahn-Teller Effect

If the ground electronic configuration of a non-linear complex is orbitally degenerate, the

complex will distort so as to remove the degeneracy and achieve a lower energy

w = weak

s = strong

blank = no distortion

Tetragonal distortion and square planar complexes

Distortion of Oh symmetry causes a change in orbital energies—and severe distortion could

cause loss of ligands!

Mainly affects d7, d

8, d

9 complexes

# of e- 1 2 3 4 5 6 7 8 9 10

high spin w w s w w s

low spin w w w w s s

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Distortion for d8 complexes can be enough to cause spin pairing in the dz2 and you lose the

ligands along the z-axis.

To accomplish this, need a strong field ligand or a strong field metal.

[NiCl4]2-

[Ni(CN)4]2-

[PdCl4]2-

Tetrahedral complexes

For Td complexes d-orbitals are split, but opposite of an Oh complex

T is smaller than o because there are fewer ligands

Td complexes are only weak field cases

EX. What is the CFSE for [CoCl4]-?

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Splitting patterns for various geometries of metal complexes. How well does this correlate with

the character tables of the correct point groups?

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MO theory and octahedral complexes (aka Ligand Field Theory) While crystal field theory is useful in making correlations between experimental evidence (e.g.,

λmax and electron configuration), it doesn’t necessarily explain why—using MO theory can held

explain the “whys”.

Complexes with σ-bonding

As with previous MO theory, there must be matching symmetry for a bond to occur.

What are the symmetries of the metal orbitals in an octahedral field? Let’s assume we have a 3d

metal (see character table).

What symmetries will the ligands adopt?

What symmetries will overlap in the MO? and as importantly, what doesn’t have the right symmetry?

eg electrons in complex are not strictly confined to the metal atom

o is still a function of t2g and eg separation.

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Complexes with π-bonding

-orbitals (bonding and antibonding) may form between d-orbitals and (filled) p-orbitals or d-

orbitals and (empty) antibonding orbitals on a ligand

d-orbital and p-orbital

Consider a ligand with a filled p-orbital (called -donor ligands)

d-orbital and antibonding -orbital

Recall CO

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What orbitals would overlap with the antibonding orbitals in CO?

Δo increases with the following trend:

-donor < weak -donor < no effects < -acceptor

I- < Br

- < Cl

- < F

- < H2O < NH3 < PR3 < CO (this is the spectrochemical series)

Ligands may be neither π-donors nor π-acceptors, but can still be strong σ-donors: CH3-, H

-

Skip sections 21.6-21.9

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Thermodynamic aspects: ligand field stabilization energies (LFSE) The data in the table of high and low spin Δoct can be plotted as follows:

This correlation with CFSE (LFSE) is found repeated in several manifestations:

Lattice energy

In a MCl2 lattice, we find the following lattice energies.

Hydration energy

This graph is for M2+

(aq) ions

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Irving-Williams Series

Ranks the stability of M2+

complexes as the following

Ba2+

< Sr2+

< Ca2+

< Mg2+

< Mn2+

< Fe2+

< Co2+

< Ni2+

< Cu2+

> Zn2+

Compare electronic structure of Ni2+

and Cu2+

How many d e-?

What's the expected e- configuration?

Comparing multiple substitution of NH3 for M(H2O)62+

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Lability and Inertness

Inert: complexes with a half life greater than 1 minute

Labile: complexes with a half life less than 1 minute

This diagram shows various lifetimes for exchange of water

M(H2O)6n+

+ H2O* [M(H2O*)(H2O)5]n+

+ H2O

some reactions are fast (Cu2+

= 10-9

sec), some are slow Cr3+

> 104 sec, ~2.8 h!)

Generalizations

1. All complexes of s-block elements are labile, with Be2+

and Mg2+

having considerably longer

t1/2 than others

2. All M3+

ions of the f-block elements are very labile

3. d10

ions with low oxidation numbers are labile

4. M2+

complexes are moderately labile, except high field Fe2+

M3+

complexes are more inert than M2+

5. Strong field d3 and d

6 octahedral complexes of first row d-metals are inert, others labile

6. In first row d-metals, the more stable the LFSE, the more inert

7. In 2nd and 3rd row d-metals, complexes are usually inert (see LFSE)

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Square planar reactions The trans effect: the effect of a coordinated ligand upon the rate of substitution of ligands

opposite to it.

It is a kinetic factor only.

If the ligand is a strong -donor or -acceptor, it will greatly increase the rate of substitution of a

ligand trans to itself

-donor: OH- < NH3 < Cl

- < Br

- < CN

-, CO, CH3

- < I

- < SCN

- < PR3 < H

-

-acceptor: Br- < I

- < NCS

- < NO2

- < CN

- < CO, C2H4

EX. Starting with [Pt(NH3)4]2+

what happens in HCl?

Ex. Starting with [PtCl4]2-

what happens in NH3?