Metal Ligand bonding

Trends in radius

Increases from right to left and down a group

Trends in electropositivity

electropositive character increases from right to left and down a group

Trends in oxidation state

Earlier metals exhibit the greatest variety in oxidation state. Higher oxidation states more commonly observed for 2nd and 3rd row metals

There are stronger bonds ____ a group

down

The 3d orbitals in the first metal are not as diffuse as the

2nd and 3rd row 4d and 5d orbitals

η - Hapticity

the number of contagious atoms of a ligand attached to a metal

κ - denticity

The number of non-contagious atoms coordinating from a ligand

μ

The number of metal atoms bridged by a ligand

Oxidation state =

Charge on the complex - sum of the charges of the ligands

the electronic structure of substances is such to cause each atom to

have essentially zero resultant charge

d electron count =

group number - oxidation state

Total valence electron count =

d-electron count + electrons donated by the ligands + number of metal-metal bonds

Metal-metal bonds

• single bonds = 1 per metal

• double bonds = 2 per metal

• triple bonds = 3

• quadruple bond = 4

Metal-metal bonding is more common for

2nd and 3rd row metals than for 1st row

The greater the no. ligands and the stronger the bonds the greater the …

thermodynamic stability of the resulting complex

The no. of ligands is limited by

ligand-ligand repulsion

Large positive and negative charges

can’t easily be supported

Continually removing electrons from a complex will result in

increasingly large ionisation energies and increasing the number of electrons will lead to large electron-electron repulsive forces

5 and 6 membered rings most stable so are

less likely to dissociate

solvation

requirement to order the solvent cage at the complex will decrease entropy and reduce overall stability

The bonding orbital looks more like the

lower energy component

The anti bonding orbital looks more like the

higher energy component

electron configuration

Transition metal valence orbitals and the 18 electron rule

𝜎 donor

All have a ‘lone pair’ of electrons to bond to an empty metal orbital

𝜎 donor, π acceptor

All have a lone pair and an empty orbital of π symmetry which can accept e^- density

𝜎 donor, π donor

All have a lone pair and filled orbits of π symmetry which can be donated to the metal

What can be both π acceptors and donors

More complex ligand

What’s more important in terms of bond strength?

σ-bonding is more important than π-bonding due to better orbital overlap

σ bond

the bond between the ligand and metal

Bonding - σ donor, π acceptor

Bonding is from a σ-only interaction and on additional π interaction between empty ligand orbitals and occupied metal orbitals

σ-donor interaction increases

electron density on the metal

π-acceptor interaction decreases

electron density

π-acceptor interaction increases

electron density on the CO ligandσ

σ-donor interaction decreases

electron density on CO ligand

π-acceptor ligands such as CO can

relieve negative charge build-up a metal centre

Experimental evidence for bonding model

• IR and Raman spectroscopy and single crystal x-ray diffraction

• Characterisation of metal carbonyls

Other ligands expected to exhibit very similar bonding to CO are

isoelectronic ligands CN^- and NO⁺

To break covalent bonds you either increase electron density

of anti bonding orbitals or decrease the electron density of bonding orbitals

Why is η¹-O₂ bent when CO is linear?

O2 has to accommodate an extra pair of electrons in the 1π_g(π^*) orbital

NO typically adopts one of two terminal coordination modes

Bent and linear

How many electrons does NO donate? (linear)

• 1 electron goes from NO to the metal, giving NO⁺ + M^-

• NO⁺ is then isoelectronic with CO, and donates 2 electrons from NO to metal

How many electrons does NO donate? (bent)

• 1 electron goes from metal to NO, giving NO^- + M⁺

• NO^- is then isoelectronic with O₂, and donates 2 electronics from NO to metal

If sufficient electron density is transferred from the metal to the σ^* orbital of H₂ the

H-H σ-bond will break and give two M-H σ-bonds

Alkenes

• π-acceptor ligands

• Form basis of many catalytic reactions

Metal oxides are used as a source of

Oxygen for the oxidation of organic compounds

π-donors and the 18 electron rule

π-acceptor ligands usually obey the 18-electron rule, this with π-donors don’t necessarily do

For π-donor ligands the metal t₂g orbitals are anti bonding so therefore it’s not

energetically favourable to fill them

Spectrochemical series

list of ligands in order of increasing ligand field strength

Electrochemical series order

CO > CN^- > PPh₃ > NH₃ > H₂O > OH^- > F^- > Cl^- > S^2- > Br^- > I^-

What helps to rationalise the stability and substitution chemistry of transition metal complexes

The trans effect and trans influence

The trans effect

a kinetic phenomenon and describes the influence of a non-labile group on the rate of substitution of a ligands trans to it

Ligand substitution

Most common reaction of coordination compounds substituting one ligand for another primary coordination sphere

Dissociation

Decreases the metal coordination number

Addition

Increases the metal coordination number

Redox reactions

Oxidation, reduction, electronic transfers

Reactions of coordinated ligands

Includes organometallic chemistry, catalysis etc.

Intimate mechanism

Usually considers the transition state of the rate-determining step

Associative mechanism

M-L bond formation is well advanced in the T.S

Dissociative mechanism

M-L bond breaking advanced in the T.S

Labile

• Complexes of d¹⁰ ions

• Complexes of 3d M(II) ions (M(III) less labile)

Inert

• d³ and low-spin d⁶ configurations

• 4d and 5d complexes due to high CFSE and better metal ligand overlap

• Chelating ligands

Activation parameters

Reaction rate examined as a function of temperature

Eyring equation allows

determination of enthalpy and entropy of activation

ΔV^‡

Volume of activation, volume difference between the initial ground state and the transition state

ΔH^‡

Activation enthalpy, bond strength difference

ΔS^‡

Activation entropy, order difference

ΔS^‡ and ΔV^‡ are both

negative

-ve ΔV^‡ indicates the transition state is

smaller than the ground state

-ve ΔS^‡ indicates the transition state is

more ordered than the ground state

Stereochemical retention of configuration is observed

during substitution

Bulky groups decrease the

rate of substitution

Most important factor of the entering group is the

Polarisability or softness

How does the leaving group affect the rate of substitution

• Hard ligands such as H₂O and Cl- leaves quickly

• Rate reflects the strength of the M-L and of the leaving group

Weaker bond

Faster reaction

Steric effects at bulky ligands will

reduce the rate of associative reactions

The electronic effect of trans ligands cause competition for bonding as they share the same d-orbitals this

will effect the M-L bond strength of the substituting ligand

The free energy of activation (ΔG^‡) of ligand substitution is the difference between the

reactant ground state and the first transition state

ΔG^‡ can be decreased by

destabilising the ground state or stabilising the transition state

Destabilisation of the ground state

• Thermodynamic effect

• Some ligands weaken the M-L bond which turns to them in the ground state

Stabilisation of the transition state

In the trigonal plane of the 5-coordinate transition state or intermediate, π-back bonding can occur between a metal d-orbital and suitable orbitals of ligand T and Y

Square planar substitution reactions are generally slow due to

Loss of crystal field stabilisation energy during the formation of trigonal bipyramidal complex from the square planar one

CFSE is greater

down a group so addition of a fifth ligand and loss of this CFSE will result in a higher activation energy

M-L bonding gets

Stronger down a group because of better orbital overlap

Interchange

Most common mechanism in octahedral complexes

As Y begins to bond to the metal X begins

to leave

Bond making and breaking occur

Simultaneously

The designations of I_d and I_a are used to

differentiate associatively from dissociatively activate processes

An associative mechanism is usually

First order in both reactant and incoming ligand

Entering group effect

The entering group has a small effect on the rate (suggests not an associative type mechanism)

LFER

linear free energy relationship

LFER are consistent with a rate determining step, which varies due to

differing bond strengths with reactions proceeding through similar transition states

In the dissociative mechanism the coordination number

is decreased in the transition state

If L is large then the steric crowding will

promote dissociation due to a lower coordination no. in the transition state

Increasing the ligand size

Increases the steric pressure at the metal resulting in a faster substitution reaction

a dissociative reaction will commonly lead to a

16e^- intermediate

An associative reaction would give a 20 electron intermediate which would be

very high in energy

Two mechanisms by which inorganic complexes transfer electrons

The outer sphere mechanism and the inner sphere mechanism

Outer sphere mechanism is important because of the analogy that can

be drawn between electron transfer in metal complexes and electron transfer in metalloenzymes