Main Group Systems

How do you calculate the metal oxidation state?

O.S. = Charge on complex - sum of the charges of the ligands.

How do you calculate the metal d-electron count?

d electrons = group number - O.S.

How do you calculate total valence electron count?

Valence electrons = d-electron count + electrons donated by ligands + number of M-M bonds

What are the principles of molecular orbital theory?

• Orbitals must be of appropriate symmetry.

• Orbitals must overlap.

• Orbitals should be of similar energy.

• Interaction energy is stronger for orbitals with good overlap and close in energy.

What does the molecular orbital diagram look like for a general transition metal complex with n-ligands?

How does bonding to a ligand affect the orbitals?

It disrupts the degeneracy of the orbitals, the extent of which is dependent upon interactions between orbitals.

• Stronger interactions means lower energy bonding MOs.

How do M-L multiple bonds form?

They contain a sigma-bond and one or two pi-bonds.

• For pi donation to occur there must be an empty metal d orbital to accept the electrons.

What is the coordination number of group 6 metals and how does it bond to other metals?

Forms a simple octahedral complex, does not bond to other metals.

Forms M(CO)6

What is the coordination number of group 7 metals and how does it bond to other metals?

Can bond to one other metal.

• As only sigma bonds are involved, the staggered conformation gives the least steric clash.

Forms M2(CO)10

What is the coordination number of group 8 metals and how does it bond to other metals?

Bonds to 3 other metals.

Forms M3(CO)12

What is the coordination number of group 9 metals and how does it bond to other metals?

Bonds to 4 other metals.

Forms M4(CO)12

What is the coordination number of group 10 metals and how does it bond to other metals?

The metal is too small to fit 6 groups around, therefore forms a tetrahedral complex.

Forms M(CO)4

Why do metals form 18 electron complexes?

This minimises orbital overlap and minimises steric clash.

How do metals form carbonyl complexes?

React the metal with excess CO at 200 atm.

How do d-orbitals interact with one another?

The least overlap gives the weakest bonding interaction, therefore they have higher energy.

Orbitals which can overlap the strongest give the strongest bonding interaction (such as dz2)

How do orbital interactions affect the energy of the d-orbitals?

As the dz2 orbitals point straight towards the ligands, when a ligand is lost in the z direction this causes the dz2 orbital to become less antibonding, therefore it falls in energy.

What is the molecular orbital diagram for the bonding of two metal complexes (e.g. Mn(CO)5)?

Each complex has one unpaired electron to donate to the other, forming a MO.

How is bond order calculated?

BO= Bonding pairs - antibonding pairs.

What is the trend in BDE of metal complexes as you go down the group?

The BDE gets stronger as you go down. This is due to the greater energy to promote an electron from the sigma bonding orbital to the sigma antibonding orbital.

What is the trend in bond strength to transition metals down a group?

As you go down the group, the bonds tend to get stronger. This is due to the d orbitals extending further as you go from 3d to 5d, allowing a closer interaction to ligands.

• This increases dz2 overlap, therefore the sigma bonds are more stable.

How do metal form quadruple bonds to one another?

The formation of quadruple bonds decreases the bond distance from around 3 A (single bond) to 2 A.

• The sigma, pi and delta bonding orbitals are all occupied.

How do delta bonds compare to pi bonds?

They are generally weaker due to poorer overlap between precursor orbitals.

What is the trend in bond order of M2 dimers?

The bond order increases, and effective bond order (calculated by quantum mechanics) increases.

• The dimers are in the 0 O.S.

What does the molecular orbital diagram of a M2 dimer look like?

The p orbitals are too high in energy so do not interact.

What bond order is expected in metal complexes?

A bond order less than 5 as the metal d orbitals are required for the M-L bonding to occur.

How are M2X8 structures formed?

d4 metal form quadruple bonds to another metal. The ligands form a square planar geometry around the metal, forming an eclipsed structure.

What is the molecular orbital diagram for a square planar complex?

The dz2 orbitals form the sigma bonds, the dxz and dyz form pi orbitals, the dxy orbitals form delta bonds (they have 2 nodes, so have 4 overlapping orbitals).

• The dx2-y2 orbitals are primarily involved in bonding to ligands.

Why is an eclipsed conformation sometimes necessary?

In some complexes, e.g. Re2Cl8^2-, the eclipsed conformation is needed for overlap of the delta orbitals.

However, in some complexes such as Os2Cl8^2-, there are two extra electrons which go into the delta* orbital, giving a bond order of 0 therefore the complex can be either eclipsed or staggered.

When are M2X6 and M2X9 structures formed?

d3 metals form triple bonds to one another.

This can give bridged (for 9 ligands) or non-bridged (eclipsed or staggered) conformations.

What does the molecular orbital diagram look like for M2X6 complexes?

The dxz and dyz orbitals become hybrids as they are mixed with dx2-y2 and dxy orbitals respectively. This causes tilting which causes the pi and pi* orbitals to have some delta character.

Do M2X6 complexes prefer eclipsed or staggered?

The eclipsed conformation gives the best orbital overlap with ligands, however the staggered conformation reduces the steric clash.

• The staggered conformation is mostly seen.

How does d-electron overlap affect magnetism?

Complexes with all electrons paired up should be diamagnetic, however the 3d orbital doesnt extend out far enough for significant orbital overlap, therefore there is little covalency and so the M-M bonds are weak and so the electrons remain unpaired.

If 5d orbitals are involved, the orbital extends further so the electrons are paired.

For 4d orbitals, the M-M distance is dependent upon the ction present in the structure. They are variable.

When do quintuple bonds form?

A quintuple bond can be formed by reduction of [Cr(µCl)Ar’]2 with slight excess of potassium graphite, producing a very short Cr-Cr bond length.

What does the molecular orbital diagram look like for quintuple bonded complexes?

There are 9 valence orbitals.

The ligand forms bonds with 4s orbitals, however the 4p are too high in energy. This leaves the 3d orbitals available for bonding to the metal.

What is the geometry of a quintuple bond?

A linear geometry is expected, however a trans bent geometry is observed, suggesting orbital mixing occurs.