Question

1. a) Do any of these complexed derived from the 18-electron rule and is so, how is this
reflected in the structure or chemical properties.

b) Use both the a) neutral ligand method and b) the donor pair method to determine if the
following compound obey the 18 electron rule.
[IrBr₂(CH₃)(CO)(PPh₃)₂]

Answer 1

d electron configuration of neutral iridium = d⁹

In the molecule iridium is attached to two bromide and one methide species. Therefore the oxidation state of iridium +3.   (CO and PPh₃ are neutral groups and are coordinatively attached to iridium)

At oxidation state +3, d electron count of iridium = (9 - 3) = 6

Electron count in neutral method:

Contribution of electron from Ir = 9

Contribution of electron from 2 bromine atoms = 2 x 1 = 2

Contribution of electron from methyl group = 1

Contribution of electron from carbonyl (CO) group = 2

Contribution of electron from 2 triphenylphosphine (PPh₃) groups = 4

Total electrons = (9 + 2 +1 + 2 + 4) = 18

Electron count in the donor pair method:

Contribution of electron from Ir = 6

Contribution of electron from 2 bromide groups= 2 x 2 = 4

Contribution of electron from methide group = 2

Contribution of electron from carbonyl group = 2

Contribution of electron from 2 triphenylphosphine groups = 4

Total electrons = (6 + 4 +2 + 2 + 4) = 18

Therefore, the complex [IrBr₂(CH₃)(CO)(PPh₃)₂] follows 18 electron rule.

Complexes that follow 18 electron rules are electronically saturated (all bonding and nonbonding orbitals are filled). They are stable and inert towards ligand exchange.