User:Derrekli/4.2 Electron counting and the 18 electron rule

The 18 electron rule is used in transition metals to determine whether a molecular complex has a full d orbital or if it can still bind with additional ligands (molecules or ions that are attached to a metal). If the molecular complex has an 18 electron count, it is called saturated. This means that additional ligands cannot bind to the transition metal because there are no empty low-energy orbitals for incoming ligands to coordinate. If the molecule has less than 18 electrons, then it is called unsaturated and can bind additional ligands.

Electron counting

To count electrons in a transition metal compound:

1. Determine the oxidation state of the transition metal and the resulting d-electron count.
   • Identify if there are any overall charges on the molecular complex.
   • Identify the charge of each ligand.
2. Determine the number of electrons from each ligand that are donated to the metal center.
3. Add up the electron counts for the metal and for each ligand.

Example

Saturated Re metal complex with 18 electron count

1. There is no overall charge on the molecule and there is one anionic ligand (CH₃^-)
   • The Re metal must have a positive charge that balances out the anionic ligand charge to equal the 0 overall molecular charge. Since there is a -1 charge contribution from the methyl ligand, the Re metal has a +1 charge.
   • Because the Re metal is in the +1 oxidation state, it is a d⁶ electron count. It would have been its regular d⁷ electron count if it had a neutral (0) oxidation state.
2. The CH₃^- ligand contributes 2 electrons. Each CO ligand contributes 2 electrons. Each PR₃ ligand contributes 2 electrons. The H2C=CH2 ligand contributes 2 electrons.
3. Adding up the electrons:
   • Re(1): 6 electrons
   • CH₃^-: 2 electrons
   • 2 x CO: 2 x 2 electrons = 4 electrons
   • 2 x PR₃: 2 x 2 electrons = 4 electrons
   • H2C=CH2: 2 electrons
   • Total: 18 electrons

In this example, the molecular compound has an 18 electron count, which means that all of its orbitals are filled and the compound is stable.

Ligand contributions

Neutral 2e donors	Anionic 2e donors	Anionic 4e donors	Anionic 6e donors
PR3 (phosphines)	X- (halide)	C3H5- (allyl)	Cp- (cyclopentadienyl)
CO (carbonyl)	CH3- (methyl)	O2- (oxide)	O2- (oxide)
alkenes	CR3- (alkyl)	S2- (sulfide)
alkynes	Ph- (phenyl)	NR2- (imide)
nitriles	H- (hydride)	CR22- (alkylidene)
	RnE- (silyl, germyl, alkoxo, amido etc.)	OR- (alkoxide, bridging ligand)
		SR- (thiolate, bridging ligand)
		NR2- (inorganic amide, bridging ligand)
		PR2- (phosphide, bridging ligand)