Both the metal and the methyl radical must donate one electron each to form our metal-ligand bond. Therefore, the methyl group is a one electron donor, not a two electron donor as it is under the ionic formalism. It remains on the metal and is counted there. Notice that this method does not give us any immediate information about the formal oxidation state of the metal, so we must go back and assign that in a separate step. Electron donation of common ligands In the table below are some common transition metal ligands and the number of electrons that each donates to a metal center.
Some ligands can donate a variable number of electrons.
For example, an alkoxide, M-OR, can donate two to six electrons depending on the hybridization of the oxygen atom. However, these formalisms are very useful to us, and both will give us the same final answer. Consider the following simple examples. Notice how some ligands donate the same number of electrons no matter which formalism we choose, while the number of d-electrons and donation of the other ligands can differ. All we have to do is remember to be consistent and it will work out for us.