Iron complexes 18 valence electron rule

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

The electrons courted for the metal atom in each of these complexes are those in its valence a and il orbitals. Metals having odd numbers of electrons obviously cannot satisfy the l8-elcctron rule by simple addition of CO (or other two-electron) ligands because the resulting moiety will necessarily also have an odd number of electrons. For example, MnlCO) and ColCOljdre both 17-electron species and, consistent with prediction, do not exist as stable molecules. However, their corresponding anions, [Mr(CO)jJ and [CoiCOlJ , are stable species and conform to the l8-elec-iron rule. 

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