Correct option is D
The 18-electron rule is a chemical rule of thumb used primarily for predicting and rationalizing formulas for stable transition metal complexes, especially organometallic compounds. The rule is based on the fact that the valence orbitals in the electron configuration of transition metals consist of five ( n−1)d orbitals, one ns orbital, and three np orbitals, where n is the principal quantum number. These orbitals can collectively accommodate 18 electrons as either bonding or non-bonding electron pairs. This means that the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period, lending stability to the complex. Transition metal complexes that deviate from the rule are often interesting or useful because they tend to be more reactive.
Two models are routinely used to count electrons, the so-called neutral -ligand method (sometimes called the covalent method) and the donor -pair method (sometimes known as the ionic method).
Neutral-ligand method: For the sake of counting electrons, each metal atom and ligand is treated as neutral. We include in the count all valence electrons of the metal atom and all the electrons donated by the ligands. If the complex is charged, we simply add or subtract the appropriate number of electrons to the total. The advantage of the neutral-ligand method is that, it is trivial to establish the electron count. The disadvantage, however, is that the method overestimates the degree of covalence and thus underestimates the charge at the metal. Moreover, it becomes confusing to assign an oxidation number to a metal, and meaningful information on some ligands is lost.
Donor-pair method: The donor-pair method requires a calculation of the oxidation number. The rules for calculating the oxidation number of an element in an organometallic compound are the same as for conventional coordination compounds.
The oxidation number of the metal atom is the total charge of the complex minus the charges of any ligands.
The number of electrons the metal provides is its group number minus its oxidation number.
The total electron count is the sum of the number of electrons on the metal atom and the number of electrons provided by the ligands.
The main advantage of this method is that with a little practice both the electron count and the oxidation number may be determined in a straightforward manner. The main disadvantage is that it overestimates the charge on the metal atom and can suggest reactivity that might be incorrect.
Using the neutral ligand method:
Option A:
: Electronic configuration of Ru is ⦏Kr⦐4d⁷5s¹. Total electron count is: 5+8+1+(2×2) =18
: Electronic configuration of Zr is [Kr] 4d²5s². Total electron count is:
(5×2) +4+(2×1) = 16
Option B:
: Electronic configuration of Ir is [Xe] 4f145d76s2. Total electron count is: 9+1+2+(2×2) =16
: Electronic configuration of Co is [Ar] 3d⁷4s².
Option C:
: Electronic configuration of Re is [Xe] 4f14 5d5 6s2. Total electron count is: 7+(5×2)+(2×1)-1=18
: Electronic configuration of Ni is [Ar] 3d⁸4s². NH3 ligand donates two electrons. Total electron count is:10+(6×2)-2=20
Option D:
: Electronic configuration of Fe is [Ar] 3d⁶4s².η3-C5H5 donates three electrons. Total electron count is 8+5+3+2=18
: Electronic configuration of [Ar] 4s²3d⁵. Total electron count is 3+7+(4×2) =18
