Chemistry - Coordination Chemistry Question with Solution | TestHub

The Effective Atomic Number (EAN) rule states that the sum of the atomic number of the metal and the electrons donated by the ligands should equal the atomic number of the nearest noble gas.

Given EAN = 36 (Krypton's atomic number).

The complex is $[\mathrm{M}(\mathrm{CO}{)}_2(\sigma -{\mathrm{C}}_5{\mathrm{H}}_5)(\pi -{\mathrm{C}}_5{\mathrm{H}}_5)]$.

Electrons donated by ligands:

CO is a 2-electron donor. There are two CO ligands: $2\times 2=4$ electrons.

$\sigma -{\mathrm{C}}_5{\mathrm{H}}_5$ (cyclopentadienyl) is a 1-electron donor (sigma bond).

$\pi -{\mathrm{C}}_5{\mathrm{H}}_5$ (cyclopentadienyl) is a 5-electron donor (eta-5 coordination).

Total electrons from ligands = $4+1+5=10$ electrons.

Let Z be the atomic number of metal M.

EAN = Z + electrons from ligands

$36=Z+10$

$Z=36-10=26$

The atomic number of metal M is 26.

Sum of digits: $2+6=8$.

The final answer is 8.