JEE Advanced 2018 · Paper 1 · Chemistry Q2

ChemistryMultiple correct+4 / -2Question 20 of 54

The correct statement(s) regarding the binary transition metal carbonyl compounds is (are) (Atomic numbers: Fe = 26, Ni = 28)

Total number of valence shell electrons at metal centre in

\ce{Fe(CO)5}

\ce{Ni(CO)4}

is 16

These are predominantly low spin in nature

Metal-carbon bond strengthens when the oxidation state of the metal is lowered

The carbonyl C−O bond weakens when the oxidation state of the metal is increased

Answer and worked solution

Official answer: B, C

(A) Fe has 8 valence electrons and five CO donate 10, so 18. Ni has 10 and four CO donate 8, so 18. Not 16. False.
(B) Strong-field CO pairs up the metal electrons, so the carbonyls are low spin (diamagnetic). True.
(C) A lower oxidation state leaves more electron density on the metal, so π back-donation into CO π* increases. This strengthens the M-C bond. True.
(D) A higher oxidation state reduces back-donation, so less electron density enters CO π*. The C-O bond becomes stronger, not weaker. False.

(B), (C)

Topics:Metal carbonyls and EAN rule Valence bond theory

JEE Advanced 2018 · Paper 1, Chemistry question 2