Among the statements \((\mathrm{a}-\mathrm{d})\), the incorrect ones are: (a) Octahedral Co (III) complexes with strong field ligands have very high magnetic moments (b) When \(\Delta_{0}<\mathrm{P}\), the d-electron configuration of Co (III) in an octahedral complex is \(\mathrm{t}_{\mathrm{eg}}^{4} \mathrm{e}_{\mathrm{g}}^{2}\) (c) Wavelength of light absorbed by \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}\) is lower than that of \(\left[\mathrm{CoF}_{6}\right]^{3-}\) (d) If the \(\Delta_{0}\) for an octahedral complex of Co (III) is \(18,000 \mathrm{~cm}^{-1}\), the \(\Delta_{\mathrm{t}}\) for its tetrahedral complex with the same ligand will be \(16,000 \mathrm{~cm}^{-1}\)

JEE Main · 2020 · 07 Jan 2020 · Shift 2 · Coordination Compounds

Among the statements

(a - d)

, the incorrect ones are:
(a) Octahedral Co (III) complexes with strong field ligands have very high magnetic moments
(b) When

Δ_{0} < P

, the d-electron configuration of Co (III) in an octahedral complex is

t_{eg}^{4} e_{g}^{2}

[Co (en)_{3}]^{3 +}

is lower than that of

[CoF_{6}]^{3 -}

(d) If the

Δ_{0}

for an octahedral complex of Co (III) is

18, 000 cm^{- 1}

, the

Δ_{t}

for its tetrahedral complex with the same ligand will be

16, 000 cm^{- 1}

Answer: (D)

Statement (a) is wrong: a strong-field

Co X^{3 +}

complex is low-spin

t_{2 g}^{6} e_{g}^{0}

, fully paired, so its magnetic moment is essentially zero rather than high. Statement (d) is wrong too: the tetrahedral splitting is

Δ_{t} = \frac{4}{9} Δ_{0} = \frac{4}{9} \times 18000 = 8000 c m^{- 1}

, not

16000

. The other two are sound — a weak field (

Δ_{0} < P

) does give the high-spin

t_{2 g}^{4} e_{g}^{2}

arrangement, and the stronger-field

[Co (en) X_{3}] X^{3 +}

absorbs at a shorter wavelength than

[CoF X_{6}] X^{3 -}

. So the incorrect statements are (a) and (d).

JEE Main · 2020 · 07 Jan 2020 · Shift 2 · Single correct option