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Chapter 24: Problem 48

The ion \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) has one unpaired electron, whereas \(\left[\mathrm{Fe}(\mathrm{NCS})_{6}\right]^{3-}\) has five unpaired electrons. From these results, what can you conclude about whether each complex is high spin or low spin? What can you say about the placement of \(\mathrm{NCS}^{-}\) in the spectrochemical series?

Short Answer

Expert verified
In conclusion, the complex \([\mathrm{Fe}(\mathrm{CN})_{6}]\mathrm{^{3-}}\) is a low spin complex due to its single unpaired electron, while the complex \([\mathrm{Fe}(\mathrm{NCS})_{6}]\mathrm{^{3-}}\) is a high spin complex with five unpaired electrons. This indicates that the thiocyanate ligand (NCS-) has a lower field strength than the cyanide ligand (CN-) in the spectrochemical series.

Step by step solution

01

Determine the oxidation state of the central metal ion

In both complexes, the central metal ion is Fe. In the first complex, the overall charge is -3, and the cyanide ligand's charge is -1, which means that Fe must have an oxidation state of +3 (since there are 6 cyanide ligands). Similarly, in the second complex, the overall charge is -3, and the thiocyanate ligand's charge is -1, which means that Fe must also have an oxidation state of +3.
02

Write the electron configurations for the Fe(III) ion

The electron configuration of Fe is [Ar]3d^6 4s^2. When it loses 3 electrons to become Fe(III), the electron configuration becomes [Ar]3d^5 with 5 empty 4s and 4p orbitals.
03

Determine the electronic configuration of the complexes

For \([\mathrm{Fe}(\mathrm{CN})_{6}]\mathrm{^{3-}}\), there is one unpaired electron. This means that out of the five d-orbitals in Fe(III), four are completely filled, and one is half-filled. This configuration is consistent with a low spin complex, as all the lower energy orbitals are filled before any higher energy orbitals are occupied. For \([\mathrm{Fe}(\mathrm{NCS})_{6}]\mathrm{^{3-}}\), there are five unpaired electrons. This means that each of the five d-orbitals in Fe(III) has one electron. This configuration aligns with a high spin complex, as the electrons occupy higher energy orbitals before completely filling the lower energy orbitals.
04

Determine the position of NCS- in the spectrochemical series

Since the \([\mathrm{Fe}(\mathrm{CN})_{6}]\mathrm{^{3-}}\) complex is low spin, we can deduce that the CN- ligand is a strong field ligand. On the other hand, the \([\mathrm{Fe}(\mathrm{NCS})_{6}]\mathrm{^{3-}}\) complex is high spin, meaning that the NCS- ligand is a weak field ligand. Therefore, NCS- must be placed on the weaker field side of the spectrochemical series compared to CN-. In conclusion, the complex \([\mathrm{Fe}(\mathrm{CN})_{6}]\mathrm{^{3-}}\) is a low spin complex, while the complex \([\mathrm{Fe}(\mathrm{NCS})_{6}]\mathrm{^{3-}}\) is a high spin complex. The thiocyanate ligand (NCS-) has a lower field strength than the cyanide ligand (CN-) in the spectrochemical series.

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Most popular questions from this chapter

The molecule methylamine \(\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)\) can act as a monodentate ligand. The following are equilibrium reactions and the thermochemical data at \(298 \mathrm{~K}\) for reactions of methylamine and en with \(\mathrm{Cd}^{2+}(a q)\) : \(\mathrm{Cd}^{2+}(a q)+4 \mathrm{CH}_{3} \mathrm{NH}_{2}(a q) \rightleftharpoons\left[\mathrm{Cd}\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)_{4}\right]^{2+}(a q)\) \(\Delta H^{\circ}=-57.3 \mathrm{~kJ} ; \quad \Delta S^{\circ}=-67.3 \mathrm{~J} / \mathrm{K} ; \quad \Delta G^{\circ}=-37.2 \mathrm{~kJ}\) $$ \mathrm{Cd}^{2+}(a q)+2 \operatorname{en}(a q) \rightleftharpoons\left[\mathrm{Cd}(\mathrm{en})_{2}\right]^{2+}(a q) $$ \(\Delta H^{\circ}=-56.5 \mathrm{~kJ} ; \quad \Delta S^{\circ}=+14.1 \mathrm{~J} / \mathrm{K} ; \quad \Delta G^{\circ}=-60.7 \mathrm{~kJ}\) (a) Calculate \(\Delta G^{\circ}\) and the equilibrium constant \(K\) for the following ligand exchange reaction: \(\left[\mathrm{Cd}\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)_{4}\right]^{2+}(a q)+2 \operatorname{en}(a q) \rightleftharpoons\) $$ \left[\mathrm{Cd}(\mathrm{en})_{2}\right]^{2+}(a q)+4 \mathrm{CH}_{3} \mathrm{NH}_{2}(a q) $$ (b) Based on the value of \(K\) in part \((a)\), what would you conclude about this reaction? What concept is demonstrated? (c) Determine the magnitudes of the enthalpic \(\left(\Delta H^{\circ}\right)\) and the entropic \(\left(-T \Delta S^{\circ}\right)\) contributions to \(\Delta G^{\circ}\) for the ligand exchange reaction. Explain the relative magnitudes. (d) Based on information in this exercise and in the "A Closer Look" box on the chelate effect, predict the sign of \(\Delta H^{\circ}\) for the following hypothetical reaction: \(\left[\mathrm{Cd}\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)_{4}\right]^{2+}(a q)+4 \mathrm{NH}_{3}(a q) \rightleftharpoons\) $$ \left[\mathrm{Cd}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}(a q)+4 \mathrm{CH}_{3} \mathrm{NH}_{2}(a q) $$

Sketch all the possible stereoisomers of (a) tetrahedral \(\left[\mathrm{Cd}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2} \mathrm{Cl}_{2}\right]\), (b) square-planar \(\left[\mathrm{IrCl}_{2}\left(\mathrm{PH}_{3}\right)_{2}\right]^{-}\), (c) octa- hedral \(\left[\mathrm{Fe}(0 \text { -phen })_{2} \mathrm{Cl}_{2}\right]^{+}\)

A palladium complex formed from a solution containing bromide ion and pyridine, \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\) (a good electronpair donor), is found on elemental analysis to contain \(37.6 \%\) bromine, \(28.3 \%\) carbon, \(6.60 \%\) nitrogen, and \(2.37 \%\) hydrogen by mass. The compound is slightly soluble in several organic solvents; its solutions in water or alcohol do not conduct electricity. It is found experimentally to have a zero dipole moment. Write the chemical formula, and indicate its probable structure.

The molecule dimethylphosphinoethane \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{PCH}_{2}-\mathrm{CH}_{2} \mathrm{P}\left(\mathrm{CH}_{3}\right)_{2}\right.\), which is abbreviated \(\left.\mathrm{dmpe}\right]\) is used as a ligand for some complexes that serve as catalysts. A complex that contains this ligand is \(\mathrm{Mo}(\mathrm{CO})_{4}(\) dmpe \()\). (a) Draw the Lewis structure for dmpe, and compare it with ethylenediammine as a coordinating ligand. (b) What is the oxidation state of Mo in \(\mathrm{Na}_{2}\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right] ?\) (c) Sketch the structure of the \(\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]^{2-}\) ion, including all the possible isomers.

In 2001, chemists at SUNY-Stony Brook succeeded in synthesizing the complex trans-[Fe(CN) \(\left._{4}(\mathrm{CO})_{2}\right]^{2-}\), which could be a model of complexes that may have played a role in the origin of life. (a) Sketch the structure of the complex. (b) The complex is isolated as a sodium salt. Write the complete name of this salt. (c) What is the oxidation state of Fe in this complex? How many d electrons are associated with the \(\mathrm{Fe}\) in this complex? (d) Would you expect this complex to be high spin or low spin? Explain.
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