H₂O as a Ligand: Weak vs Strong Field Cases

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The Aqua Anomaly: $H_2O$ as a Ligand

In the Spectrochemical Series, $H_2O$ sits right on the borderline between Weak Field and Strong Field. Its behavior depends entirely on the metal's oxidation state and its position in the periodic table.

JEE Advanced Hack: If you see $H_2O$ with a $+2$ metal, it is almost always a Weak Field Ligand (pairing does not occur). If you see it with $Co^{3+}$ or any $4d/5d$ metal, it forces pairing!

1 The Default Rule (3d Series)

For almost all 3d transition metals in $+2$ and $+3$ oxidation states (e.g., $Fe^{2+}$, $Fe^{3+}$, $Co^{2+}$, $Ni^{2+}$, $Mn^{2+}$), $H_2O$ acts as a Weak Field Ligand (WFL). The crystal field splitting energy ($\Delta_o$) is less than pairing energy ($P$). They form High Spin complexes.

2 The Cobalt(+3) Exception

Due to the exceptionally high polarizing power and effective nuclear charge ($Z_{eff}$) of the $Co^{3+}$ ion, the splitting energy ($\Delta_o$) increases drastically. With $Co^{3+}$, $H_2O$ acts as a Strong Field Ligand (SFL), forcing electrons to pair up (Low Spin, Diamagnetic $d^6$).

3 The 4d/5d Series Rule

For heavier transition metals from the 4d and 5d series (like $Pd$, $Pt$, $Rh$, $Ir$), $\Delta_o$ is intrinsically 30-50% larger than in 3d metals. Therefore, ALL ligands (including $H_2O$ and even Halogens) act as Strong Field Ligands and form Low Spin complexes.

No complexes found matching your search.

$$\ce{[Fe(H2O)6]^2+}$$

WFL

Oxidation StateFe(II) $\rightarrow 3d^6$

Configuration$t_{2g}^4 \ e_g^2$

Spin StateHigh Spin

Unpaired ($n$)4

Mag. Moment ($\mu$)4.90 BM

$$\ce{[Fe(H2O)6]^3+}$$

WFL

Oxidation StateFe(III) $\rightarrow 3d^5$

Configuration$t_{2g}^3 \ e_g^2$

Spin StateHigh Spin

Unpaired ($n$)5

Mag. Moment ($\mu$)5.92 BM

$$\ce{[Co(H2O)6]^2+}$$

WFL

Oxidation StateCo(II) $\rightarrow 3d^7$

Configuration$t_{2g}^5 \ e_g^2$

Spin StateHigh Spin

Unpaired ($n$)3

Mag. Moment ($\mu$)3.87 BM

$$\ce{[Co(H2O)6]^3+}$$

SFL (Exception)

Oxidation StateCo(III) $\rightarrow 3d^6$

Configuration$t_{2g}^6 \ e_g^0$

Spin StateLow Spin (Inner)

Unpaired ($n$)0

Mag. Moment ($\mu$)0 BM (Diamagnetic)

$$\ce{[Ni(H2O)6]^2+}$$

WFL

Oxidation StateNi(II) $\rightarrow 3d^8$

Configuration$t_{2g}^6 \ e_g^2$

Spin StateHigh Spin

Unpaired ($n$)2

Mag. Moment ($\mu$)2.83 BM

$$\ce{[Mn(H2O)6]^2+}$$

WFL

Oxidation StateMn(II) $\rightarrow 3d^5$

Configuration$t_{2g}^3 \ e_g^2$

Spin StateHigh Spin

Unpaired ($n$)5

Mag. Moment ($\mu$)5.92 BM

$$\ce{[Mn(H2O)6]^3+}$$

WFL

Oxidation StateMn(III) $\rightarrow 3d^4$

Configuration$t_{2g}^3 \ e_g^1$

Spin StateHigh Spin

Unpaired ($n$)4

Mag. Moment ($\mu$)4.90 BM

$$\ce{[Cr(H2O)6]^3+}$$

WFL (Technically)

Oxidation StateCr(III) $\rightarrow 3d^3$

Configuration$t_{2g}^3 \ e_g^0$

Spin StateInner Orbital ($d^2sp^3$)

Unpaired ($n$)3

Mag. Moment ($\mu$)3.87 BM

$$\ce{[Rh(H2O)6]^3+}$$

SFL (4d Metal)

Oxidation StateRh(III) $\rightarrow 4d^6$

Configuration$t_{2g}^6 \ e_g^0$

Spin StateLow Spin

Unpaired ($n$)0

Mag. Moment ($\mu$)0 BM (Diamagnetic)

$$\ce{[Pt(H2O)4]^2+}$$

SFL (5d Metal)

Oxidation StatePt(II) $\rightarrow 5d^8$

ConfigurationSquare Planar

Spin StateLow Spin ($dsp^2$)

Unpaired ($n$)0

Mag. Moment ($\mu$)0 BM (Diamagnetic)

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