Concept Explanation: Identifying Paramagnetic Compounds

Paramagnetic compounds have unpaired electrons and are attracted to a magnetic field. To determine if a compound is paramagnetic, we analyze the electron configuration of the central metal ion, considering its oxidation state and the geometry imposed by the ligands (which affects the splitting of d-orbitals in crystal field theory).

Step-by-Step Analysis:

Step 1: List the compounds and identify the central metal ion and its oxidation state.

1. [Ni(CO)₄]: Nickel carbonyl. Ni is in 0 oxidation state. Electron configuration of Ni(0) is [Ar] 3d⁸ 4s². With strong field ligand CO, it forms tetrahedral complex. In tetrahedral field, d⁸ has two unpaired electrons? Actually, for Ni(0) d¹⁰? Wait, Ni atomic number 28: Ni⁰ has 28 electrons. Configuration: 1s²2s²2p⁶3s²3p⁶3d⁸4s²? But in coordination compounds, the 4s electrons are lost before 3d? For Ni(0), it is d¹⁰? Let's calculate: Ni atom has 28 electrons. In Ni(CO)₄, Ni is in zero oxidation state, so it has 28 electrons. The configuration is [Ar] 3d⁸ 4s²? But CO is a strong field ligand, and it causes pairing. Actually, Ni(CO)₄ is diamagnetic because it has tetrahedral geometry and d¹⁰ configuration? How? Ni(0) must have 28 electrons. The common configuration for Ni(0) is 3d¹⁰? Yes, because in atoms, the 4s orbital is filled before 3d, but for Ni⁰, the configuration is [Ar] 3d⁸ 4s²? That would be paramagnetic. However, in Ni(CO)₄, the Ni is considered to have configuration 3d¹⁰ due to back bonding? Actually, it is known to be diamagnetic, so it has no unpaired electrons. So, it is not paramagnetic.
2. [NiCl₄]^2-: Tetrachloronickelate(II). Ni²⁺ has d⁸ configuration. Cl^- is weak field ligand. Tetrahedral geometry. For d⁸ in tetrahedral field, high spin, two unpaired electrons. Paramagnetic.
3. [Co(NH₃)₄Cl₂]Cl: Here, Co is in +3 oxidation state (since NH₃ is neutral, Cl is -1, so to balance charge, Co is +3). Co³⁺ has d⁶ configuration. NH₃ is strong field ligand. Octahedral geometry. Low spin d⁶ has no unpaired electrons. Diamagnetic.
4. Na₃[CoF₆]: Hexafluorocobaltate(III). Co³⁺ d⁶. F^- is weak field ligand. Octahedral geometry. High spin d⁶ has four unpaired electrons. Paramagnetic.
5. Na₂O₂: Sodium peroxide. Contains O₂^2- ion. This ion has all electrons paired (bond order 1). Diamagnetic.
6. CsO₂: Cesium superoxide. Contains O₂^- ion. This ion has one unpaired electron (bond order 1.5). Paramagnetic.

Step 2: Count the paramagnetic compounds.

From above: [Ni(CO)₄] is diamagnetic. [NiCl₄]^2- is paramagnetic. [Co(NH₃)₄Cl₂]Cl is diamagnetic. Na₃[CoF₆] is paramagnetic. Na₂O₂ is diamagnetic. CsO₂ is paramagnetic.

So, paramagnetic are: [NiCl₄]^2-, Na₃[CoF₆], CsO₂. That is 3 compounds.

Final Answer: The total number of paramagnetic compounds is $3$.

Related Topics:

• Crystal Field Theory: Explains how ligands affect the d-orbitals of metal ions, leading to splitting and electron pairing.
• Oxidation States: Determining the charge on the central metal ion is crucial for finding its d-electron count.
• Magnetic Properties: Diamagnetic (all electrons paired) vs. Paramagnetic (unpaired electrons present).
• Coordination Geometry: Tetrahedral vs. Octahedral complexes have different splitting patterns.
• Superoxide and Peroxide Ions: O₂^- has an unpaired electron, while O₂^2- does not.

Key Formulae and Theory:

For a metal ion in a complex:

• Number of d electrons = Atomic number - oxidation state (for first row transition metals, subtract from 21 to 30? Actually, for Ni, atomic number 28, Ni²⁺ has 26 electrons, so d electrons = 8).
• In tetrahedral field, splitting is small, so often high spin.
• In octahedral field, with strong field ligands (like CO, NH₃), pairing occurs (low spin). With weak field ligands (like Cl^-, F^-), high spin.
• For dⁿ configurations:
  • d⁶ octahedral low spin: paired (diamagnetic).
  • d⁶ octahedral high spin: 4 unpaired (paramagnetic).
  • d⁸ tetrahedral: always two unpaired (paramagnetic).
  • d¹⁰: all paired (diamagnetic).
• For O₂^- superoxide: bond order = 1.5, unpaired electron.
• For O₂^2- peroxide: bond order = 1, all paired.