Question

Which of the following complex is octahedral, diamagnetic and the most stable?

• A. K₃[CO(CN)₆]
• B. [Ni(NH₃)₆]Cl₂
• C. The correct option is(A): Na₃[COCl₆]
• D. [Co(H₂O)₆]Cl₂

Answer 1

The Correct Option is A

To analyze the properties of K$_3$[Co(CN)$_6$], let's break it down:

Oxidation state of cobalt: The overall charge on the complex is 0. Let $x$ represent the oxidation state of cobalt: \[ 3 + x - 6 = 0 \implies x = +3 \] So, Co is in the +3 oxidation state.

Electronic configuration of \(Co^{3+}\): Cobalt's ground state is [Ar] \(3d^7 4s^2\). 
After losing 3 electrons, the configuration becomes \(3d^6\).

Ligand strength: CN$^-$ is a strong field ligand (SFL) as per the spectrochemical series. Strong field ligands cause significant splitting of the d-orbitals, leading to pairing of electrons in the lower energy orbitals.

Electron pairing: In the presence of CN$^-$, the 3d electrons pair as follows: \[ \text{Before pairing: } \uparrow \uparrow \uparrow \uparrow \uparrow \uparrow \] \[ \text{After pairing: } \uparrow \downarrow \uparrow \downarrow \uparrow \downarrow \] As all electrons are paired, the complex is diamagnetic.

Geometry: The coordination number is 6, and with CN$^-$ being a strong ligand, the geometry is octahedral.

Stability: CN$^-$ forms strong bonds with the metal center due to its strong field nature, making K$_3$[Co(CN)$_6$] the most stable complex among the options.

Thus, K$_3$[Co(CN)$_6$] is octahedral, diamagnetic, and the most stable.