The CFSE for octahedral [CoCl₆ ] ^4– is 18,000 cm^–1. The CFSE for tetrahedral [CoCl₄ ] ^2– will be 

[CoCl_6​]⁴⁻ and [CoCl_4​]²⁻ have octahedral and tetrahedral geometries respectively.

For [(CoCl₆​]^4−, the CFSE Δ_o​=18000cm⁻¹.

The relationship between the crystal field stabilization energies for octahedral and tetrahedral field is ${∆}_{\mathrm{t}} =\frac{4}{9} {∆}_0$

Substitute value in the above expression to determine the value of CFSE in [CoCl_4​]²⁻

$$\begin{gathered} {∆}_{\mathrm{t}} =\frac{4}{9}\times 18000 {\mathrm{cm}}^{1-} \\ {∆}_{\mathrm{t} }=8000 {\mathrm{cm}}^{1-} \end{gathered}$$