[Rutherford]

From diammonium hexachloroplatinate(IV) we can prepare Pt(NH₃)₂Cl₂ which occurs in cis (ΔH_f=–467.4 kJ/mol; ΔG_f=–228.7kJ/mol) and trans (ΔH_f=–480.3 kJ/mol; ΔG_f=–222.8 kJ/mol) form. Is the cis form or trans form thermodynamically more stable?
I thought trans because more energy is released when it is made, but it is cis. Why is it cis?

[Schrödinger]

You need to look at the Gibbs energy of formation. The one with the lower Gibbs energy is more stable.

In this case, just consider a hypothetical reaction in which cis and trans are getting interconverted. What is the change in Gibbs free energy for both of these reactions (cis to trans AND trans to cis)? One will be negative and one will be positive. That should give you an idea as to which compound is thermodynamically more stable.

[Rutherford]

If I understood, when talking about stability, ΔG has a bigger priority than ΔH. Thanks for the help.

[Schrödinger]

Gibbs free energy. The name kinda says it. The more the 'free energy', the more work it can do in order to reduce it further. Hence the instability.