Coordination compounds Which of the following complex is diamagnetic?    
  
  
  $\ce{[Cr(Cl3)(NMe3)]}$
  $\ce{[Co(EDTA)]-}$
  $\ce{[Cu(CN)(NO2)(NH3)(Py)]}$
  $\ce{[Fe(NH3)6]^2+}$
  


For (4) I found that since $\ce{NH3}$ is a weak field ligand it cannot pair the electrons of $\ce{Fe^2+}$. Thus, (4) will remain paramagnetic.

But, for the rest of the options, I'm unable to clearly understand if they are paramagnetic or diamagnetic. I need help here.
1) Cr3+ has 3 electrons in 3d orbital. Obviously, it is a tetrahedral splitting, so configuration will be eg2 t2g1 which shows it is paramagnetic.

2) Here you need to know that for Co3+ all complexes are low spin except [CoF6]3- and [Co(H2O)3F3]. So, the complex given will be low spin with all 6 electrons paired &amp; thus it will be diamagnetic.

3) Cu2+ has 3d9 configuration. So, it can never be diamagnetic as one electron will always be unpaired.

4) What you thought is right. But I would like to add that NH3 doesn't always behave like a weak field or strong field ligand. It depends on many factors which can't be explained using CFT.

Hope it helps!