On adding an excess of $NH_3$ solution to $CuSO_4$ solution,the dark blue colour is due to the formation of which complex?

Arrange the following complexes in the correct increasing order of their electrical conductivity in aqueous solution:
$(I) [Pt(NH_3)_6]Cl_4$
$(II) [Cr(NH_3)_6]Cl_3$
$(III) [Co(NH_3)_4Cl_2]Cl$
$(IV) K_2[PtCl_6]$

Given below are two statements:
Statement $I$ : $N(CH_3)_3$ and $P(CH_3)_3$ can act as ligands to form transition metal complexes.
Statement $II$: As $N$ and $P$ are from same group,the nature of bonding of $N(CH_3)_3$ and $P(CH_3)_3$ is always same with transition metals.
In the light of the above statements,choose the most appropriate answer from the options given below:

Write the correct order of the spectrochemical series for the given ligands.

Which of the following is a spin-free complex?

Which among the following cations forms a least stable complex if the ligand is same?