The $IUPAC$ name of the complex ion formed when gold dissolves in aqua regia is

The $IUPAC$ name of $K_2[OsCl_5N]$ is:

Write the $IUPAC$ names of the following coordination compounds:
$(a)$ $[Pt(NH_3)_2Cl(NO_2)]$
$(b)$ $K_3[Cr(C_2O_4)_3]$
$(c)$ $[CoCl_2(en)_2]Cl$
$(d)$ $[Co(NH_3)_5(CO_3)]Cl$
$(e)$ $Hg[Co(SCN)_4]$

Which formula and name combination is incorrect?

The sum of the coordination number and the oxidation number of $M$ in $[M(en)_2C_2O_4]Cl$ is

What is the correct $IUPAC$ name of the coordination compound $[Co(NH_3)_4(OH)_2Co(NH_3)_4]^{+4}$?