Write the formula and name of 

(a) a complex ion having Cr³⁺ as the central ion and two NH₃ molecules and four Cl^– ions as ligands 

(b) a complex ion of iron(III) having a coordination number of 6 and CN^– as ligands 

(c) a coordination compound comprising two types of complex ions: one a complex of Cr(III) with ethylenediamine (en), having a coordination number of 6; the other, a complex of Ni(II) with CN^–, having a coordination number of 4.