JEE Main · 2025 · 07 Apr 2025 · Shift 2 · Coordination Compounds

Primary valency is the oxidation state of the metal and secondary valency is its coordination number. In $[\mathrm{Co}(\mathrm{en}{)}_2\mathrm{C}{\mathrm{l}}_2]\mathrm{Cl}$ cobalt is $+3$ and is surrounded by two bidentate ethylenediamine molecules together with two chlorides, a coordination number of six, so its valencies are three and six. In $[\mathrm{Pt}(\mathrm{N}{\mathrm{H}}_3{)}_2\mathrm{Cl}(\mathrm{N}{\mathrm{O}}_2)]$ platinum is $+2$ with four donor atoms about it, so two and four. In $[\mathrm{Mg}(\mathrm{EDTA}){]}^{2-}$ magnesium is $+2$ and the hexadentate EDTA wraps it through six donor sites, so two and six. In $\mathrm{Hg}[\mathrm{Co}(\mathrm{SCN}{)}_4]$ cobalt is bound by four thiocyanate groups, a coordination number of four, and completes the remaining pairing of three and four. The complex with ethylenediamine therefore goes with three and six, the platinum complex with two and four, the mercury thiocyanato-cobaltate with three and four, and the magnesium-EDTA complex with two and six.