(a) CoCl₃.4NH₃, (b) CoCl₃.5NH₃, (c) CoCl₃.6NH₃ and (d) CoCl(NO₃)₂.5NH₃.
Number of complex(es) which will exist in cis-trans form is/are _______________.

Solution

Chemistry ma’am · 3 steps

Turn each formula into an octahedral complex, then look for two like ligands that can be cis or trans.

1. Build each complex. Cobalt(III) has coordination number 6. NH₃ fills the sites first, and anions take the remaining sites. Whatever is left over stays outside the bracket as a counter-ion.
2. When cis-trans exists. An octahedral complex shows cis-trans isomerism only if two identical ligands can sit either side by side (cis, 90°) or opposite each other (trans, 180°). Type MA₄B₂ allows this. Types MA₆ and MA₅B have only one possible arrangement.

   Compound	Complex	Type	Cis-trans?
   (a) CoCl₃.4NH₃	[Co(NH₃)₄Cl₂]Cl	MA₄B₂	Yes: the two Cl can be cis or trans
   (b) CoCl₃.5NH₃	[Co(NH₃)₅Cl]Cl₂	MA₅B	No
   (c) CoCl₃.6NH₃	[Co(NH₃)₆]Cl₃	MA₆	No
   (d) CoCl(NO₃)₂.5NH₃	[Co(NH₃)₅Cl](NO₃)₂ or [Co(NH₃)₅NO₃]Cl(NO₃)	MA₅B	No (shows ionisation isomerism instead)

3. So. Only (a), [Co(NH₃)₄Cl₂]Cl, exists in cis and trans forms. The answer is 1.

The common slip: 2. Counting (d) as well gives 2. Its two possible forms swap Cl⁻ and NO₃⁻ between inside and outside the bracket, which is ionisation isomerism, not cis-trans.

The idea to keep. In an octahedral complex, cis-trans needs a pair of like ligands that can be adjacent or opposite. MA₄B₂ has it; MA₅B and MA₆ never do.

Answer from the NTA final answer key. Solution by PrepWiser.