Question

A metal complex having composition Cr(NH₃)₄Cl₂Br has been isolated in two forms A and B. The form A reacts with AgNO₃ to give a white precipitate readily soluble in dilute aqueous ammonia, whereas B gives a pale yellow precipitate soluble in concentrated ammonia. Write the formula of A and B and state the hybridisation of chromium in each. Calculate their magnetic moments (spin-only value). 

Answer 1

A metal complex having composition Cr(NH₃)₄Cl₂Br have two forms A and B. 

Form 'A' gives white ppt with AgNO₃, hence it must have chloride ion in form of non-complex ion ie. outside the complex sphere as

The precipitate of AgCl is soluble in NH₄OH due to formation of complex salt.

 Similarly, form B gives pale yellow precipitate of AgBr which is sparingly soluble in NH₄OH. Hence, form 'B' is [Cr(NH₃)₄Cl₂]Br.

In both complexes, chromium is present as central ion and its oxidation number is + 3. So in these