Consider the following test for a group-IV cation. M2+ + H2S A (Black precipitate) + byproduct A + aqua regia B + NOCl + S + H2O B + KNO2 + CH3COOH C + byproduct The spin-only magnetic moment value of the metal complex C is BM (Nearest integer)
Answer
0
Solution
Group-IV black sulphide dissolved in aqua regia and then tested with KNO2 / acetic acid is the test for Co2+:
Co2+ + H2S CoS (A, black) + 2H+
CoS + aqua regia CoCl2 (B) + NOCl + S + H2O
CoCl2 + KNO2 + CH3COOH K3[Co(NO2)6] (C, yellow ppt) + NO +
In C, cobalt is Co3+ (3d6); NO2- is a strong-field ligand, so the electrons pair up as t2g6 eg0 (low spin, d2sp3).
Hence unpaired electrons n = 0, so μ = 0 BM.
Answer: 0