Which complex has a metal ion with zero unpaired electrons in an octahedral field?
[Co(CN)6]3− (Co₃+, d6) with strong field CN− is low spin (t2g6), with 0 unpaired electrons.
Ref: NCERT Class 12 Chemistry > Chapter 5: Coordination Compounds > Topic: Isomerism - Optical Isomerism and Linkage Ionisation Solvate