In the metal complex Na[Co(CO)2(CN)4], how many d electrons are present in the cobalt (Co) atom?

• A. 7
• B. 8
• C. 6
• D. 5

Answer: (C)

Verified step by step guidance

1. Identify the oxidation state of the cobalt (Co) atom in the complex Na[Co(CO)2(CN)4]. Start by noting that Na is +1, CO and CN are neutral and -1 ligands respectively, and the overall complex is neutral.
2. Assign charges to the ligands: CO is a neutral ligand (charge 0), and CN is a negatively charged ligand with charge -1. Since there are 4 CN ligands, their total charge contribution is -4.
3. Set up the equation for the overall charge of the complex ion: charge of Co + (2 × charge of CO) + (4 × charge of CN) = charge of complex ion. Since the complex ion is balanced by Na+ outside, the complex ion charge is -1.
4. Solve for the oxidation state of Co using the equation: Co charge + 2(0) + 4(-1) = -1, which simplifies to Co charge - 4 = -1, so Co charge = +3.
5. Determine the number of d electrons in Co by subtracting the oxidation state from the atomic number of Co (which is 27). The number of d electrons = 27 - 3 = 24, but since Co is in the 3d series, the d electrons correspond to the electrons in the 3d subshell, which is 9 - oxidation state = 9 - 3 = 6 d electrons.