Answer: 18 (Ni: 10 d+4s electrons; 4 CO donate 2e each = 8; total 10+8=18).
- A 18 (Ni: 10 d+4s electrons; 4 CO donate 2e each = 8; total 10+8=18)
- B 16, the electron count typical of square planar d<sup>8</sup> complexes instead
- C 20, an electron count that would exceed the stable noble gas total
- D 12, an electron count well below the stable 18-electron configuration
Correct answer: A. 18 (Ni: 10 d+4s electrons; 4 CO donate 2e each = 8; total 10+8=18)
Explanation: Ni(0) has 10 valence electrons (3d<sup>10</sup> 4s0 after redistribution); 4 CO × 2e = 8e; total = 18. Ni(CO)<sub>4</sub> obeys the 18e rule.
Aqueous solutions of transition metal ions show characteristic colours caused by electrons absorbing visible light to jump between split d-orbitals; the exact colour depends on the metal, its oxidation state, and surrounding ligands.
Concept context
Transition metals (d-block) and lanthanides/actinides (f-block). Known for coloured compounds, variable oxidation states, complex formation, and catalytic properties. Frequently tested in JEE and NEET.