Q7(a)(i). Metal carbonyls; Mond's process; tetrahedral Ni(CO)₄

Metal carbonyls

1. Complexes in which CO is a ligand, bonded to the metal through carbon; binary carbonyls contain CO only: Ni(CO)₄, Fe(CO)₅, Cr(CO)₆ (mononuclear), Mn₂(CO)₁₀, Co₂(CO)₈, Fe₂(CO)₉ (polynuclear, with M–M bonds and sometimes bridging CO).
2. Synergic bonding: CO donates its lone pair (on C) into an empty metal orbital (σ), and the filled metal d orbitals give electrons back into the empty π* orbitals of CO (π back-donation). Each strengthens the other. Back-donation lowers the C–O bond order: ν(CO) falls from 2143 cm⁻¹ (uncombined CO) to 1850–2120 cm⁻¹ (terminal).
3. Metals are in low oxidation states (often 0), and most stable carbonyls obey the 18-electron rule: Ni 10+4×2=18; Fe 8+5×2=18; Cr 6+6×2=18.

Mond's process

1. Impure nickel (made by reducing nickel oxide ore with H₂ and CO) is treated with CO at about 50°C and 1 atm: Ni(s) + 4CO(g) → Ni(CO)₄(g)
2. Ni(CO)₄ (b.p. 43°C) is a gas at this temperature and is carried away from the impurities, which do not react under these mild conditions (Fe and Co form carbonyls only slowly, at high pressure).
3. The gas is passed over pellets heated to about 220°C; the reaction reverses: Ni(CO)₄ → Ni + 4CO. Very pure nickel deposits, and CO is recycled. The process exploits the reversibility of a single equilibrium at two temperatures.

Why Ni(CO)₄ is tetrahedral

1. Valence bond: Ni(0) is 3d⁸4s². CO is a strong-field ligand; the 4s electrons are pushed into 3d, giving 3d¹⁰. The empty 4s and three 4p orbitals form sp³ hybrids, which point to the corners of a tetrahedron, and accept four CO lone pairs.
2. Crystal field: a d¹⁰ ion has no CFSE in any geometry, so the shape is fixed by ligand–ligand repulsion alone: four ligands are farthest apart at 109.5°, a tetrahedron.
3. Magnetism and dipole: all electrons are paired (diamagnetic), and the molecule has zero dipole moment; a square-planar or distorted shape with d¹⁰ Ni gives no gain.
4. Vibrational spectrum: the four C–O stretches of a T_d molecule transform as A₁+T₂; only T₂ is IR active, so the IR spectrum shows one C–O band (Raman: two, A₁ and T₂). A square-planar D₄ₕ structure would give a different pattern (A_(1g)+B_(1g)+Eᵤ).
5. Electron diffraction confirms four equal Ni–C bonds and linear Ni–C–O at tetrahedral angles.

Application. Mond's process brought nickel ore from Canada to Wales for purification; the same reversibility makes Ni(CO)₄ extremely toxic (it releases CO and Ni inside the body), so it is handled only in closed systems.