ACS: Inorganic Chemistry

Inorganic Chemistry items probe periodicity, bonding models (VSEPR, molecular orbital, crystal field), symmetry, coordination chemistry, and descriptive trends across the d and p blocks.

Periodic trends

• Effective nuclear charge rises across a period, shrinking radii and raising ionization energy; radii grow down a group as n increases.
• Ionization-energy anomalies occur at Be > B and N > O, reflecting filled 2s and half-filled 2p subshells.
• Lattice energy scales with the product of ionic charges divided by internuclear distance; small, highly charged ions give high-melting solids.
• Electron affinity is larger for Cl than for F because the small F atom suffers greater electron–electron repulsion.

Structure and bonding

• VSEPR treats lone pairs as more repulsive than bonding pairs: SF4 is see-saw, ClF3 is T-shaped, XeF4 is square planar.
• MO bond order = (bonding − antibonding)/2; O2 has two unpaired π* electrons and is paramagnetic, while N2 has bond order 3.
• Assign point groups by locating the highest-order rotation axis, then checking for perpendicular C2 axes or a horizontal mirror plane.

Coordination chemistry

• Spectrochemical series: I− < Br− < Cl− < F− < H2O < NH3 < en < CN− ≈ CO.
• Octahedral splitting places t2g at −0.4Δo and eg at +0.6Δo; tetrahedral Δt ≈ (4/9)Δo, and 3d tetrahedral complexes are essentially always high spin.
• High spin versus low spin depends on Δo compared with the pairing energy P; 4d and 5d metals almost always pair.
• The chelate effect and the 18-electron rule guide stability predictions.

Worked example

High-spin Fe2+ is d6: t2g^4 eg^2, giving four unpaired electrons and a spin-only moment μ = √(n(n+2)) = √24 ≈ 4.90 μB. Its LFSE is 4(−0.4Δo) + 2(+0.6Δo) = −0.4Δo. The low-spin form t2g^6 is diamagnetic (μ = 0) with LFSE = −2.4Δo plus two pairing energies, and is favored only when Δo > P.

Traps

• Miscounting d electrons: Fe2+ is d6, not d8.
• Applying octahedral LFSE values to tetrahedral complexes.
• Assuming every d6 ion is low spin; ligand field strength decides.
• Treating multiple bonds in VSEPR as equivalent to lone pairs.
• Predicting a Jahn–Teller distortion for a complex with a nondegenerate ground state.