15.3 Transition Metals: Configurations, Colors & Common Reactivity

15.3 Transition Metals: Configurations, Colors & Common Reactivity

Quick Summary: Transition metals are characterized by partially filled d-orbitals, conferring variable oxidation states, catalytic utility, coordination complex formation, and paramagnetic behavior. When transition metals ionize, outer 4s electrons are lost before 3d electrons. Under Crystal Field Theory (CFT), an octahedral ligand field splits degenerate d-orbitals into lower-energy t2g and higher-energy eg sets. The magnitude of crystal field splitting (Δ_oct) matches visible light wavelengths; photon absorption promotes d-d electronic transitions, imparting the complementary color to the complex. High-spin versus low-spin electron configurations depend on ligand field strength in the spectrochemical series. Intensely colored d0 oxoanions (MnO4-, Cr2O7^2-) derive their color from ligand-to-metal charge-transfer (LMCT) transitions.

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1. Characteristic Properties of First-Row Transition Metals

First-row d-block transition metals (Sc to Zn, Z = 21 to 30) possess distinctive physical and chemical properties rooted in incomplete d-subshells:

1. Variable Oxidation States: Adjacent oxidation states differ by units of one (e.g., Manganese exhibits +2, +3, +4, +6, and +7). This behavior occurs because 3d and 4s electrons experience similar nuclear shielding and have comparable binding energies.
2. Coordination Complex Formation: Small, highly charged transition metal cations provide vacant hybrid orbitals that accept lone pairs from Lewis bases (ligands) such as H2O, NH3, Cl-, and CN-.
3. Catalytic Utility: Readily accessible oxidation states and surface d-orbital bonding facilitate low-activation-energy pathways. Familiar catalysts include Fe in the Haber ammonia synthesis, V2O5 in sulfuric acid manufacture, and Pt/Pd/Rh in automotive converters.
4. Paramagnetism: Unpaired d-electrons generate net magnetic dipole moments, drawing complexes into external magnetic fields.

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2. Electron Configurations and Ionization Rules

Neutral chromium and copper display ground-state configurations with half-filled or fully filled d-subshells:

• Chromium (Z = 24): [Ar] 4s1 3d5 (favored by exchange energy over 4s2 3d4)
• Copper (Z = 29): [Ar] 4s1 3d10 (favored over 4s2 3d9)

Ionization: Loss of 4s Electrons Before 3d

When first-row transition metals ionize, valence 4s electrons are removed before 3d electrons:

M^0([Ar] 4s2 3d^n) → M^2+([Ar] 3d^n) + 2 e- → M^3+([Ar] 3d^(n-1)) + 3 e-

Although the 4s subshell fills first in neutral atoms, filling 3d orbitals lowers their energy below 4s through nuclear penetration. The 4s electrons experience greater radial extent and are stripped first:

• Iron: Fe^0 ([Ar] 4s2 3d6) → Fe2+ ([Ar] 3d6) → Fe3+ ([Ar] 3d5)
• Titanium: Ti^0 ([Ar] 4s2 3d2) → Ti2+ ([Ar] 3d2) → Ti4+ ([Ar] 3d0)
• Copper: Cu^0 ([Ar] 4s1 3d10) → Cu+ ([Ar] 3d10) → Cu2+ ([Ar] 3d9)

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3. Crystal Field Theory (CFT) and the Origin of Colors

Crystal Field Theory models coordination complexes by treating coordinating ligands as negative point charges interacting electrostatically with the metal ion's d-orbitals.

Octahedral Splitting Pattern

In an isolated gaseous ion, all five d-orbitals are degenerate. In an octahedral complex, six ligands approach along the Cartesian axes (±x, ±y, ±z):

• eg set (dx2-y2, dz2): Lobes point directly at approaching ligands, experiencing severe electrostatic repulsion (+0.6 Δ_oct above barycenter).
• t2g set (dxy, dyz, dxz): Lobes point between axes at 45° angles, experiencing less repulsion (-0.4 Δ_oct below barycenter).

Free Ion           Spherical Field                Octahedral Field
                                                --- ---  eg (+0.6 Δ_oct)
                                               /
----- (5 d)  --->  ----- ----- ----- -----  ---
                                               \
                                                --- --- ---  t2g (-0.4 Δ_oct)
                                                |<--- Δ_oct --->|

The gap between sets is the crystal field splitting energy, Δ_oct (or 10 Dq).

d-d Transitions and Complementary Colors

The energy gap Δ_oct falls in the visible range (400-700 nm): Δ_oct = hν = hc / λ. Absorbing a photon promotes an electron from t2g to eg. The transmitted light perceived is the complementary color:

• Absorbing red (~650 nm, small Δ_oct) yields cyan / blue (e.g., hydrated [Cu(H2O)6]2+).
• Absorbing yellow (~580 nm) yields violet / dark blue (e.g., [Cr(H2O)6]3+).
• Ions with d0 (Sc3+, Ti4+) or d10 (Zn2+, Cu+, Ag+) configurations are colorless because d-d electronic promotions are impossible.

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4. Spectrochemical Series, Spin States & Magnetism

Ligands are ranked by their ability to split d-orbitals in the spectrochemical series:

I- < Br- < Cl- < F- < OH- < H2O < NH3 < en < NO2- < CN- < CO

• Weak-field ligands (halides, H2O): Small Δ_oct.
• Strong-field ligands (NH3, CN-, CO): Large Δ_oct.

High-Spin vs Low-Spin Configurations (d4 to d7)

Electron distribution balances Δ_oct against spin-pairing energy (P):

• Weak Field (Δ_oct < P): High-Spin Complex. Electrons occupy eg before pairing in t2g, maximizing unpaired electrons. For Fe2+ (3d6) in [Fe(H2O)6]2+: t2g^4 eg^2 → 4 unpaired electrons (paramagnetic).
• Strong Field (Δ_oct > P): Low-Spin Complex. Electrons pair in t2g before populating eg. For Fe2+ in [Fe(CN)6]4-: t2g^6 eg^0 → 0 unpaired electrons (diamagnetic).

Magnetic Moments: Gouy Balance

A Gouy balance measures magnetic attraction. Spin-only magnetic moment scales with unpaired electrons n:

μ_s = √(n(n + 2)) μ_B

(where μ_B is the Bohr magneton). An ion with n = 4 yields μ_s = √24 ≈ 4.90 μ_B; diamagnetic species (n = 0) yield μ_s = 0.

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5. Diagnostic Colors and Charge-Transfer Bands

Certain d0 species display vivid colors that cannot arise from d-d transitions:

• Permanganate (MnO4-): Mn(VII) is 3d0, yet deep purple.
• Chromate (CrO4^2-): Cr(VI) is 3d0, bright yellow.
• Dichromate (Cr2O7^2-): Cr(VI) is 3d0, vivid orange.

These colors arise from Ligand-to-Metal Charge Transfer (LMCT): absorption of a photon transfers an electron from an oxygen 2p orbital to an empty metal 3d orbital. Because LMCT transitions are Laporte-allowed (Δl = ±1), molar absorptivities are thousands of times higher than forbidden d-d transitions, producing brilliant colors.

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6. Reference Tables

Complementary Color Wheel Reference

Diagnostic Transition Metal Aqueous Colors