Transition metals form coordination compounds far more readily than s-block or p-block elements. This tendency — complex formation — is the reason you see deeply coloured solutions, catalytic cycles, and biologically critical metalloenzymes throughout d-block chemistry.
Why d-block? Three properties converge: (1) small ionic radii with high charge density, (2) vacant or partially filled d-orbitals available to accept lone pairs from ligands, and (3) variable oxidation states that let the same metal accommodate different ligand sets. NCERT Class 12 Chemistry Chapter 4 (page 18) explicitly notes that the tendency to form complexes is a general characteristic of transition elements, arising from their small size, high ionic charge, and availability of d-orbitals for bond formation.
Charge density argument. A high charge-to-radius ratio polarises incoming ligands strongly, stabilising the metal–ligand bond. Compare: Na⁺ (large, +1) forms few stable complexes; Fe³⁺ (small, +3) forms hundreds.
d-orbital availability. Ligand lone pairs donate into empty or half-filled d-orbitals (σ-donation) and in some cases accept electron density back (π-back-bonding in carbonyls). Main-group ions lack accessible, energetically suitable empty orbitals of comparable energy, so their complex formation is limited.
Variable oxidation states matter. Because different oxidation states change the number of available d-orbitals and the effective nuclear charge, the same metal can stabilise both hard (F⁻, OH⁻) and soft (CN⁻, CO) ligands depending on its oxidation state.
Watch-out for NEET: Questions often ask why a specific d-block element forms more complexes than its s/p-block neighbour, or why higher oxidation states favour complex formation. The answer always routes back to charge density + orbital availability — not merely "has d-electrons."
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is NOT a reason for the high tendency of transition metals to form coordination compounds?
Among the following ions, which would have the greatest tendency to form stable coordination complexes?
The complex formation tendency of transition metals is attributed to their high charge density. Which factor directly increases the charge density of a cation?
Why do main-group elements like Na and Mg form far fewer complexes than transition elements like Fe and Co?
Consider two ions: Cr³⁺ (ionic radius 62 pm) and Cr²⁺ (ionic radius 73 pm). Which forms more stable complexes with a given set of ligands, and why?
Which statement correctly explains why Fe³⁺ forms a greater variety of complexes than Fe²⁺?
Zn²⁺ has a completely filled d-orbital (d¹⁰ configuration). Despite this, it forms complexes like [Zn(NH₃)₄]²⁺. Which factor primarily enables Zn²⁺ to form complexes?
Among Ti²⁺ (d²), V²⁺ (d³), Fe²⁺ (d⁶), and Cu²⁺ (d⁹), which ion has the maximum number of vacant d-orbitals available for accepting ligand electrons?
Given
Consider Co³⁺ and Co²⁺. Co³⁺ ionic radius = 55 pm, Co²⁺ ionic radius = 65 pm. Both have partially filled d-orbitals.
Required
Determine which ion has the greater tendency to form stable complexes, and by what factor the charge densities differ (approximating ions as spheres).
Concept
Complex formation tendency correlates with charge density (charge/volume). Higher charge density → stronger polarisation of ligand electron clouds → more stable metal–ligand bonds.
Formula
Charge density ∝ Z/r³ (for spherical ions, where Z = ionic charge, r = ionic radius). Ratio = (Z₁/r₁³) ÷ (Z₂/r₂³)
Substitution
For Co³⁺: Z = 3, r = 55 pm For Co²⁺: Z = 2, r = 65 pm Ratio (Co³⁺ / Co²⁺) = (3/55³) ÷ (2/65³)
Calculation
55³ = 166,375 65³ = 274,625 Co³⁺ charge density ∝ 3/166,375 = 1.803 × 10⁻⁵ Co²⁺ charge density ∝ 2/274,625 = 7.284 × 10⁻⁶ Ratio = 1.803 × 10⁻⁵ / 7.284 × 10⁻⁶ = 2.47 Note: The ionic radii (55, 65 pm) and charges (3, 2) are exact given values in this problem — they do not limit significant figures. The ratio is reported to 3 significant figures.
Final answer
Co³⁺ has approximately 2.5 times the charge density of Co²⁺, and therefore a significantly greater tendency to form stable coordination complexes.
Common trap
Students sometimes compare only the charges (3 vs 2 → 1.5× difference) and forget that the radius difference contributes cubically. The actual advantage is ~2.5×, not 1.5×.
Similar NEET-style question
"Arrange Fe²⁺, Fe³⁺, and Na⁺ in decreasing order of tendency to form complexes. Justify using charge density and orbital availability."
Small size + high charge density + empty d-orbitals → strong coordination tendency. Form coordinate bonds with ligands via lone pairs. Ligand types: monodentate, bidentate, polydentate.
-- NCERT Class 12 Chemistry, Ch. 4, p. 18Magnetic moment from n unpaired electrons. 1 unpaired: 1.73 BM; 5: 5.92 BM.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
| mu | magnetic moment | Bohr magneton |
Predicts paramagnetic moment of d-block ion. n unpaired electrons in d-orbitals.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
| mu | magnetic moment | BM |
Catalogues common stable oxidation states across first-row transition metals.
| Symbol | Quantity | SI Unit |
|---|---|---|
| OS | oxidation state | - |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student assumes Mn²⁺ is the product regardless of medium. Acidic: → Mn²⁺ (5e⁻). Neutral/weakly basic: → MnO₂ (3e⁻). Strongly basic: → MnO₄²⁻ (1e⁻).
Question gives KMnO4 oxidation in unspecified or specific medium.
Always check medium. In acidic: Mn(+7) → Mn(+2). In neutral: → Mn(+4) (MnO₂). In basic: → Mn(+6) (manganate). The number of electrons (n) in Nernst calculations depends accordingly.
Root cause: concept gap
Acidic: → Mn²⁺ (5e⁻). Neutral/weakly basic: → MnO₂ (3e⁻). Strongly basic: → MnO₄²⁻ (1e⁻).
Root cause: concept gap
Lanthanoid contraction (imperfect 4f shielding) makes 5d elements similar in size to 4d. Zr/Hf, Nb/Ta, Mo/W have nearly identical chemistry.
10 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
The correct order of decreasing basic strength of the given amines is:
The pair of lanthanoid ions which are diamagnetic is
Which one of the following statements is correct?
Gadolinium has a low value of third ionisation enthalpy because of
Tritium, a radioactive isotope of hydrogen, emits which of the following particles? (β–)
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
uses acidic formula in basic medium
Assumes Mn²⁺ regardless of medium
predicts large difference due to period shift
Expects 5d to be much larger than 4d
misses d0 d10 stability
Doesn't account for closed-shell stability
Test yourself on this topic with real past-paper questions:
Practice this topic →Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.