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Among the lanthanide ions La3+, Ce3+, Eu3+, and Lu3+, which has the greatest tendency to form coordination complexes?

1. La3+
2. Ce3+
3. Eu3+
4. Lu3+

Correct answer: Lu3+

Solution

All four ions carry a 3+ charge. Due to lanthanide contraction, the ionic radius decreases steadily from La3+ (largest) to Lu3+ (smallest). A smaller ionic radius gives a higher charge-to-radius ratio, making the ion a stronger Lewis acid and thus increasing its tendency to accept electron pairs from ligands. Therefore Lu3+ has the maximum complex-forming tendency.

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