Transition Metals: Why They Behave as a Group

The single structural cause

Transition metals have partly filled d orbitals whose energies lie close to those of the outermost s orbital. Because the two sets are close, electrons can be removed from either, and the d electrons remain available to participate in bonding and in electronic transitions.

Variable oxidation states

Because the energy gap between the d and s orbitals is small, successive ionisation energies increase gradually rather than sharply. Several oxidation states are therefore accessible for the same element.

Two patterns are examined:

• The highest oxidation state increases to the middle of the series, where the maximum number of d electrons is available, then decreases as the d orbitals contract and hold their electrons more tightly.
• Lower oxidation states become more stable toward the right of the series, for the same reason.

Predicting which oxidation state is most stable for a given element is a standard question, and it follows from where the element sits in the series.

Colour

A partly filled d set allows an electron to be promoted between the split d levels, absorbing visible light. The complex appears the complementary colour.

Two consequences follow directly and are asked constantly: a d⁰ or d¹⁰ species is colourless because no such transition is available, and the colour of a given metal ion changes with the ligand because the ligand changes the splitting.

Intensely coloured compounds of metals in high oxidation states are usually coloured by charge transfer rather than d–d transitions, which is why they are far more intense than typical d–d colours.

Magnetism

Unpaired d electrons make most transition metal compounds paramagnetic, and the spin-only formula relates the moment to the number of unpaired electrons. Measuring the moment therefore determines the electron configuration and hence the spin state.

A diamagnetic transition metal complex is informative precisely because it is unusual: it indicates either a d⁰ or d¹⁰ configuration, or a low-spin arrangement with all electrons paired.

Catalytic activity

Transition metals catalyse reactions by two routes:

• Changing oxidation state readily, allowing them to accept and donate electrons within a catalytic cycle.
• Forming and breaking bonds to reactants, providing a surface or a coordination site where the reactants are held and activated.

Both depend on the accessibility of several oxidation states and on the availability of d orbitals for bonding — the same structural feature again.

Other characteristic properties

The high melting point argument is worth stating carefully: it is the participation of d electrons in metallic bonding, in addition to the s electrons, that gives stronger bonding than in a comparable main-group metal.

Frequently asked questions

Why do transition metals show variable oxidation states?

Because the d and s orbital energies are close, so successive ionisation energies rise gradually and several states are accessible.

Why are some transition metal compounds colourless?

Because they have a d⁰ or d¹⁰ configuration, leaving no possible d–d transition.

Why are transition metals good catalysts?

Because they change oxidation state readily and can bind reactants using available d orbitals, both of which are needed to complete a catalytic cycle.

Why do they have higher melting points than main-group metals?

Because d electrons contribute to metallic bonding alongside the s electrons, giving stronger bonding overall.