The formation constants (log K 1) of lanthanide­(III) (Ln) ions except for Pm­(III) and the Y­(III) cation have been measured with the ligand TPEN (N,N,N′,N′-tetra-2-picolylethylenediamine). These log K 1 values show a typical variation with ionic radius, with a local maximum at Sm­(III) and a local minimum at Gd­(III), with an overall increase in log K 1 from La­(III) to Lu­(III) as the ionic radius decreases. The log K 1 for the Y­(III)/TPEN complex is much lower than expected from its ionic radius, while the literature log K 1 for Am­(III) is much higher. The latter effect is thought to be due to greater covalence in the M–L (metal–ligand) bond than for Ln­(III) ions: the low log K 1 for Y­(III) is interpreted as being due to lower covalence. The f → f transitions in the Nd­(III) and Pr­(III) complexes were examined for effects that might indicate the participation of f orbitals in M–L bonding. The intensity of the f → f transitions in the Nd­(III)/TPEN complex was greatly increased compared to that of the Nd3+ aqua ion, which appeared to be due to additional sharp peaks, possibly parity forbidden transitions where parity rules were broken by covalence in the M–L bond. The Pr­(III)/TPEN complex showed that all of the f → f transitions shifted to longer wavelengths by some 5 nm, with modest increases in intensity. The effects seen in the f → f transitions of Nd­(III) and Pr­(III) with TPEN with its six nitrogen donors were present to a much smaller extent in the EDTA and other complexes with fewer nitrogen donors. The changes in the f → f transitions of the TPEN complexes of Er­(III) and Ho­(III) were small, suggesting a smaller contribution of f orbitals to M–L bonding in these heavier Ln­(III) ions. The intense Laporte allowed f → d transitions in Ce­(III) complexes show large shifts to longer wavelengths as complexes of, for example, EDTA with increasing numbers of nitrogen donors, suggesting the participation of both f and d orbitals, or either, in M–L bonding. The nature of M–L bonding in M­(III)/TPEN complexes was further investigated via density functional theory calculations.