How can f-block mono-cations behave as mono-cations of d-block transition metals?

The electronic structures of LnNH+ species are studied by DFT (B3LYP) quantum calculations for the Ln = La, Eu and Gd 4f-block elements (lanthanides). Ln=N triple bonds of essentially d character are formed for La and Gd, which explains why La+ and Gd+ behave like ions of d-block transition metals, as experimentally evidenced by mass spectrometry, and why the Ln+ reactivity is correlated with its electron-promotion energy: the present theoretical study is a support to such a correlation and qualitative knowledge. The Ln+ + NH3 → LnNH3+ → transition state → HLn=NH2+ → transition state → Ln(triple bond)NH+ + H2 reaction pathway is calculated. The formation of HLn=NH2+ corresponds to the formation of new covalent bonds associated with more electron pairing and corresponding lowering of the spin multiplicity-spin crossing reaction. It is in this step that low electron-promotion energy is required to promote a 4f electron into a 5d orbital as is typical for La+ and Gd+. A similar geometry, bonding and electronic configuration are calculated for NpNH+ an actinide complex observed by mass spectrometry with higher participation of 5f orbitals (20% and 25% for the s and p bonds, respectively) as compared to the 4f orbitals (3% and 8%) of GdNH+: Gd+ and Np+ are the only lanthanide and actinide mono-cations with one s- and one d-valence electrons in their ground states. (authors)