Fe(CH₄)_n⁺ and Ni(CH₄)_n⁺ clusters: Experimental and theoretical bond energies for n=1-6

Measurements are reported for sequential clustering of CH₄ to Fe⁺ and Ni⁺ ions under equilibrium conditions. Detailed density functional theory calculations were performed to provide structural and electronic configuration information and to help analyse and interpret the experimental data. The calculations indicate that the first two CH₄ ligands add on opposite sides of the Fe⁺ core ion in an η³ configuration, in an η² configuration for Ni⁺, and induce significant s/d hybridization on both of the metal centers. This hybridization both reduces Pauli repulsion and fosters sigma donation from the ligands into the 4s orbital on M⁺. Another major covalent interaction is the donation from CH₄ into the singly occupied d orbital(s) on M⁺ for both η² and η³ configurations. For Fe⁺, the change of spin state, from ⁶D (3d⁶4s¹) to ⁴F (3d⁷), takes place during the clustering of the first methane ligand. The clustering of the third CH₄ to Fe⁺ and Ni⁺, unlike Co⁺(CH₄)₃, is not impeded by the s/d hybridization present for n=1 and 2. The interactions of all three CH₄ ligands with the Fe⁺ and Ni⁺ core are essentially the same. The m/z 120 peak [nominally Fe⁺(CH₄)₄] and the m/z 122 peak [nominally Ni⁺(CH₄)₄] were formed irreversible in the temperature range from 270 to 170 K, probably due to the persistent impurity we reported earlier for the Co⁺ system. The n=5 and 6 ligands are very weakly bound and begin a second solvation shell. Calculations suggest the n=6 cluster forms a pseudo octahedral complex.