Abrupt Change from Ionic to Covalent Bonding in Nickel Halides Accompanied by Ligand Field Inversion.

The electronic configuration of transition metal centers and their ligands is crucial for redox reactions in metal catalysis and electrochemistry. We characterize the electronic structure of gas-phase nickel monohalide cations via nickel L2,3-edge X-ray absorption spectroscopy. Comparison with multiplet charge-transfer simulations and experimental spectra of selectively prepared nickel monocations in both ground- and excited-state configurations are used to facilitate our analysis. Only for [NiF]+ with an assigned ground state of 3Π can the bonding be described as predominantly ionic, while the heavier halides with assigned ground states of 3Π or 3Δ exhibit a predominantly covalent contribution. The increase in covalency is accompanied by a transition from a classical ligand field for [NiF]+ to an inverted ligand field for [NiCl]+, [NiBr]+, and [NiI]+, resulting in a leading 3d9 L̲ configuration with a ligand hole (L̲) and a 3d occupation indicative of nickel(I) compounds. Hence, the absence of a ligand hole in [NiF]+ precludes any ligand-based redox reactions. Additionally, we demonstrate that the shift in energy of the L3 resonance is reduced compared to that of isolated atoms upon the formation of covalent compounds.

Soft X-ray absorption and fragmentation of tin-oxo cage photoresists.

"Tin-oxo cage" organometallic compounds are considered as photoresists for extreme ultraviolet (EUV) photolithography. To gain insight into their electronic structure and reactivity to ionizing radiation, we trapped bare gas-phase n-butyltin-oxo cage dications [(BuSn)12O14(OH)6]2+ in an ion trap and investigated their fragmentation upon soft X-ray photoabsorption by means of mass spectrometry. In complementary experiments, the tin-oxo cages with hydroxide and trifluoroacetate counter-anions were cast in thin films and studied using X-ray transmission spectroscopy. Quantum-chemical calculations were used to interpret the observed spectra. At the carbon K-edge, a distinct pre-edge absorption band can be attributed to transitions in which electrons are promoted from C1s orbitals to the lowest unoccupied molecular orbitals, which are delocalized orbitals with strong antibonding (Sn-C σ*) character. At higher energies, the most prominent resonant transitions involve C-C and C-H σ* valence states and Rydberg (3s and 3p) states. In the solid state, the onset of continuum ionization is shifted by ∼5 eV to lower energy with respect to the gas phase, due to the electrostatic effect of the counterions. The O K-edge also shows a pre-edge absorption, but it is devoid of any specific features, because there are many transitions from the different O1s orbitals to a large number of vacant orbitals. In the gas phase, formation of the parent [(BuSn)12O14(OH)6]3+ radical ion is not observed at the C K-edge nor at the O K-edge, because the loss of a butyl group from this species is very efficient. We do observe a number of triply charged photofragment ions, some of which have lost up to 5 butyl groups. Structures of these species are proposed based on quantum-chemical calculations, and pathways of formation are discussed. Our results provide insight into the electronic structure of alkyltin-oxo cages, which is a prerequisite for understanding their response to EUV photons and their performance as EUV photoresists.