Multi-reference character and Ce 4f orbital contributions in terminal multiple CeZ bonds of Cp2CeZ (Z = CH2, CH−, NH, O, F+) complexes

- Terminal multiple cerium-main group element bonding is analyzed.
- Weak covalent Ce f orbital contributions are detected.
- The oxidation state of cerium in organometallic systems is investigated.

The geometric and electronic structures of bis(cyclopentadienyl)cerium compounds of the type Cp2CeZ (Z = CH2, CH−, NH, O, F+) are analyzed paying special attention to the terminal metal-ligand multiple bonding in the CeZ units. Complete active space self-consistent field calculations were performed, followed by unitary transformations in the active orbital space in order to monitor the weights of the leading configurations, as well as the Ce and Z character of the orbitals. It is shown that all compounds, except the closed-shell Ce(IV) complex Cp2CeF+, have an open-shell singlet ground state and the 4f orbitals are important for their electronic structure. In case of Z = CH2, CH− the singlet ground state consists to more than 90% of the Cefπ1Zpπ1 configuration when using nearly pure cerium 4f and carbon 2p orbitals of the Z group, implying Ce(III) systems. In contrast, the systems containing Z = NH, O revealed a more mixed Ce(III)/Ce(IV) ground state wavefunction. The interactions of the active orbitals were characterized by calculating the expectation values of the charge fluctuation operator and the local spin operator. The results are evaluated by comparison with the values achieved by stretching the covalent bond of a H2 molecule. The CH2 and the CH− complexes show, besides a CeZ σ bond, a quite weak covalent Ce fZ p orbital interaction in π symmetry, while the other two complexes (Z = NH, O) exhibit a stronger covalent interaction with a noticeable ionic character because of the higher electronegativities of nitrogen and oxygen.

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