The molecular IrF5-, IrF6- anions and M[IrF6] (M = Na-Cs) ion pairs were prepared by co-deposition of laser-ablated MF with IrF6 and isolated in solid Ne or Ar matrices. The free anions were obtained by co-deposition of IrF6 with laser-ablated metals (Ir, Pt) as electron sources. The products were characterized in a combined analysis of matrix IR spectroscopy and electronic structure calculations using two-component quasi-relativistic DFT methods accounting for scalar-relativistic and spin-orbit coupling effects as well as multi-reference configuration-interaction approaches with SOC. Inclusion of SOC is crucial in the prediction of spectra and properties of IrF6- and its alkali-metal ion pairs. The observed IR bands and the computations show that the IrF6- anion adopts an Oh structure in a nondegenerate ground state stabilized by SOC effects, and not a distorted D4h structure in a triplet ground state as suggested by scalar-relativistic calculations. The corresponding "closed-shell" M[IrF6] ion pairs with C3v symmetry are stabilized by coordination of an alkali metal ion to three F atoms, and their structural change in the series from M = Na to Cs was proven spectroscopically. There is no evidence for the formation of IrF7, IrF7- or M[IrF7] (M = Na-Cs) ion pairs in our experiments.
Keywords: IR spectroscopy * matrix isolation * d4 transition-metal * spin-orbit coupling * quantum chemistry.
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