  The mixed-valent iridate Ba3InIr2O9 has been discussed as a promising
candidate for quantum spin-liquid behavior. The compound exhibits Ir$^{4.5+}$
ions in face-sharing IrO6 octahedra forming Ir2O9 dimers with three t2g holes
per dimer. Our results establish Ba3InIr2O9 as a cluster Mott insulator. Strong
intra-dimer hopping delocalizes the three t2g holes in quasi-molecular dimer
states while inter-dimer charge fluctuations are suppressed by Coulomb
repulsion. The magnetism of Ba3InIr2O9 emerges from spin-orbit entangled
quasi-molecular moments with yet unexplored interactions, opening up a new
route to unconventional magnetic properties of 5d compounds. Using
single-crystal x-ray diffraction we find the monoclinic space group C2/c
already at room temperature. Dielectric spectroscopy shows insulating behavior.
Resonant inelastic x-ray scattering (RIXS) reveals a rich excitation spectrum
below 1.5 eV with a sinusoidal dynamical structure factor that unambiguously
demonstrates the quasi-molecular character of the electronic states. Below 0.3
eV, we observe a series of excitations. According to exact diagonalization
calculations, such low-energy excitations reflect the proximity of Ba3InIr2O9
to a hopping-induced phase transition based on the condensation of a
quasi-molecular spin-orbit exciton. The dimer ground state roughly hosts two
holes in a bonding j=1/2 orbital and the third hole in a bonding j=3/2 orbital.
