<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">Ba</mml:mi> <mml:mn>4</mml:mn> </mml:msub> <mml:msub> <mml:mi>SbRu</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="no

${\mathrm{Ba}}_{4}{\mathrm{SbRu}}_{3}{\mathrm{O}}_{12}$ crystallizes with a structure that consists of clusters of three face-sharing Ru-centered octahedra $({\mathrm{Ru}}_{3}{\mathrm{O}}_{12})$ connected via Sb-centered octahedra. Rietveld refinements of both x-ray and neutron powder diffraction data confirm the absence of Sb/Ru antisite mixing. Variable temperature neutron diffraction measurements reveal a phase transition on cooling below 100 K from the aristotype structure with $R\overline{3}m$ symmetry to a monoclinic structure with $\mathrm{C}2/m$ symmetry. The phase transition is driven by displacements of $\mathrm{B}{\mathrm{a}}^{2+}$ cations and, as such, only subtly perturbs the geometry of the ${\mathrm{Ru}}_{3}{\mathrm{O}}_{12}$ clusters. The Ru $4d$ orbitals overlap to form delocalized molecular orbitals that span the trioctahedral cluster. At high temperature (T \ensuremath{\ge} 200 K), the cluster adopts an $S=3/2$ intermediate spin state, but upon cooling to low temperature, susceptibility data suggest a gradual transition to an $S=1/2$ low spin state. The presence of $\mathrm{S}{\mathrm{b}}^{5+}$ ions with a [Kr] $4{d}^{10}$ electron configuration minimizes interlayer superexchange interactions between clusters, thereby maintaining the frustration of the 2D triangular network. ${\mathrm{Ba}}_{4}{\mathrm{SbRu}}_{3}{\mathrm{O}}_{12}$ shows no signs of magnetic ordering down to 0.3 K, yet Curie-Weiss fitting of high-temperature susceptibility suggests strong antiferromagnetic interactions $({\ensuremath{\theta}}_{\mathrm{CW}}=\ensuremath{-}378 \mathrm{K})$. The low-temperature specific heat evolves linearly with temperature, suggestive of a gapless quantum spin liquid. The combination of small magnetic quantum number, minimal chemical disorder, geometric frustration, and lack of long-range magnetic order makes ${\mathrm{Ba}}_{4}{\mathrm{SbRu}}_{3}{\mathrm{O}}_{12}$ an intriguing quantum spin liquid candidate that merits further study.