  We report the crystal structure and magnetic behavior of the $4d^3$
spin-$\frac32$ silicophosphate MoP$_3$SiO$_{11}$ studied by high-resolution
synchrotron x-ray diffraction, neutron diffraction, thermodynamic measurements,
and ab initio band-structure calculations. Our data revise the crystallographic
symmetry of this compound and establish its rhombohedral space group ($R\bar
3c$) along with the geometrically perfect honeycomb lattice of the Mo$^{3+}$
ions residing in disconnected MoO$_6$ octahedra. Long-range antiferromagnetic
order with the propagation vector $\mathbf k=0$ observed below $T_N=6.8$ K is a
combined effect of the nearest-neighbor in-plane exchange coupling $J\simeq
2.6$ K, easy-plane single-ion anisotropy $D\simeq 2.2 $ K, and a weak
interlayer coupling $J_c\simeq 0.8$ K. The 12% reduction in the ordered
magnetic moment of the Mo$^{3+}$ ions and the magnon gap of $\Delta\simeq 7$ K
induced by the single-ion anisotropy further illustrate the impact of
spin-orbit coupling on the magnetism. Our analysis puts forward single-ion
anisotropy as an important ingredient of $4d^3$ honeycomb antiferromagnets
despite their nominally quenched orbital moment.
