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Extended solid-state materials based on the hexagonal perovskite framework are typified by close competition between localized magnetic interactions and quasi-molecular electronic states. Here, we report the structural and magnetic properties of the new six-layer hexagonal perovskite Ba 3 CaMo 2 O 9 . Neutron diffraction experiments, combined with magnetic susceptibility measurements, show that the Mo 2 O 9 dimers retain localized character down to 5 K and adopt nonmagnetic spin-singlet ground states. This is in contrast to the recently reported Ba 3 SrMo 2 O 9 analogue, in which the Mo 2 O 9 dimers spontaneously separate into a mixture of localized and quasi-molecular ground states. Structural distortions in both Ba 3 CaMo 2 O 9 and Ba 3 SrMo 2 O 9 have been studied with the aid of distortion mode analyses to elucidate the coupling between the crystal lattice and electronic interactions in 6H Mo 5+ hexagonal perovskites.