The low-temperature structures of M(pyz)V4O10 (M = Co, Zn) in the spin-gapped state have been investigated by single-crystal X-ray diffraction and electronic structure calculations and provide evidence for a new type of spin-antiferroelectric transition for M = Zn. This new type of phase transition involves the onset of charge ordering within spin ladders that can be mediated by the metal-organic chains via a modification of the inter-ladder interactions.