Niu (2013) Designing a new organic-inorganic arsenomolybdate by modification of traditional Keggin-type building fragment, Journal of Coordination Chemistry, 66:4, 725-736, A rare monocapped trivacant Keggin fragment [A-α-As III As V Mo 9 O 34 ] 6-has been captured and an organic-inorganic hybrid arsenomolybdate (AM) (H 2 en)[Ni(en) 2 (H 2 O)]{[Ni(en) 2 ][As III As VMo 9 O 34 ]}•3H 2 O (1) was obtained under hydrothermal conditions. The structure of 1 was determined by X-ray single-crystal diffraction and further characterized by IR spectroscopy, X-ray powder diffraction, scanning electron microscope with energy dispersive X-ray, and X-ray photoelectron spectroscopy. Single-crystal structural analysis indicates that 1 contains the peculiar monocapped trivacant [A-α-As III As V Mo 9 O 34 ] 6-fragment derived from the rare trivacant Keggin-type [A-α-As VMo 9 O 34 ] 9-unit capped by a triangular pyramidal {AsO 3 } group. Compound 1 represents the first Ni-substituted inorganic-organic AM containing the [A-α-As III As V Mo 9 O 34 ] 6-subunit.