Two sandwich arsenomolybdates based on the new building block As(III)Mo7O27 (9-): [Cr-2(AsMo7O27)(2)](12-) and [Cu-2(AsMo7O27)(2)](14-)

The polyanions [Cr-2(AsMo7O27)(2)](12-) (1a) or [Cu-2(AsMo7O27) (2)](14-) (2a) have sandwich-like structures wrapping two transition metals between two [As(III) Mo7O27](9-) fragments, and the fragment is unprecedented and can be viewed as a mono-capped hexavacant B-alpha-Keggin subunit with a central AsO3 group.