Heteromultimetallic hydride clusters containing both rare-earth and d-transition metals are of remarkable interest in terms of both their structure and reactivity. Well-defined hydride clusters of this type are also of particular interest as molecular models for hydrogen storage alloys, in view of the fact that rare earth/d-transition metal alloys such as LaNi5Hn are excellent hydrogen-storage materials. In this work, we plan to determine the precise structures of a novel family of molecular heterometallic hexanuclear polyhydride clusters of the core structure type of Y4M2H14 and Y5MH13 (M = Mo, W), which reversibly release and uptake hydrogen gas in a similar manner as for the core structure type Y4MoH11.