A four-coordinate hydrogen atom has been unambiguously located, by single-crystal neutron diffraction for the first time, in the center of the tetrahedral metal complex Y4H8(CP')(4)-(THF) [CP'=C5Me4(SiMe3)]. The core of the molecule consists of a tetranuclear cluster with one interstitial, one face-bridging, and six edge-bridging hydride ligands. The compound was prepared via the reaction of YCp'(CH2SiMe3)(2)-(TH F) with gaseous H-2. Neutron data were collected on a 4 mm(3) crystal at the Quasi-Laue diffractometer VIVALDI at ILL (Grenoble)(1a) and on an 8 mm(3) crystal at the SXD diffractometer at ISIS (Didcot). The final agreement factor is R = 8.9% for 4171 reflections. The existence of 4-coordinate hydrogen now completes the series of high-connectivity hydride ligands located in the interstitial cavities of molecular cluster complexes. We had previously reported the existence of 6-coordinate hydrogen in the octahedral cavity of [HCo6(CO)(15)](-) in 1979, and 5-coordinate hydrogen in the square pyramidal cavities of [H2Rh13(CO)(24)](3-) in 1997, also via single-crystal neutron analyses.