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Pentacoordinate hydrogen atoms were identified by single-crystal neutron diffraction analysis of [N(CH3)4]3[H2Rh13(CO)24]. The hydrogen atoms are located in square pyramidal cavities of the Rh13 cluster, in positions almost coplanar with the Rh4 faces on the surface of the cluster. They are slightly displaced inward, toward the central rhodium atom of the cluster, with average H-Rh(central) and H-Rh(surface) distances of 1.84(2) and 1.97(2) angstroms, respectively. This result shows that hydrogen, which normally forms only one bond, can be attached to five other atoms simultaneously in a large metal cluster.

Keywords: Neutron diffraction / Donor-acceptor complex / Bridging hydride ligand / Rhenium / Chromium A neutron diffraction analysis was carried out at 120 K on a single crystal of [K(18-crown-6)][(PPh 3 ) 2 ReH 6 Cr(CO) 3 ] · THF in order to locate the hydride ligands. Three of the hydrides are bound terminally to the Re atom while the other three bridge the Re-Cr bond; the bridging hydrides are closer to the Re atom. This compound can be thought of as a donor-

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