A four-coordinate H 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 (see figure).  The four individual Y–H distances to the unique interstitial hydride ligand are 2.184(16), 2.189(16), 2.221(13) and 2.168(12)Ĺ.  The compound was prepared via the reaction of YCp'(CH₂SiMe₃)₂(THF) with PhSiH₃ and gaseous H₂, and an initial x-ray analysis suggested the present geometry. [1]   Neutron data were collected on a 4 mm³ crystal at the Quasi-Laue diffractometer VIVALDI at ILL (Grenoble), and at the SXD difractometer at ISIS (Didcot).

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 H in the octahedral cavity of [HCo₆(CO)₁₅]^– in 1979, [2]  and 5-coordinate H in the square pyramidal cavities of [H₂Rh₁₃(CO)₂₄]^3– in 1997, [3] via single-crystal neutron analyses.     

[1] Tardif, O., Nishiura, M., Hou, Z., Organometallics, 2003, 22, 1171. 

[2] Hart, D.W., Teller, R.G., Wei, C.W., Bau, R., Longoni, G., Campanella, S., Chini, P., Koetzle, T.F., Angew. Chem. Int. Ed., 1979, 18, 80.  [

[3] Bau, R., Drabnis, M.H., Xie, Z., Garlaschelli, L., Klooster, W.T., Koetzle, T.F., Martinengo, S., Science, 275, 1099 (1997).