small difference between the solvolysis rates of system 2 and the corresponding fluorenylidene derivatives78 argues against high contribution of this effect.

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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.

Multinuclear n.m.r. studies (1 H,' H-{103Rh}, l3C, 13C-{103Rh), and lo3Rh) show that protonation of [Rh6(C0)15]2and [Rh6(C0)15C]2-at low temperatures gives [Rh,H(CO),,]-and [Rh6H(CO)15C]-which contain terminal and triangular face-bridging hydrides respectively; warming these solutions to room temperature results in loss of hydrogen and formation of [ Rhl 2(CO)30]2-and [ Rhl 2( CO), , ( C),], -respectively.