Page 286 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

Metal Particles and Cluster Compounds 543

5
This causes the ring centroids of the other three (η -C 5 H 5 ) Hydrides are known to coordinate both as interstitial
ligands, each associated with a cluster vertex, to be bent and surface ligands. As a surface ligand, a hydride may be
5
out of the Rh 3 plane. These three (η -C 5 H 5 ) ligands are terminally bound or it may bridge two or three metals. As
bent toward the face of the Rh 3 triangle which is capped in boron cluster compounds the hydride ligands preferen-
by the smaller hydride ligand. tially select the more highly coordinated (µ 3 -H) position,
From a practical point of view the cyclopentadienyl participating in multicenter bonding rather than the termi-
ligand may be important for several reasons. Clusters can nal hydride position. In fact, terminal hydrides are quite
be stabilized with relatively few ligands and these lig- rare. Of the two hydrides in H 2 Os 3 (CO) 11 one is termi-
ands may be capable of changing their mode of coordi- nally bound while the other is a bridging µ 2 -H. The Os
nation. The change in coordination is accompanied by atom with the terminal hydride is also one of the bridged
change in the electron donation made to the metal cen- atoms. Removal of one CO to produce H 2 Os 3 (CO) 10
ter. In mononuclear–cyclopentadienyl chemistry the con- causes the terminal hydride to adopt a bridging mode.
5
3
version of (η -C 5 H 5 ) → (η -C 5 H 5 ) is important in cat- Both hydrides now bridge the same Os Os bond. In or-
alytic reactions; perhaps conversions of this sort will prove der to remain electronically saturated this doubly bridged
to be important in cluster–cyclopentadienyl chemistry as Os Os bond must have a bond order of two. As in organic
well. From an aesthetic point of view the symmetry inher- chemistry this double bond is a seat of reactivity. Many
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ent to the (η -C 5 H 5 ) ligand in consort with the symme- new triosmium clusters may be derived from this reactive
try of a cluster core makes these compounds particularly cluster.
attractive. Asaninterstitialligandthehighestcoordinationnumber
The importance of ligand unsaturation such that there is of a hydride observed to date is six. This situation exists in
an abundance of electrons available to a metal center has [(µ 6 -H)Nb 6 I 11 ], the first compound in which the existence
been demonstrated. The extreme of unsaturation is the of an interstitial hydride was established. The hydride oc-
bare atom, and bare atoms constitute an important class cupies the octahedral cavity created by the Nb 6 core. Al-
of cluster ligands. Bare atoms that have been observed ternatively, an interstitial hydride may occupy a tetrahe-
as cluster ligands include: from Group I H; from Group dral cavity as is the case in [(µ 4 -H)Os 10 (µ 6 -C)(CO) 24 ] −
IV C, Si, Ge, and Sn; from Group V N, P, As, and Sb; (Fig. 32). The arrangement of Os atoms in this cluster is
from Group VI O, S, and Se and all of the Group VIII such that both an octahedral cavity and tetrahedral cavities
halogens except At. exist.FourtetrahedralcavitiesarecreatedbyfourOs(CO) 3
Two types of bare atom ligands are observed: intersti- units capping four faces on an octahedral Os 6 core. The
tially bound and surface bound. When interstitially bound hydride cannot occupy the central octahedral cavity as it
or encapsulated by a cluster framework, all of the valence is already occupied by a carbon atom or carbido ligand
electrons of that atom are donated to the cluster. As an which brings us to Group IV atomic donors.
atom’s ability to accommodate lone pairs of electrons in- All of the Group IV elements, except lead, have been
creases (i.e., electronegativity increases) there is an in- observed as atomic donor ligands. For Sn, Ge, and Si there
creased tendency to adopt surface over interstitial coor-
dination. This is best demonstrated by the halogens for
which there are no known examples of interstitial coordi-
nation.
Hydride ligands are typically introduced to a cluster by
the protonation of an anionic precursor as in the following
reactions:
+H +
[Co 6 (CO) 15 ] 2− −→ [HCo 6 (CO) 15 ] −
←−
−H +
+H +
[Ru 6 (CO) 18 ] 2− −→ [HRu 6 (CO) 18 ] −
←−
−H −
+H +
[Re 3 (CO) 12 ] 3− −→ [HRe 3 (CO) 12 ] 2−
←−
−H +
+H +
−→ [H 2 Re 3 (CO) 12 ] −
←− FIGURE 32 Structure of [(µ 4 -H)Os 10 (µ 6 -C)(CO) 24 ] . An inter-
−
−H +
stitial hydride occupies one of four tetrahedral cavities while the
+H + central octahedral cavity is occupied by a carbide. (Terminal CO
[Os 3 (CO) 12 ←− [HOs 3 (CO) 12 ] +
−→
−H + are not shown.)