A new class of transition
metal cluster is described, [Rh
6(PR
3)
6H
12][BAr
F4]
2 (R =
iPr (
1a), Cy(
2a); BAr
F4 = [B{C
6H
3(CF
3)
2}
4]
-). These clusters are unique in that they have structures exactly like thoseof early transition
metal clusters with edge-bridging
![](/i<font color=)
mages/gifchars/pi.gif" BORDER=0 >-donor ligands rather than the structures expected forlate transition
metal clusters with
![](/i<font color=)
mages/gifchars/pi.gif" BORDER=0 >-acceptor ligands. The solid-state structures of
1a and
2a have beendeter
mined, and the 12 hydride ligands bridge each Rh-Rh edge of a regular octahedron. Pulsed gradientspin-echo NMR experi
ments show that the clusters re
main intact in solution, having calculatedhydrodyna
mic radii of 9.5(3) Å for
1a and 10.7(2) Å for
2a, and the for
mulation of
1a and
2a wasuna
mbiguously confir
med by ESI
mass spectro
metry. Both
1a and
2a take up two
molecules of H
2 toafford the cluster species [Rh
6(P
iPr
3)
6H
16][BAr
F4]
2 (
1b) and [Rh
6(PCy
3)
6H
16][BAr
F4]
2 (
2b), respectively, ascharacterized by NMR spectroscopy, ESI-MS, and, for
2b, X-ray crystallography using the [1-H-CB
11Me
11]
-salt. The hydride ligands were not located by X-ray crystallography, but
1H NMR spectroscopy showed a15:1 ratio of hydride ligands, suggesting an interstitial hydride ligand. Addition of H
2 is reversible: placing
1b and
2b under vacuu
m regenerates
1a and
2a. DFT calculations on [Rh
6(PH
3)
6H
x]
2+ (
x = 12, 16) supportthe structural assign
ments and also show a
molecular orbital structure that has 20 orbitals involved withcluster bonding. Cluster for
mation has been
monitored by
31P{
1H} and
1H NMR spectroscopy, and
mechanis
ms involving heterolytic H
2 cleavage and eli
mination of [HP
iPr
3]
+ or the for
mation of tri
metallicinter
mediates are discussed.