B-frame-supported bimetallics. Isoelectronic arachno-structured [(PMe2Ph)4Pd2B8H10] and closo-structured [(PMe3)4(CO)2Ir2B8H 8]

Article abstract of DOI:10.1016/S0020-1693(98)00353-3

Reaction of [IrCl(CO)(PMe₃)₂] with [NEt₄][nido-B₉H₁₂] gives the ten-vertex eight-boron cluster species [(PMe₃)₄(CO)₂Ir₂B₈H ₈]. This formally has a closo cluster electron count compatible with its closo structure. By contrast, ten-vertex eight-boron [(PMe₂Ph)₄Pd₂B₈H₁₀], isolated from [4-(NH₂Ph)-arachno-B₉H₁₃] and [PdCl₂(PMe₂Ph)₂], has a closo electron count but an arachno structure. The origins of this apparent anomaly are briefly addressed and enunciated.

Full text of DOI:10.1016/S0020-1693(98)00353-3

Inorganica Chimica Acta 285 (1999) 290±295
B-frame-supported bimetallics. Isoelectronic arachno-structured
[(PMe2Ph)4Pd2B8H10] and closo-structured [(PMe3)4(CO)2Ir2B8H8]
a,b
a
a
a,*
È
Jonathan Bould , Paul A. Cooke , Udo Dor¯er , John D. Kennedy ,
Lawrence Bartonb, Nigam P. Rathb, Mark Thornton-Petta
aSchool of Chemistry, University of Leeds, Leeds LS2 9JT, UK
bDepartment of Chemistry, University of Missouri at St Louis, St Louis, MO 63121, USA
Received 4 June 1998; accepted 24 July 1998
Abstract
Reaction of [IrCl(CO)(PMe3)2] with [NEt4][nido-B9H12] gives the ten-vertex eight-boron cluster species [(PMe3)4(CO)2Ir2B8H8]. This
formally has a closo cluster electron count compatible with its closo structure. By contrast, ten-vertex eight-boron [(PMe2Ph)4Pd2B8H10],
isolated from [4-(NH2Ph)-arachno-B9H13] and [PdCl2(PMe2Ph)2], has a closo electron count but an arachno structure. The origins of this
apparent anomaly are brie¯y addressed and enunciated. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Palladium complexes; Iridium complexes; Borane complexes
1. Introduction
ited that are more open than the formal `Wade's rules'
electron count may imply. To assess this type of phenom-
enon, it is useful to have pairs of comparison species [20]. In
this paper we report the isolation and structural character-
isation of one such pair, consisting of ten-vertex closo-
structured [(PMe3)4(CO)2Ir2B8H8] (1), and ten-vertex arach-
no-structured [(PMe2Ph)4Pd2B8H10] (2), of which each has a
formal closo ten-vertex electron count.
In 1971 it was recognised that most of the then known
polyhedral boron-containing cluster structures could be
systematically described in terms of (a) closed deltahedral
geometries, (b) such closed deltahedral geometries with one
vertex missing, and (c) such closed geometries with two
vertices missing [1,2]. These geometries are generally
described as closo, nido and arachno respectively. It was
then rapidly realised that, if the cluster bonding electrons are
counted up according to simple empirical rules, then the
closo, nido and arachno n-vertex structures are respectively
associated with 2n ‡ 2, 2n ‡ 4 and 2n ‡ 6 cluster bonding
electrons [3,4]. This useful combination of observations is
now well recognized, and is commonly known as `Wade's
rules'. There is currently a developing interest in exceptions
and apparent exceptions to these rules [5±12]. One aspect of
this interest focuses on metallaborane and metallaborane
cluster structures that also incorporate late transition-ele-
ment square-planar centres [7±9]. Here some interest has
involved the phenomenon of cluster structures that have
similar symmetries, but that have differing formal electron
counts [13±19]. There is also interest in the converse phe-
nomenon, i.e. formally isoelectronic structures that exhibit
quite different geometries. Frequently, structures are exhib-
2. Experimental
2.1. General
Reactions were carried out in dry solvent (thf) under dry
nitrogen, but subsequent manipulatory and separatory pro-
cedures were carried out in air. [4-(NH2Ph)-arachno-B9H13]
[21], [PdCl2(PMe2Ph)2] [22], [NEt4][nido-B9H12] [23] and
[Ir(CO)Cl(PMe3)2] [24] were prepared as described else-
where. Preparative thin-layer chromatography (TLC) was
carried out using 1 mm layers of silica gel G (Fluka, type
GF254) made from water slurries on glass plates of dimen-
sions 20 Â 20 cm2, followed by drying in air at 808C.
2.2. Nuclear magnetic resonance spectroscopy
NMR spectroscopy was performed at ca. 5.9 and 9.4 T
1
*Corresponding author. Tel.: +44-113-233-6414; fax: +44-113-233-
6565; e-mail: johnk@chem.leeds.ac.uk
(®elds corresponding to 250 and 400 MHz H NMR fre-
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00353-3

J. Bould et al. / Inorganica Chimica Acta 285 (1999) 290±295
291
31
0.63 (2H), ꢀ(1H)(PMe2) ‡1.43 with 2Jꢀ P 1H† ˆ
quencies respectively) using commercially available instru-
mentation and using techniques and procedures as described
and enunciated elsewhere [25±27]. Chemical shifts ꢀ are
given in ppm relative to Ä ˆ 100 MHz for ꢀ(1H)
(Æ0.05 ppm) (nominally TMS), Ä ˆ 32.083972 MHz for
ꢀ(11B) (Æ0.5 ppm) (nominally [F3BOEt2] in CDCl3) [28],
and Ä ˆ 40.480730 MHz for ꢀ (31P) (Æ0.5 ppm) (nominally
85% H3PO4). Ä is as de®ned by McFarlane [29].
31 Hz, ꢀ(1H)(PPh) 7.28±7.18; ꢀ(31P) 12.1.
2.5. Single-crystal X-ray diffraction analysis
Crystal data and re®nement parameters for both struc-
tures are listed in Table 3. Orange irregular crystals of
[(PMe3)4(CO)2Ir2B8H8] (1) and pale yellow rectangular
prisms of [(PMe2Ph)4Pd2B8H10] (2) that were suitable for
the single-crystal X-ray work were both grown from chloro-
form/hexane. Crystallographic measurements for com-
pound 1 were carried out at 218 K using a Siemens
SMART CCD area-detector diffractometer with narrow
!-rotation frames, and for compound 2 at 160 K using a
Stoe STADI-4 four-circle diffractometer operating in the
!±ꢃ scan mode. In both cases, graphite-monochromated Mo
2.3. Preparation of [(PMe3)4(CO)2Ir2B8H8] (1)
The preparative procedure was essentially that described
in the literature for the synthesis of arachno-[(CO)H-
(PMe3)2IrB8H12] [28], using [IrCl(CO)(PMe3)2] (350 mg,
860 mmol) and [NEt4][nido-B9H12] (0.25 g, 1000 mmol)
dissolved in CH2Cl2 (25 ml), with the exception that, before
separation of the products using thin-layer chromatography
(TLC), the reaction mixture was washed through silica gel
with CH2Cl2 (200 ml), leaving a trace of yellow±red mate-
rial on the silica gel which was then removed by extraction
with MeCN. The MeCN extract was applied to a TLC plate
and developed using MeCN/CH2Cl2 (20:80) as the mobile
phase, giving a red±yellow band (RF ˆ 0.25) identi®ed as
[2,2,6,6-(PMe3)4-2,6-(CO)2-closo-2,6-Ir2B8H8] (1) (3 mg,
0.4% yield). Crystals suitable for X-ray diffraction analysis
were obtained by hexane diffusion into a solution of the
compound in CH2Cl2. IR data (3 M te¯on IR card): ꢁ(CO)
1950 and 1932 cm 1, ꢁ(BH) 2465 cm 1. Mass spectrum
(70 eV EI): high-mass limit m/z 841 corresponding to
Ê
Ka radiation (ꢄ ˆ 0.71073 A) was used. An absorption
correction was applied to both data sets after correction
for Lorentz and polarisation effects, and was empirical
(SADABS) [31] for compound 1, and semi-empirical (based
on psi-scans) for compound 2.
Both structures were solved by heavy-atom methods
SHELXS-86 [32] and were re®ned by full-matrix least-
squares (against all the unique F2 data) using SHELXL-93
[33] (Programs part of the SHELXTL-PLUS (5.03) program
suite [34] for compound 1). In both cases, half a molecule of
the complex was located in the asymmetric unit. For com-
pound 2, the half molecule was near a C2 axis running along
the b axis at x ˆ 0 and z ˆ 0.25, passing through the
midpoint of the plane de®ned by atoms B(5), B(50), B(7)
and B(70), and the midpoint of the B(1)±B(3) vector, and for
compound 1 it was near a two-fold axis along the b axis at
x ˆ 0, z ˆ 0.25. Re®nement was similar for both structures.
All non-hydrogen atoms were re®ned with anisotropic
displacement parameters. Phenyl rings were restrained to
be ¯at and of overall C2v symmetry. All organophosphine
hydrogen atoms were constrained to idealized positions
with a riding model and were assigned isotropic displace-
ment parameters. Cluster hydrogen atoms were located on
Fourier-difference syntheses and were re®ned freely.
‡
12
C
14
1H43 31P4 16O2 11B8 193Ir2 [M H peak]. NMR data
(CDCl3, 294±300 K), ordered as ꢀ(11B) [ꢀ(1H) in square
brackets], are as follows: ‡32.7 [‡6.42, d, splitting 22.7 Hz,
31
3
probably from Jꢀ P 1H†], 1.3 [‡0.64], 3.4 [‡2.97],
32.4 [ 2.75, d, splitting 31.2 Hz, probably from
1
3Jꢀ P 1H†]; additionally, ꢀ(1H)(PMe3) ‡1.92 and ‡1.69,
31
31
2
both d, both with Jꢀ P 1H† ˆ 31 Hz; ꢀ(31P) 31.5 and
52.3, any coupling 2Jꢀ P 31P† not resolved (< ca. 3 Hz).
2.4. Preparation of [(PMe2Ph)4Pd2B8H10] (2)
A
sample of [4-(NH2Ph)-arachno-B9H13] (40 mg,
200 mmol) was dissolved in thf (10 ml), and then NaH
(60% active, 17 mg, 410 mmol) was added at room
temperature. The solution was stirred for 20 min at
room temperature, and then [PdCl2(PMe2Ph)2] (91 mg,
200 mmol) was added. After stirring for 4 h the more volatile
components were removed, the solid residue redissolved
in CH2Cl2 (ca. 10 ml) and the products separated and
puri®ed by repeated preparative TLC. Development with
CH2Cl2 gave [6,6,9,9-(PMe2Ph)4-arachno-6,9-Pd2B8H10]
(2) (67 mg, 39%, RF ˆ 0.4) as a yellow TLC band, which
was removed by extraction with CH2Cl2, and recrystallised
from CH2Cl2/n-hexane. NMR data (CDCl3, 294±300 K),
ordered as assignment ꢀ(11B) [ꢀ(1H) in square brackets], are
as follows: BH(2,4) ‡35.3 [‡5.53], B(5,7,8,10) ‡11.0
[‡3.26], B(1,3) 12.8 [‡1.77]; additionally ꢀ(1H)(ꢂ-H)
3. Results and discussion
The ®rst compound that is the subject of this report,
[2,2,6,6-(PMe3)4-2,6-(CO)2-closo-2,6-Ir2B8H8] (1), was
obtained in a very small yield (0.4%) as a by-product from
the reaction of the [nido-B9H12] anion with [Ir(CO)Cl-
(PMe3)2] that has previously been described [30] to give
arachno-[(CO)H(PMe3)2IrB8H12]. The formulation of 1
readily follows from the results of single-crystal X-ray
diffraction analysis (Fig. 1, Tables 1 and 3). These results
reveal a classical closo ten-vertex bicapped square antipris-
matic cluster structure [1,2], with the iridium atoms of the
two {Ir(CO)(PMe3)2} centres in adjacent positions, one in
each of the two different tropical belts (schematic IA). The

292
J. Bould et al. / Inorganica Chimica Acta 285 (1999) 290±295
demonstrates a clear classical (2n ‡ 2)-cluster-electron
closo-[B10H10]2 analogy.
Within the context of this closo structure, the detail of
compound 1 does however indicate interesting features.
Thus, within each tropical belt, the iridium±boron distance
of ca. 260 pm that ¯anks the iridium atom in the other
tropical belt is at the long end of iridium-to-boron distances
typical for polyhedral iridaboranes [36], and the inter-iri-
dium distance is also long at nearly 300 pm (hatched lines in
schematic structure IB). Each of these two iridium±boron
stretches is suggestive of ten-vertex `isonido' stretching
[10±12,37]. In 1, the two iridium centres generate two such
stretchings. This phenomenon, coupled with the long inter-
iridium distance, demonstrates an incipient distortion
towards ten-vertex nido character (schematic IC). There
is a closely related double stretching in formally closo ten-
vertex [(C5Me5)2Rh2B7H7NEt] (schematic cluster structure
ID) [38]. These stretchings are also similar to those involved
in the initial stages of the double diamond-square-diamond
process associated with the inter-enantiomeric ¯uxionalites
of classically closo ten-vertex metalladicarbaboranes (sche-
matic structures IE and IF) [39,40]. However, in spite of
these deviations, the compound is reasonably in the classical
closo ten-vertex category, both from the structural [1,2] and
the electron-counting [3,4] points of view.
Fig. 1. ORTEP-type [35] drawing of the crystallographically determined
molecular structure of [(PMe3)4(CO)2Ir2B8H10] (1). The atomic number-
ing scheme shows the crystallographic two-fold symmetry of 1. In the
formal IUPAC numbering system (Table 1), Ir(20), B(10), B(30), B(40) and
B(50) become Ir(6), B(10), B(9), B(8) and B(7) respectively.
Table 1
Selected interatomic distances (pm) and angles (8) for [2,2,6,6-(PMe3)4-
2,6-(CO)2-closo-2,6-Ir2B8H8] (1)a
Ir(2)±C(1)
Ir(2)±P(2)
Ir(2)±B(1)
Ir(2)±B(3)
B(1)±B(3)
B(1)±B(4)
B(1)±B(5)
B(4)±B(8)
O(1)±C(1)
1.893(6)
2.283(2)
2.144(6)
2.304(7)
1.731(9)
1.692(9)
1.770(9)
1.773(14)
1.146(6)
Ir(2)±P(1)
Ir(2)±Ir(6)
Ir(2)±B(5)
Ir(2)±B(7)
B(3)±B(4)
B(3)±B(7)
B(3)±B(8)
B(4)±B(5)
2.331(2)
2.9408(4)
2.578(6)
2.331(6)
1.798(10)
1.840(9)
1.787(9)
1.803(9)
P(1)±Ir(2)±P(2)
C(1)±Ir(2)±P(2)
P(1)±Ir(2)±B(5)
B(3)±Ir(2)±B(5)
B(3)±B(4)±B(5)
94.54(6)
87.8(2)
C(1)±Ir(2)±P(1)
C(1)±Ir(2)±B(1)
P(2)±Ir(2)±B(3)
B(4)±B(3)±Ir(2)
B(4)±B(5)±Ir(2)
95.6(2)
168.4(3)
84.9(2)
89.67(14)
65.9(2)
103.9(4)
93.9(3)
95.5(4)
a Estimated standard deviations are given in parentheses.
NMR properties were entirely consistent with the crystal-
lographically determined molecular structure, indicating
that the bulk material was represented by the crystal
selected. The cluster structure of compound 1 is in accord
with the Williams±Wade structure-geometry ± electron-
counting `Wade's rules' paradigm [1±4]. The two formally
`octahedral' iridium(III) units each contribute three elec-
trons and three orbitals to the cluster bonding scheme, and
each of the eight {BH} vertices two electrons and three
orbitals. This generates a 22-electron count and thence
This, however, is not the case with the second compound,
formulated as [6,6,9,9-(PMe2Ph)4-arachno-6,9-Pd2B8H10]
(2), which was obtained in moderately good yield (39%)
from the reaction of [4-(NH2Ph)-arachno-B9H13] with NaH

J. Bould et al. / Inorganica Chimica Acta 285 (1999) 290±295
293
Table 3
Crystal data, together with data collection and refinement parameters, for
the crystallographically determined structures of 1 and 2
1
2
Formula
C
16H46B8Cl6Ir2O2P4
C32H54B8P4Pd2
160
Temperature (K)
M
218(2)
1077.99
0.30 Â 0.15 Â 0.10
orthorhombic
Pbcn (No. 60)
12.7926(1)
15.0723(1)
19.4712(2)
±
861.91
Crystal dimensions
Crystal system
Space group
Ê
a (A)
Ê
b (A)
0.56 Â 0.27 Â 0.11
monoclinic
C2/c (No. 15)
13.7909(9)
15.1212(8)
19.0736(11)
92.672(5)
3973.2(4)
4
Ê
c (A)
ꢅ (8)
U (A )
Ê 3
3754.32(5)
4
Z
F (000)
2056
1752
Fig. 2. ORTEP-type [35] drawing of the crystallographically determined
molecular structure of [(PMe2Ph)4Pd2B8H10] (2). Hydrogen atoms have
been assigned a small arbitrary radius for clarity. The atomic numbering
scheme shows the crystallographic two-fold symmetry possessed by
compound 2. In the formal IUPAC numbering system (Table 2), Pd(60),
B(30), B(20), B(50) and B(70) become Pd(9), B(1), B(4), B(8) and B(10)
respectively.
3
Dc (mg m
1
ꢂ (mm
)
1.907
1.441
)
7.697
2.09±28.99
0
1.089
2.0±25.0
3.6
Data collection range (8)
Decay (%)
Measured reflections
Unique data
Observed reflections
32011
3519
4777
4775
3497
2959
[I > 2s(I)]
No. of independent
parameters
188
232
followed by [PdCl2(PMe2Ph)2]. This reaction was investi-
gated as part of a program of exploratory investigation into
the reactions of amine-substituted boranes with metallic
reagents, in which either nitrogen incorporation in the
cluster to generate azametallaboranes, or loss of the nitro-
gen-substituted boron vertex to generate cluster compounds
with lower boron-atom counts, may be expected [38,41±43].
This reaction is clearly in the latter category. As with the
diiridium compound 1, the constitution of the dipalladium
compound 2 was con®rmed by single-crystal X-ray work
(Fig. 2, Tables 2 and 3). The NMR properties are entirely
consistent with the crystallographically determined mole-
Rint
0.0920
0.0422
Tmax, Tmin
0.90, 0.46
0.1016
0.0901
0.394, 0.337
0.0824
0.0577
!R2 (all data)
wR2 (observed data)
R1 (all data)
0.0776
0.0443
R1 (observed data)
Goodness of fit, S
Weighting parameters
Max. residual electron
0.0388
0.0291
1.003
0.050
1.132
0.0218, 12.3937
‡0.447
‡3.173(close to Ir2)
3
Ê
density (e A
)
Min. residual electron
3.199 (close to Ir2)
0.483
3
Ê
density (e A
)
cular structure, indicating that the bulk material was repre-
sented by the crystal selected. The NMR data also exhibit
clear similarities with the previously reported diplatinum
and palladium±platinum mixed-metal analogues [44,45].
Compound 2 is clearly not of closo geometrical aspect. It
has an open classical ten-vertex arachno boat shape (sche-
matic IIA), as typi®ed by the arachno-[B10H14]2 anion
itself (schematic IIB), and has two bridging hydrogen atoms
in the 5,10 and 7,8 `gunwale' positions, typical for arachno
Table 2
Selected interatomic distances (pm) and angles (8) for [6,6,9,9-(PMe2Ph)4-
arachno-6,9-Pd2B8H10] (2)a
Pd(6)±P(1)
Pd(6)±B(2)
Pd(6)±B(7)
B(1)±B(2)
B(1)±B(3)
B(1)±B(5)
B(3)±B(4)
B(3)±B(7)
B(5)±B(10)
2.3379(9)
2.251(4)
2.245(4)
1.768(6)
1.739(8)
1.792(5)
1.768(6)
1.786(6)
1.875(6)
2.3411(9)
2.251(4)
Pd(6)±B(5)
B(2)±B(3)
B(2)±B(5)
B(2)±B(7)
B(3)±B(8)
B(7)±B(8)
1.769(6)
1.762(5)
1.773(5)
1.792(5)
1.875(6)
1
ten-vertex species. Further, its cluster 11B and H NMR
properties show clear parallels with the arachno-[B10H14]2
anion [12,44,45]. However, this arachno structure, together
with the other arachno ten-vertex characteristics, apparently
belies its cluster electron count. Eight cluster {BH} units
each contribute two electrons to the cluster bonding scheme,
two bridging hydrogen atoms one electron each, and each
{Pd(PMe2Ph)2} unit two electrons, generating a 22-electron
count. This would correspond to a (2n ‡ 2)-electron closo
total for a ten-vertex cluster.
P(1)±Pd(6)±P(2)
P(1)±Pd(6)±B(2)
P(1)±Pd(6)±B(5)
P(1)±Pd(6)±B(7)
B(2)±Pd(6)±B(7)
B(5)±Pd(6)±B(7)
98.43(3)
124.93(10)
88.71(10)
171.36(10)
46.44(14)
83.74(14)
P(2)±Pd(6)±B(2)
P(2)±Pd(6)±B(5)
P(2)±Pd(6)±B(7)
B(2)±Pd(6)±B(5)
132.45(10)
167.88(10)
89.74(10)
46.09(14)
a Estimated standard deviations are given in parentheses.

294
J. Bould et al. / Inorganica Chimica Acta 285 (1999) 290±295
been invoked (schematic structure IIG) [30,44,45,56]. This
could be regarded, depending on semantics, as either metal-
to-cluster back-donation or as a contribution from a higher
metal valence state. Comparison of representations IIE, IIF
and IIG show that again it is not generally essential to
invoke this in terms of the basic inter-boron bonding
topology electronics of the cluster, or of its basic arachno
character. Nevertheless, molecular orbital calculations sug-
gest that it may have some validity in some species [57], and
here the ¯exibility of transition-element fragments to be
isolobal with more than one main-group element fragment
should be recognised [58]
This arachno-structure±closo-count anomaly arises as
follows. For the classically arachno [B10H14]2 anion, a
formal anionic {BH2} unit (schematic IIC) contributes
two electrons and two orbitals to the ten-boron cluster
bonding proper. Application of Wade's rules [3,4] requires
that the electron pairs in each of the two endo B±H bonds
(xx in schematic IIB) be counted in the cluster-electron
count. In the dipalladium compound 2, the neutral square-
planar {Pd(PMe2Ph)2} palladium(II) unit (schematic IID)
has a two-orbital two-electron contribution to the cluster
bonding, analogous to that of a {BH2} unit. However, it
has no electron pair in the endo position for electron
counting, even though the basic bonding within the cluster
(e.g. schematics IIE and IIF) will be the same in each case.
This type of phenomenon will be quite general for square-
planar 16-electron transition-element centres, and will lead
to cluster electron-counts that are two fewer than the
classical Wadian for each such centre in the cluster
[8,9,12,46]. In 2, there are two such centres, and so the
electron count is four less than that otherwise expected for
the ten-vertex arachno cluster shape.
Although the reasons for this apparent anomaly are long
recognised [46±51], a comparison of this pair of compounds
nicely illustrates its origins, which can still lead to confusion
(a) when clusters may, for example, be categorically
described as closo purely on the basis of perceived electron
count [54], even though the observed geometry is patently
much more open [49,50], as with the arachno-structured
compound 2 in this present work, and (b) when attempts are
unnecessarily made to locate extra electron pairs to redress
the presumed electron de®ciency.
Some attempts to ®nd `extra' electrons have involved the
postulation of interactions between the metal centre and
hydrogen atoms bound to other groups when these are
adjacent in crystal structures [5,6,52,53]. However, such
positionings can generally derive from ef®cient packing
[8,9,12,55], and it is uncertain whether any interaction is
otherwise thermodynamically signi®cant. Bonding compar-
isons as in IIE and IIF, and the known independent stability
of square-planar transition-element centres, mitigate any
need generally to invoke such processes to ®nd extra
electron pairs. Alternatively, a cluster bonding involvement
of the electron-pair that is in the otherwise non-bonding pd-
hybrid of the formal square-planar system has sometimes
.
4. Supplementary material
Atomic coordinates, displacement parameters, inter-
atomic distances and angles between interatomic vectors
for 1 and 2 are deposited with the Cambridge Crystal-
lographic Data Centre (CCDC), deposition number
102286 for 1 and 102287 for 2. Any request to the CCDC
for this material should quote the full literature citation.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft for a
È
Fellowship to U. Dor¯er, and the EPSRC (UK) (grant Nos.
GR/J/56929 and GR/K/05818) for support. We also
acknowledge ACS-PRF support (grant No. 31001-AC3)
and UM-St. Louis instrumentation grants NSF No. CHE-
9309690 and DOE No. DE-FG02-92CH10499.
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