Macropolyhedral boron-containing cluster chemistry. A novel triple-cluster structural motif. Isolation and characterisation of contiguous twenty-vertex

Article abstract of DOI:10.1016/S0022-328X(02)01552-8

Twenty-vertex [(PPh₃)₄ClPd₄B₁₆H₁₇(PPh₃)] (1),¹ from the thermolysis of [(PPh₃)₂PdB₈H₁₂] (2)¹ in refluxing toluene solution, followed by CH₂Cl₂ extraction, has a core nido five-vertex {Pd₂B₃} unit that has opposing triangular faces in common with an arachno ten-vertex {Pd₂B₈} unit and a nido eleven-vertex {Pd₃B₈} unit.

Full text of DOI:10.1016/S0022-328X(02)01552-8

Journal of Organometallic Chemistry 657 (2002) 262ꢀ266
/
www.elsevier.com/locate/jorganchem
Macropolyhedral boron-containing cluster chemistry
A novel triple-cluster structural motif. Isolation and characterisation
of contiguous twenty-vertex [(PPh3)4ClPd4B16H17(PPh3)2]1
Michael G.S. Londesborough, Colin A. Kilner, Mark Thornton-Pett,
John D. Kennedy *
The Department of Chemistry of the University of Leeds, Leeds LS2 9JT, UK
Received 30 October 2001; accepted 23 April 2002
Abstract
Twenty-vertex [(PPh3)4ClPd4B16H17(PPh3)] (1),1 from the thermolysis of [(PPh3)2PdB8H12] (2)1 in refluxing toluene solution,
followed by CH2Cl2 extraction, has a core nido five-vertex {Pd2B3} unit that has opposing triangular faces in common with an
arachno ten-vertex {Pd2B8} unit and a nido eleven-vertex {Pd3B8} unit. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Thermolysis; Cluster chemistry; Macropolyhedral; Fusion modes; Metallaborane; Palladium-boron cluster
Although several thirteen-vertex closed metallcarba-
boranes have been known for some time [1], any truly
extensive contiguous polyhedral boron-containing clus-
ter chemistry that extends beyond the twelve-vertex
barrier that is typified by the stable twelve-vertex
icosahedron requires the intimate fusion of smaller
clusters [2]. To develop this potentially extensive field,
there is merit in the examination for: (a) new cluster
fusion modes and the architectures that they engender
[3,4]; (b) generation of larger contiguous clusters beyond
the eighteen-vertex limit of the most readily accessible
macropolyhedral skeletons of the syn and anti isomers
[11ꢀ19]. A problem here is that generic routes to cluster
fusion are lacking. However, it is found empirically that
several arachno nine-vertex species, specifically
/
[B9H13(SMe2)] [20], [SB8H12] [8ꢀ10], [SB8H10(SMe2)]
/
[21], [(CO)(PMe3)2HIrB8H12] [12] and [(PMe2Ph)2-
PtB8H12] [15,16], will engender macropolyhedral species
upon mild thermolysis in refluxing solvents such as
toluene, benzene, cyclohexane, etc. Of these arachno
nine-vertex species, [(PMe2Ph)2PtB8H12] in particular
gives a very interesting variety of structural types
[15,16], including fourteen-vertex [(PMe2Ph)2Pt2B12H18]
[15], seventeen-vertex [(PMe2Ph)PtB16H18(PMe2Ph)]
[22], seventeen-vertex [(PMe2Ph)4Pt3B14H16] [23] and
eighteen-vertex [(PMe2Ph)2Pt2B16H15(C6H4Me)(PMe2-
Ph)] [16]. This variety in complexity of product structure
must arise from a subtle balance of mechanism. It is
thence tempting to speculate that the course of this last
reaction may be changed, and thence that the variety of
products with new structural characteristics may be
extended, by modifications of the [(PMe2Ph)2PtB8H12]
starting substrate. Such modifications may be accom-
plished, for example, by the use of the other elements in
the platinum group, viz. palladium and nickel, and also
by the use of different exo-cluster ligands on the metal
centre. We now have preliminary results to show that
this is indeed the case, and here we now report the
of B18H22 [5ꢀ/7], and of the anti and iso isomers of
S2B16H16 [8ꢀ10]; and (c) macropolyhedral assembly
/
independent of these B18H22 and S2B16H16 systems
* Corresponding author
E-mail address: johnk@chem.leeds.ac.uk (J.D. Kennedy).
1
A cursive IUPAC nomenclature for compound 1 is m-8,9-chloro-
9ƒ,5,7,9 - tetrakis(triphenylphosphine) - arachno-6ƒ,9ƒ - dipalladadeca-
borano-{5ƒ,6ƒ,2ƒ: 4?,1?,5?}-nido-1?,2?-dipalladapentaborano-{1?,2?,3?:
8,7,3}-nido-7,8,9-tripalladaundecaborane, and, for compound 2, 4,4-
bis(triphenylphosphine)-arachno-4-palladanonaborane. This article
was freely submitted for publication without royalty. By acceptance
of this paper, the publisher and/or recipient acknowledges the right of
the authors to retain non-exclusive, royalty-free license in and to any
copyright covering this paper, along with the right to reproduce all or
part of the copyrighted paper.
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 5 5 2 - 8

M.G.S. Londesborough et al. / Journal of Organometallic Chemistry 657 (2002) 262ꢀ
/266
263
structural characterisation of an interesting twenty-
vertex species of formulation [(PPh3)4Pd4ClPdB16H17-
(PPh3)2] (compound 1)1. This is isolated from the
products of thermolysis of [4,4-(PPh3)2-arachno-4-
PdB8H12] (compound 2)1 in toluene solution.
Thus, initially, cis-[PdCl2(PPh3)2] (360 mg, 510 mmol)
was quickly added to a stirred solution of K[B9H14] (120
mg, 770 mmol) in dry MeOH (ca. 20 ml). The immediate
yellow suspension was stirred at room temperature for
30 min, the yellow solid filtered off and washed with a
small quantity of methanol to remove traces of excess
K[B9H14], and then dried to give yellow micro-crystals.
These were readily identified by NMR spectroscopy,2
and confirmed by a single-crystal X-ray diffraction
analysis (Fig. 1),3 as [(PPh3)2PdB8H12] (compound 2,
320 mg, 440 mmol, 86%). It is clearly of the nine-vertex
arachno-4-metallanonaborane configuration that is
well-recognised [24], for example in the platinum
analogue [(PMe2Ph)2PtB8H12] [25]. Thence, a suspen-
sion of compound 2 (300 mg, 410 mmol) in dry toluene
Fig. 1. Drawing of the crystallographically determined molecular
structure of [(PPh3)2PdB8H12] (compound 2).3 Note that, although
H(6,7) and H(7,8) freely refined approximately to the bridging
positions, it is probable that one atom is bridging and the other atom
is in an endo position [43], and therefore that this structure is subject to
some effective disorder in this regard. In solution, the endo and
bridging hydrogen atoms associated with the B(6)ꢀ
/
B(7)ꢀB(8) notch
/
will interchange character, giving, on time-average, an apparent mirror
plane symmetry through Pd(4), B(1) and B(7) as manifested in the
2
Measured NMR data for [(PPh3)4ClPd4B16H17(PPh3)2]
(compound 1) and [(PPh3)2PdB8H12] (compound 2) are as follows,
2
NMR properties. Selected interatomic distances in A are: from Pd(4)
˚
CDCl3 at 295ꢀ
[d(1H)/ppm of attached exo-H atom in square brackets] BH(1) ꢁ
[ꢁ4.60], BH(7) ꢁ16.0 [ꢁ3.88], BH(5,9) ꢁ08.8 [ꢁ2.86], BH(6,8)
24.1 [ꢁ1.62], and BH(2,3) ꢂ28.7 [ꢁ
0.72]; additionally d(1H) for
H(Ph) at ꢁ7.5 to ꢁ7.0 ppm (unresolved multiplets), for H(m-5,6;m-8,9)
at ꢂ2.54 ppm, and for H(endo/m-6,7;7,8) ca. ꢁ0.4 ppm, with
d(31P)(PPh3) at ꢁ
24.7 ppm; compound 1 is as yet unassigned in this
/
300 K: compound 2, ordered as assignment d(11B)/ppm
to P(1) 2.3635(6), to P(2) 2.3528(6), to B(1) 2.229(3), to B(5) 2.218(3),
and to B(9) 2.230(3), with B(5)ꢀ
B(7)ꢀB(8) 1.847(5) and B(8)ꢀB(9) 1.837(4). Selected angles at Pd(4)
are as follows: P(1)ꢀP(2) 100.57(2)8, P(1)ꢀB(5) 84.88(8)8, P(2)ꢀB(9)
90.26(8)8 and B(5)ꢀB(9) 84.81(11)8.
/
B(6) 1.845(4), B(6)ꢀB(7) 1.882(5),
/
/22.1
/
/
/
/
/
/
/
/
/
/
ꢂ/
/
/
/
/
/
/
/
/
/
(ca. 20 ml) was heated at reflux for 1 h, and the toluene
was then removed in vacuo. The residue was dissolved in
dichloromethane, with slight effervescence, and the
mixture then subjected to preparative TLC in air (silica
gel G, Fluka GF 254), revealing several coloured
products. Purification of a predominant green compo-
preliminary work, but has tentative 11B and 1H resonance positions as
follows, listed as d(11B)/ppm [d(1H)/ppm of an attached H atom
(either exo or m) in square brackets] ꢁ
14.7 [ꢁ0.02], ca. ꢁ14.4 (broad) [ꢁ
[ꢁ4.51], ca. ꢁ11.0 (broad) [ꢁ3.00], ca. ꢁ
directly-bound H], ca. ꢁ6.50 (broad) [ꢁ1.97], ca. ꢁ
4.0 (broad) [ꢁ2.04, ꢁ1.47 m?], ca. ꢁ2.2 (broad) [ꢁ
[ꢁ1.59], 16.7 [ꢁ1.26], 18.1 [ꢁ0.67] and 18.3 [ꢁ
additionally d(1H)(Ph) at ꢁ
7.5 to ꢁ7.0 ppm (unresolved multiplets)
and d(1H)(m-H) at ꢁ0.27 ppm associated with d(11B) ꢁ
16.5 to ꢁ11.0
ppm, with d(31P)(PdPh3) P(1) and P(2) both at ca. ꢁ
33.8 ppm, P(6)
and P(9) at ꢁ29.2, and ꢁ
24.1 ppm with 3J(31Pꢀ31P) ca. 15 Hz, and
P(4) at ca. ꢁ
9.5 ppm (broad quartet, 1J(31Pꢀ11B) ca. 110 Hz).
Crystal data for [(PPh3)4ClPd4B16H17(PPh3)2] (compound 1), as
the CHCl3 sesqui-solvate C91.5H93.5B16Cl5.5P5Pd4× (CHCL3)1.5: Mꢃ
2141.55, triclinic (deep redꢀgreen prism from C6H14 CHCl3), 0.65ꢄ
0.49ꢄ0.42 mm, space group P1; aꢃ14.98470(10), bꢃ18.6685(2), cꢃ
1.4763(2) A, aꢃ 106.8560(5), gꢃ100.6430(4)8, Uꢃ
1.342 Mg mꢂ3, Zꢃ
2, MoꢀKa, lꢃ
150(2) K, R1ꢃ0.0498 for 17 115 reflections with
2s(I), and wR2ꢃ0.1539 for all 20 595 unique reflections; CCDC
reference number 172802. Crystal data for [(PPh3)2PdB8H12
(compound 2), as the C6H6 hemi-solvate C36H42B8P2Pd×/12C6H6: Mꢃ
768.57, triclinic (yellow rod from C6H6 ꢀC6H14 CHCl3), 0.48ꢄ
11.0840(2), bꢃ11.7122(2), cꢃ
87.0810(9), gꢃ63.3610(9)8, Uꢃ
1.319 Mg mꢂ3 , Zꢃ
2, MoꢀKa, lꢃ
150(2) K, R1ꢃ0.0356 for 7159 reflections with
2s(I), and wR2ꢃ0.0969 for all 7576 unique reflections; CCDC
/
31.8 [ꢁ
1.88 m?],ꢁ
8.0 (broad) [no apparent
4.4 [ꢁ3.49], ca.
2.05], ꢂ13.5
0.88];
/
5.72], ca. ꢁ
/
16.5 [ꢁ
4.41], ꢁ
/
2.96],
ꢁ/
/
/
/
/
14.0 [ꢁ
/
/13.1
/
/
/
/
/
/
/
/
ꢁ/
/
/
/
/
/
nent (RF ca. 0.6, CH2Cl2ꢀC6H14 4:1) by repeated TLC,
/
/
ꢂ/
/
ꢂ/
/
ꢂ/
/
followed by crystallisation by diffusion of hexane into a
dichloromethane solution, gave an air-stable crystalline
/
/
/
/
/
/
solid, of an apparent dichroic greenꢀred appearance.
/
/
/
/
This was characterised by a single-crystal X-ray diffract-
ion analysis (Fig. 2)3 as [(PPh3)4ClPd4B16H17(PPh3)]
(compound 1, 40 mg, 20 mmol, 13.5%). Multinuclear
NMR spectroscopy2 was consistent with the crystallo-
graphically determined molecular structure, showing
that the crystal used for the X-ray work was represent-
ative of the bulk sample.
The new macropolyhedral compound [(PPh3)3-
ClPd3B16H17Pd(PPh3)] (compound 1, Fig. 2) has a
most interesting condensed twenty-vertex macropolyhe-
dral cluster structure (schematics I). It is somewhat more
condensed than the several species [15,16] from the
platinum system mentioned above, and therefore of
somewhat different character. The molecule contains
several unusual and unique features. Although, in
architectural terms, the twenty-vertex {Pd4B16} skeletal
/
/
3
/
/
/
ꢀ/
/
¯
/
/
/
/
˚
/
105.7950(4), bꢃ
5301.07(8) A , Dcalc
mꢃ
0.922 mmꢂ1, Tꢃ
I ꢀ
/
/
/
3
˚
˚
0.71073 A,
ꢃ/
/
/
/
/
/
/
/
/
]
/
/
ꢀ/
/
¯
0.42ꢄ
/
0.1 mm, space group P1; aꢃ
/
/
/
˚
17.0127(3) A, aꢃ
/
78.8610(11), bꢃ
1935.32(6) A , Dcalc
mꢃ
0.590 mmꢂ1, Tꢃ
I ꢀ
/
/
/
3
˚
˚
0.71073 A,
ꢃ/
/
/
/
/
/
/
/
/
reference number 172803. Methods and programs were standard
[44,45].

264
M.G.S. Londesborough et al. / Journal of Organometallic Chemistry 657 (2002) 262ꢀ266
/
neutral [C4B18H22] and the [B20H18]2ꢂ anion [27ꢀ
29],
/
which can in principle be regarded in terms of sub-
cluster units fused to a central, more open, arachno-type
four-vertex unit. The nido {1,2-M2B3} geometry of the
five-vertex sub-cluster IC is long-recognised in single-
cluster chemistry, as in Fehlner’s [(CO)6Fe2B3H7] spe-
cies [30], although its incidence in a fused-cluster species
is unprecedented. Because of the overall novelty of
compound 1, there is no close parallel with any known
cluster species. The overall cluster structure perhaps has
its closest architectural relationship in those of double-
cluster [(PMe2Ph)4Pt3B14H16] (compound 3, schematic
II) [15,23], with its nido eleven-vertex {Pt2B9} sub-
cluster, and [(PMe2Ph)2PtB26H24] (compound 4, sche-
matic III) [3,31], with its fused triple-cluster architecture.
However, neither of these has a central structural
feature such as this unique square-pyramidal five-vertex
core. Although compound 4 does have a central nido
cluster, this is of conventional eleven-vertex nido char-
acter, and is based on overlapping pentagonal pyrami-
dal motifs, quite different from the quadrilateral
pyramidal nido five-vertex shape, and it also exhibits
the more usual edge-fusions with its conjoined clusters,
rather than face-fusion (schematic III).
Fig. 2. Drawing of the crystallographically determined molecular
structure of [(PPh3)4ClPd4B16H17(PPh3)2] (compound 1),3 with Pꢀ
phenyl atoms, except the ipso carbon ones, omitted for clarity.
/
˚
Selected interatomic distances in
A are: from Pd(6) to Pd(6?)
2.9017(4), to P(6) 2.3309(11), to H(6,2?) 1.91(6), to B(2) 2.235(5), to
B(2?) 2.515(4), to B(5) 2.181(5), and to B(7) 2.226(5); from Pd(6?) to
Pd(9) 2.9221(4), to Cl(6,9) 2.4618(10), to H(5,1?) 2.17(6), to B(2?)
2.184(5), to B(5?) 2.070(4), to B(7) 2.211(5), to B(7?) 2.294(5), and to
B(8) 2.359(5); from Pd(9) to P(9) 2.3394(10), to Cl(6,9) 2.4930(10) to
B(4) 2.205(4), to B(8) 2.225(4), and to B(10) 2.175(5); and from Pd(9?)
to P(1) 2.3674(11), to P(2) 2.3297(11), to B(4?) 2.228(5), to B(8?)
2.249(5), and to B(10?) 2.230(5); with B(2?)ꢀ
1.890(6), and B(4)ꢀP(4) 1.946(5). Selected angles are: at Pd(6), Pd(6?)ꢀ
B(5) 108.98(13)8 and P(6)ꢀH(6,2?) 88.8(19)8; at Pd(6?), B(5?)ꢀB(7?)
84.12(17)8, Pd(9)ꢀPd(6) 82.030(11)8, Cl(6,9)ꢀB(2?) 150.89(12)8, and
B(7?)ꢀB(8) 168.93(16)8; at Pd(9), P(9)ꢀCl(6,9) 89.39(4)8, Cl(6,9)ꢀB(10)
161.66(14)8, and Pd(6?)ꢀB(10) 109.00(13)8; and at Pd(9?), P(1)ꢀP(2)
99.70(4)8 and B(8?)ꢀB(10?) 84.08(17)8; with Pd(9)ꢀCl(6,9)ꢀPd(6?)
72.27(3)8. Note the bridging hydrogen atoms at B(5?)ꢀB(10?) and
B(7?)ꢀB(8?) on the right-hand sub-cluster, which are consistent with
arachno ten-vertex character [35], and the bridging hydrogen atoms at
B(2?)ꢀPd(6) and B(5?)ꢀB(7) on the open face of the central five-vertex
{Pd(6)ꢀB(2?)B(5?)ꢀB(7)ꢀPd(6?)} square pyramid, which, together with
/
B(5?) 1.805(6), B(5?)ꢀ
/
B(7)
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
the geometry, is consistent with nido five-vertex character. The
disposition of the bridging atoms around the open face of the left-
hand eleven-vertex sub-cluster, together with the overall geometry, is
consistent with nido eleven-vertex character.
framework IA essentially consists of a nido eleven-vertex
{Pd3B8} skeletal unit IB conjoined with an arachno ten-
vertex unit ID, their intimacy of conjunction is such that
they are effectively conjoined by and fused to a linking
nido five-vertex {Pd2B3} unit IC. The overall metalla-
borane skeleton therefore can be described in terms of a
core nido five-vertex {Pd2B3} unit that has opposing
triangular faces fused in common with an arachno ten-
vertex {Pd2B8} unit and a nido eleven-vertex {Pd3B8}
unit. The phenomenon of the intervening nido five-
vertex unit shows an interesting intermediacy between
the very condensed tetrahedral borons-only core in
globular species such as [(PPh3)3(PPh2)2Pd3B20-
H16Pd(PPh3)] [11], [(PMe3)2IrB26H24Ir(CO)(PMe3)2]
[14] and [(C5Me5)3Ir3B18H15(OH)] [26], and the much
more open type of inter-cluster linkage in species such as
The nido eleven-vertex {Pd3B8} unit of compound 2
has some parallels in the nido eleven-vertex {Pt3B8} sub-
cluster in [(PMe2Ph)4Pt3B14H16] (compound 3, sche-
matic II), although the relative dispositions of the three
metal centres differ between these two sub-clusters. As
far as we are aware, isolated three-metal nido eleven-
vertex {M3B8} single-cluster compounds have not been
reported, although examples of two-metal nido eleven-
vertex {M2B9} species are known [32(a),32(b)], and a
nido-shaped eleven-vertex {M2B9} feature a distinguish-
able in species such as in [(PPh3)2Cl2(Ph2PC6H4)-
Rh2SB9H8] [32c]. This last compound also exhibits a
four-electron chlorine bridge between its adjacent metal
atoms, similar to the {Pd(9)ꢀ
/
ClꢀPd(6?)} bridge in
/
compound 2 reported here. This is also very unusual
in boron-containing cluster chemistry, and, as far as we

M.G.S. Londesborough et al. / Journal of Organometallic Chemistry 657 (2002) 262ꢀ
/266
265
are aware, these are the only two examples. This four-
electron bridge is, however, related to the four-electron
{M(Ph2P)M} bridges seen in [(CO)(PPh3)Os(PPh2)Pt-
(PPh3)B5H7Ph] [33] and in the [(PPh3)3(PPh2)2Pd3-
B20H16Pd(PPh3)] species mentioned above [11], and we
have also recently observed other new {Pd(PPh2)Pd}-
bridging examples in our laboratories, in species tenta-
tively identified as eleven-vertex nido-type [(PPh3)2-
(PPh2)Pd2B9H8(OEt)2] and eight-vertex arachno-type
[(PPh3)2(PPh2)Pd2B6H9], which we hope to be able to
describe fully in due course [34]. The incidence of the
chlorine bridge in an ostensibly chlorine-free initial
reaction merits comment. It presumably arises from
subsequent halocarbon solvent interaction with an
initially formed reactive macropolyhedral palladabor-
ane, perhaps involving an inter-metal hydrogen
The identification of other products from this and
related systems involving further metal and ligand
variation may thence offer valuable ‘snapshots’ of
structural intermediates. In this context, we currently
devise experiments with this in mind, and we hope to be
able to report on some such intermediate condensation
products in due course. In sum, it is clear that even
relatively subtle variations of metal and ligand in these
arachno metallaborane species, as well as less subtle
ones [9,12], will yield an increased variety of new
macropolyhedral skeletal architectures, and well por-
tends much interesting mechanistic and structural devel-
opment in this contemporaneously burgeoning field [2ꢀ
/
4,38ꢀ42].
/
bridge*an effervescence was observed upon treatment
/
1. Supplementary material
of the reaction product mixture with CH2Cl2.
In the {Pd3B8} unit, the grouping of the three
palladium atoms into a string is also noteworthy. Such
metal-atom grouping also rare in larger metallaborane
cluster chemistry [24] and is reminiscent of ‘composite
cluster’ phenomena in species such as [(C5Me5)2-
Ir2CSB6H8] [35], in which boron atoms on one hand,
and heteroatoms on the other, group together in
separate domains within a single-cluster. This palla-
dium-atom grouping, together with the {Pd3} string in
globular [(PPh3)3(PPh2)2Pd3B20H16Pd(PPh3)] [11] and
the {Pd2} units in the more open nido-type [(PPh3)2-
(PPh2)Pd2B9H9(OEt)2] and arachno-type [(PPh3)2-
(PPh2)Pd2B6H12] species just mentioned, may indicate
a tendency for palladium atoms in particular to group
together, which may presage interesting future com-
posite-cluster species.
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 172803 and 172802 for
compounds [(PPh3)4ClPd4B16H17(PPh3)2] (1) and
[(PPh3)2PdB8H12] (2). Copies of this information may
be obtained free of charge from The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (Fax: ꢁ44-
/
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
Acknowledgements
We thank the UK EPSRC for support (grants J56929,
L49505 and M83360) and for a studentship to MGSL,
and Dr Jonathan Bould for stimulating discussions.
The arachno ten-vertex {Pd2B8} sub-cluster of com-
pound 2 (schematic ID) has clearer precedent, being
long known for the single-cluster species [(PMe2Ph)4-
Pt2B8H10] [25], and more recently established also for
the palladium system in the direct analogue
[(PMe2Ph)4Pd2B8H10] [36], as well as being recognised
for other {M2B8} systems [24], such as [(Et2NC-
S2)2Au2B8H10] [37]. Its incidence here in compound 2
may have some relevance to mechanism, as the
[(PMe2Ph)4Pt2B8H12] molecular unit is known to be
formed from [(PMe2Ph)2PtB8H12] under basic condi-
tions [24]. In general terms, however, the mechanisms of
formation of these highly condensed products from the
[(PMe2Ph)2PtB8H12] system [15,16] and, now, from the
corresponding [(PPh3)2PdB8H12] system, remain ob-
scure. Although there may be some significance in the
retention in compound 1 of two basic arachno-shaped
{B8} units that mimic the {B8} geometry in the
[(PPh3)2PdB8H10] starting compound 2, the incorpora-
tion of the two extra Pd centres in the product 2 remains
unexplained.
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