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Analysis of the Bonding in [Ni5(5-S)(2-SR)5] an Unprecedented Pentanuclear Sulfide Cluster.

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Analysis of the Bonding in
N&d"2-SR)5I
-9
an Unprecedented
Pentanuclear Sulfide Cluster **
Fu-Wah Cheung and Zhenyang Lin*
In the past decade, much progress has been
made in the synthetic chemistry of metal
chalcogenide clusters." - These clusters can
act as model compounds for the active sites of
certain metalloproteins and metal sulfide catal y s t ~ . [ ~They
]
also exhibit many interesting
structural types and bonding modes. Chalcogen atoms in metal chalcogenide clusters can
occupy terminal or bridging positions (pn,
n = 2-6). The existence of numerous coordination modes of chalcogenide ligands gives rise
to the question whether they are part of the
polyhedral framework. In other words, the
electronic structures of these clusters is difficult
to describe. As Dance and Fisher noted in their
review,"] the diversity of compositions and
structures known for metal chalcogenide clusters makes the establishment of a relationship
between the observed geometry and bonding
theory a true challenge.
Such relationships have been established for
Figure 3. Ball-and-stick (top) and space-filling (bottom) representation of the crystal structure of 3
boranes and metal carbonyl clusters: in the
viewed along the axis (left) and from the side (right); hydrogen atoms omitted for clarity.
1970s, Williams and Wade[8-91 formulated
well-known n + 1 rule, which describes the relationship between the structure of boranes and carboranes and
191 Crystal data for 3: C,,H,,N,,S6.4CH,CN; diffractometer and data collection: STOE IPDS (200 K), graphite-monochromated Mo,. radiation
the number of skeletal electrons, Subsequently, Wade and Min( A = 0.71073 A), monoclinic, space group C2/c, a = 33.748(8), b = 16.836(9),
gos[9- 111 recognized the similarities between the bonding in
c = 14.665(4)A, fl = 106.52(2)", V =7988(5) A', 2 = 4, p = 0.234 mm-',
these main group clusters and that in many transition metal
~(000)= 34OO,p,,,,, =1.345 Mgm-', 2 0 , ~=
~
-26<h141, -2o<k117,
- 1611515. Structure solution and refinement: A total of 6498 reflections
carbonvl clusters. Since then. the geometries of these various
were collected, 4391 independent [R(int) = 0.08361, of which 3799 were used to
clusters have been systematized by a set of rules known collecfit 413 parameters with 4 restraints. The structure was refined against F 2
tively as the polyhedral skeletal electron pair theory (PSEPT),
(full-matrix least squares), GOF(FZ= S) =1.065, R1 = 0.1480 [Rl = 0.2324
which describes the relationship between the skeletal structure
(all data)], wR2 = 0.3860 [wR2 = 0.461 1 (all data)], max./min. residual density: +0.896/ - 0.399 e k ' . All atoms in rings (except for central pyrimidine
and the total number of valence electrons in clusters.["] Other
ring) are calculated as pertaining to a regular hexagon. All hydrogen positions
rules have also been put forward to account for these structural
are calculated. Crystallographic data (excluding structure factors) for the strucrelationships.["* 131 While these rules have proved helpful in
ture(s) reported in this paper have been deposited with the Cambridge Crystalunderstanding the structure and bonding of transition metal
lographic Data Centre as supplementary publication no. CCDC-100400.
Copies of the data can be obtained free of charge on application to The Direccarbonyl clusters, they can not readily be extended to metal
tor, CCDC, 12 Union Road, Cambridge CBZIEZ, UK (fax: int. code
chalcogenide clusters.' 14] For example, the recently synthesized
+ (1223)336-033, e-mail: deposit@chemcrys.cam.ac.uk).
pentagonal cluster 1 formally contains nickel atoms of oxida[lo] D. J. Williams, A. M. Colquhoun, C. A. O'Mahoney, J. C h e m SOC.Chem.
tion state + 1.2. A formal nickel
Commun. 1994, 1643.
oxidation state of + 1.6 is found
tBu
in [Ni,,Se, ,(SeMe)
- ,r31 although this cluster contains simiNI-:----NI.
1BU
;s'
\ / ;'
lar Ni,(p,-Se) structural units. A
\
,*s,,:
simple description of the bonding
,.,.
in these compounds was regarded
as impossible.[31
,/*N'\s
Here we describe a simple
tBu
rBu
bonding model, based on the or1[Ni,(ps-S) (p2-SBu)]4 0 ,
/"\
1-
i '.. 1 7
[*I
Dr. Z. Lin, F.-W. Cheung
Department of Chemistry
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon (Hong Kong)
Fax: Int. code +2358-1594
e-mail: chzlin@usthk.ust.hk
[**I This work was supported by the Research Grants Council of Hong Kong. We
would like to thank Dr.Virginia Anne Unkefer for carefully reading and checking the manuscript.
Angew. Chem. In[. Ed. Engl. 1997,36,No. 17
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
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bital interaction concept,[' 5 1 for the unprecedented pentagonal
cluster 1. We hope that this analysis will serve as a foundation
for a similar orbital interaction concept that will explain the
bonding situation in all Ni-S and Ni-Se clusters.
On examining the structure of the [Ni,S(StBu),]- cluster, one
can see that each nickel metal atom is bonded to two tert-butyl
sulfide ligands. In other words, [Ni,S(StBu),]- can be regarded
as being derived from five ML, fragments (M = Ni, L = tBuS
ligand) and a ps-S capping atom. The molecular orbitals of an
ML, fragment are shown in Figure l a . The frontier orbitals of
the fragments that are involved in the metal-metal interaction
are depicted with their orbital lobes. Because of the high 3 d - 4 ~
promotion energy, the contribution of the p, orbital to the
metal-metal bonding is ignored in the following discussion of
the orbital interactions for the sake of simplicity. Figure I b
tween the pentagonal [Ni,(p,-StBu),] fragment and the capping
p,-sulfur atom, three ( a , +el) of the five metal-metal bonding
orbitals stabilize the three p orbitals of the capping sulfur atom,
and three additional antibonding orbitals are generated (see
Figure 1c). The e2 orbitals remain nonbonding as there are no
orbitals of the same symmetry on the capping atom. A preliminary MO calculation['61 on the model compound [Ni,(p,-S)(pz-StBu),]- showed that the e2orbitals are indeed the HOMOs
(Figure 2). The HOMO-LUMO gap is 0.28 au, which is remarkably large.
Figure 2. Contour maps of the two HOMOs (e2) of the [Ni,(S(SH),]- cluster complex. The contours of the plots decrease by a factor of 1/2, and the Iowest contour
is 0.000976563.
(a)
(b)
(C)
(4
Figure 1. MO diagrams for [Ni,S(SrBu);] in which SrBu groups are represented by
X. a) Frontier orbitals of an ML, fragment; b) orbitals of Ni,X,; c) orbitals of
Ni,SX,; d) p valence orbitals of the capping p, sulfur atom.
shows the energy levels of the molecular orbitals (MOs) derived
from the linear combinations of five ML, fragment orbitals. In
the lower part of the MO diagram, 30 MOs are derived mainly
from the M-L 0 bonds (two per fragment)and the t,, and d,,
orbitals. These 30 orbitals are fully occupied and, therefore,
their contributions to the metal-metal bonding can be neglected. The interactions of the frontier orbitals of the five fragments
gives rise to five bonding (a, +el +e,) and five antibonding
(at e: + e t ) MOs. The five metal-metal bonding orbitals correspond formally to the five metal-metal edge bonds.
Figure IC shows the resulting MO energy levels for "is(p5-S)(pz-StBu),]-. In the point group CSvrthe orbitals of the
capping sulfur atom have the transformation properties of
al(p,) and e,(p,, p,) symmetries (see Figure 1d). The sulfur p,
orbital is of appropriate symmetry to interact strongly with the
a, orbital of the pentagonal fragment. Similarly, the p, and py
orbitals of sulfur are of appropriate symmetry to interact
strongly with the e, orbitals. As a result of the interaction be-
+
1848
0 WILEY-VCH Veriag GmbH, D-69451 Weinhelm, 1997
The [Ni,S(StBu),]- cluster has 70 valence electrons. Figure 1b shows that the 30 molecular orbitials are completely
filled with 60 electrons. These 60 electrons do not take part in
Ni - Scapping
and M -M interactions. Therefore, ten valence elecand M- M bonding. If these
trons are available for Ni - Scapping
extra ten valence electrons are excluded, one would expect each
Ni atom to have a d8 electron configuration. When the ten
valence electrons are added, six electrons are formally assigned
to the capping S2 ligand. The remaining four electrons, which
occupy the e2 metal- metal bonding orbitals, should be assigned
to the five Ni atoms. Each nickel atom therefore has formally
8.8 d electrons (d8.' configuration), and a formal mean oxidation state of + 1.2 can be assigned. As discussed above, because
the p, orbital of each Ni atom is hardly involved in the bonding,
the [Ni,S(StBu),]- cluster actually fulfills the 16-electron rule.
Based on the analysis of [Ni,S(StBu)J complex, we can now
describe the bonding in the novel selenolato-bridged nickel cluster 2 (Scheme 1).r31Thecentralunit ofthe [Ni,,Sel,(SeMe),o]2anion is a pentagonal Nil, antiprism in which the two pentagonal faces are capped by two p,-Se ligands. The remaining ten
nickel atoms form a ten-membered ring that encompasses the
pentagonal antiprism and in which the nickel atoms bridge the
edges along the prismatic axis. The ten p4-Se ligands cap the
square faces formed by the Ni,, metal core. Five of them are
above the ten-membered ring, and the other five are below the
ring (see Scheme 1). In such an arrangement, the central antiprismatic unit of the [Ni,,Se1,(SeMe),,J2- complex can be
regarded as a combination of two [Ni,(ps-Se)(pz-Se),1 units,
similar to the above-mentioned [Ni,(p5-S)(p2-StBu)J complex.
The outer Nil, ring is bridged by ten p,-SeMe groups, and
therefore each nickel atom in the Nil, ring is coordinated by two
SeMe- and two Se2- ligands in a distorted tetrahedral fashion.
If the metal-metal interaction between the two pentagonal
0570-0833/97/361?-1848 $17 SO+.SO/O
Angew. Chem. Int. Ed. Engi. 1997,36,No. 17
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respectively. This result supports the above conclusion that the
outer Ni atoms have a higher atomic charge than the inner ones.
The bonding similarity between the two Ni,(p,-Se) pentagonal
units of this Ni,, cluster and the [Ni,(p,-S)(p,-SR),]- cluster is
also supported by their similar net atomic charges. The net
atomic charge of the Ni atoms in the latter is 0.1 5.
In summary, contrary to the common belief that a simple
description of the bonding situation in the Ni-S( Se) clusters is
not possible, we have demonstrated that the cluster
[Ni,S(StBu),]- can be understood in terms of a schematic MO
diagram; [Ni,S(SBu'),]- has a total number of 70 valence electrons and thus obeys the 16-electron rule. Based on the mean
oxidation number of nickel atoms in [Ni,S(StBu),]-, [Ni2,Set,(SeMe),,]2- can be regarded as containing ten Ni atoms of
oxidation state + 1.2 and ten of oxidation state +2. We are
now examining other Ni-S(Se) clusters such as [Ni,Se4C1,(PEt,Me)6],[21 [Ni,S(SC,H,),]and [Ni,2(S2)6S8]3-[201
to
determine whether we can find a simple bonding description for
them.
+
n
Nizo metal core
+ 2 ps-Se
+ 10 p 4 - S e
+ 10 p 2 - S e M e
Received: January 21. 1997 [Z10017IE]
German version: Angew. Chem 1997, 109. 1933-1936
Keywords: bond theory
nickel S ligands
-
- clusters
mixed-valent compounds
-
I. Dance, K. Fisher, Progr. Inorg. Chem. 1994, 41, 637
D. Fenske, H. Krautscheid, M. Muller, Angen. Chem. 1992, 104,309; Angew.
Chem. Int. Ed. Engl. 1992, 31, 321.
D. Fenske, A. Fischer, Angew. Chem. 1995, 107, 340; Angeu. Chem. Int. Ed.
Engl. 1995, 34. 307.
D. Fenske, J. Ohmer, J. Hachgenei, Angew. Chem. 1985, Y7.993; Angeu. Chem.
Inr. Ed. Engl. 1985, 24, 993.
T. Kriiger, B. Krebs, G. Henkel, Angew. Chem. 1989, 101. 54; Angew Chem.
Int. Ed. Engl. 1989, 28, 61
A Miiller. G. Henkel, Chem. Commun. 1996, 1005.
The Biornorganrc Chrmisfry ofNickel (Ed.: J. R. Lancaster). VCH. Weinheim,
Scheme 1
1988.
units and that between the central Nil, antiprismatic unit and
the encompassing ten-membered ring are weak, one would expect that the bonding in the Ni5(p5-S) unit in the Ni,, cluster
should be similar to that of the [Ni,(p5-S)(p,-StBu),]- cluster.
In other words, if the three building blocks [two Ni,(p5-S) units
and one Nil, ten-membered ring] are simply connected by the
ten p4-Se bridging ligands and no metal-metal interactions
occur, an oxidation state of + 1.2 can be assigned to each Ni
atom of the central pentagonal antiprismatic unit. As
[Ni2,Se,2(SeMe)lo]2-contains 20 Ni atoms with a total charge
of + 32, each Ni atom in the ten-membered ring should have a
formal oxidation state of + 2.0. If we examine the structure of
[Ni,oSe,2(SeMe)to~Zin detail, this bonding description seems
quite plausible, as the Ni-Ni distances within each of the two
pentagonal Ni,(p5-Se) units (ca. 2.47 A) are much shorter than
those between the two Ni,(p,-Se) units (ca. 2.62
The NiNi distances within the Nil, ten-membered ring (ca. 2.57 A) and
between the Nit, ten-membered ring and the two pentagonal
Ni,(p,-Se) units (ca. 2.53 A) are also noticeably longer. The
assignment of a formal oxidation state of + 2.0 for each Ni atom
in the ten-membered ring is also reasonable, since several trinuclear Ni clusters, for example [N~,S(S~BU),(CN),]~-,[~~
[Ni,S(SMe),12 -,["I
[Ni,S(SCH,C,H4CH,S),)Z-,['81and
[Ni,S(SPh),]' , [ 1 9 ] contain tetrahedrally coordinated Ni" centers.
A preliminary ab initio calculation[161on [Nizo(ps-Se),(p4Se),o(pz-SeH)lo]-gave net atomiccharges of +0.12 and +0.23
for the inner Ni atoms [those in the two Ni5(p5-Se)pentagonal
units] and the outer Ni atoms (those in the ten-membered ring),
R. E. Williams, Adv. Inorg. Chem. Radrochem 1976, 18. 67
K. Wade, Adv. Inorg. Chem. Radiochem. 1976, 18, I
D. M. P Mlngos, Nature (London) Phys. Sci. 1972, 236. 99.
a) D. M. P. Mingos, R. L. Johnston, Slrucr. Bonding (Berlin) 1987, 68, 29;
b) D. M. P Mingos, T. See, Z. Lin, Chem. Re!,. 1990. YO, 383.
B. K. Teo. Inorg. Chem. 1984,23, 1251.
R. B. King, Chemical Application of Topological and Graph Theory, Elsevier,
Amsterdam, 1983.
P. Alemany, R. Hoffmann, J Am. Chem. SOC.1993, 115. 8290.
T. A. Albnght, J. K. Burdett, M. H. Whangbo, Orbital ln/craction.s in Chemisrr)., Wiley, New York, 1985.
Single-point calculations for [Ni5(p5-S)(p,-SH),]- with the experimentally determined geometry were performed at the Restricted Hartree-Fock level. The
basis set used for sulfur and hydrogen atoms was 6-31G'. For the nickel atom,
an effective core potential (ECP) with LANL2DZ basis set was employed. For
[Ni,,(ps-Se),(p4-Se),,(/r,-SeH),~]-, an ECP with LANL2MB basis set was
employed for both Ni and Se atoms.
G. Henkel, M. Kriege, K. Matsumoto, J. Chem. Soc. Dalton Trans. 1988, 657.
W. Tremel, B. Krebs, G. Henkel, Inorg. Chim. Artn 1983. 80, L31.
K. Matsumoto, H. Nakano, S. Ooi, Chem. Lett. 1988, 828.
C. L. Cahill, K. Tan, R. Novoseller, J. B. Parise, Chem. Commun. 1996,1677.
-
AngeM Chrm I n / Ed Engl 1997, 36, No 17
0 WILEY-VCH
Verlag GmbH, D-69451 Wemheim, 1997
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