Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 145-148, 2005

ArticlesRecent Progress in the Chemistry of Rhenium Cluster Complexes and Its Relevance to theProspect of Corresponding Technetium Chemistry
Yoichi Sasaki*
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received: July 31, 2005; In Final Form: September 30, 2005
Metal-metal bonded cluster complexes represent one of the important groups of compounds of group 7 elements,
technetium and rhenium. While the chemistry of rhenium-rhenium bonded cluster complexes has developed
extensively in these years, corresponding chemistry of technetium remains largely unrevealed. A general rule toclassify the metal-metal bonded structural cores in terms of the number of d electrons, is presented, and existing
Re-Re bonded cluster complexes are summarized accordingly. Recent progress of hexarhenium cluster complexes
has been introduced as a typical example. From these overviews on the Re-Re cluster complexes, it is suggested
that the chemistry of Tc-Tc bonded cluster complexes is highly promising.

1. Introduction
Technetium and rhenium belong to group 7 elements and theirchemistry is expected to be similar to each other in many
respects.1−3 While the chemistry of rhenium coordination compounds developed extensively in these years, corresponding
chemistry of technetium still remains largely unrevealed due todifficulty in handling of and in accessibility to this radioactive
element. In order to discuss the prospect of still unrevealed technetium chemistry, it is very useful to survey current status ofrhenium chemistry.
Rhenium that is located just below technetium in the periodic
table, has the most versatile coordination chemistry among transition elements. This is because rhenium, as a member of group
7 elements in the middle of transition metals in the table, shows
characteristics of both early and late transition metal complexes.
While rhenium tends to form direct metal-metal bond to givevarious cluster complexes, numerous stable monomeric complexes are also known.1−3
Rhenium takes wide range of oxidation states from +1 to +7
under mild conditions. Various air stable complexes are known
in the oxidation states of +1, +3, +4, +5, and +7. Thus rhenium
takes the widest range of stable oxidation states among thetransition elements. For example, with a rather common tripodal ligand, tris(2-pyridylmethyl)amine (tpa), rhenium gives
stable complexes in all these oxidation states with the cooperation of appropriate co-ligands. The typical tpa complexes are,[ReI (CO) 3 (κ3-tpa)] +,4 [ReIIICl 2 (tpa)] +, 5 [ReIV2 (µ-O) 2 (tpa) 2] 4+,6
[ReVO(OCH 2CH 2O)(κ3 -tpa)] +,7 and [ReVII (O) 3 (κ3 -tpa)] +.7 A
similar hexadentate polypyridyl-amine ligand, N,N,N’,N’-tetrakis(2-pyridylmethyl)ethylenedimanine (tpen) gives unusual
seven-coordinate species, [ReIVO(tpen)] 2+ and [ReVO(tpen)]3+.8
This ligand takes a unique tridentate mode with ReVII (O)3.9, 10
These complexes are only limited examples showing the versatility of rhenium coordination compounds. Such structural
versatility should be inherently found in technetium chemistry,
although the research is limited so far.
Here I will focus on metal-metal bonded cluster compounds,
because, while such compounds represent significant part of
the rhenium chemistry, corresponding technetium compounds
have not been paid much attention so far. A general idea ofsummarizing the metal-metal bonded cluster complexes will be*Corresponding author. E-mail: ysasaki@sci.hokudai.ac.jp.
FAX: +81-11-706-3447.

given first, and then the current status of the area of rhenium
and technetium cluster complexes will be briefly surveyed.
Then, as a typical example of the progress of rhenium clustercompounds, octahedral hexarhenium complexes will be
reviewed. From these brief summaries, I will try to show how
the chemistry of technetium cluster complexes would be versatile and promising area.
2. Structures of Rhenium-Rhenium Bonded Cluster Complexes
Rhenium-rhenium bonded cluster complexes are known for theoxidation states from +2 to +6. Core structures of such clustercomplexes depend on the number of d electrons and therefore theoxidation states. Since each Re-Re bond is formed by the donation of one d electron each from Re atoms, the maximum number
of Re-Re bond formed by each Re atom depends on its d electron
number. For a multiple bond, the number may be counted as its
bond order; for example, for the Re-Re quadruple bond, the number of Re-Re bonds is counted as four.11 For the multiply bonded
dimers, one σ, one or two π, and one δ bonds are possible, energyof the σ, π, and δ bonds decreasing in this order.
Figure 1 shows the core structures of some typical metalmetal bonded cluster complexes associated with d electron
numbers. Although the correlation in Figure 1 only provides asimplified view, it would help to summarize systematicallyvarious types of metal-metal bonded cluster core structures.
For a series of metal-metal single bonded clusters, metal ionswith d 2 , d 3, and d 4 electron numbers form the metal-metal
bonded core structures with triangle, tetrahedral, and octahedral shapes, respectively.11 Figure 2 shows the existing examd1
M

M

d2
M

M

M
M

d4

d3
M

M
M

M

M

M

M

M

M

M
M

M

d5

M

M

M
M

M
M

M

M

Figure 1. Correlation between d-electron number and typical M-M
bonded cluster core structures.

© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

146 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
Re(VI) d1
O
Me
Me

H2 O
C

Re

Re
O

Re(IV) d3

Re(V) d2
O
N

Me

O

Re
N

Me

N

N

N

N

N

Re
O

N

N

Re
N

O

N

O

Re
O

Re(III) d4

N

N

Cl

N

Cl

Cl

Re

Re

Cl

Cl

S

Cl

Cl

S
Ph3P Re
S

Ph3P

Re
Cl

Cl

Re

Re
S

S

S

Re

PPh3

S

Cl
S

Re

PR3

Cl

Re

Re

3P
PRR
3

Cl

Tc(VI) d1
N

PR3

O

Tc

Tc(IV) d3

N

Tc
O

Tc(V) d2

Tc(III) d4

O

Tc

Tc
O

Cl

Cl

Cl

Tc

Tc

Cl

Cl

Tc(II) d5

Cl

Tc

Tc

Cl

Cl

Re

Re

S

Re(II) d5

Cl

Cl
Cl

Sasaki

Cl

Cl

Cl

Re

Re
Cl

Cl

Cl

Figure 3. Correlation between d-electron number and the structureof some Tc-Tc bonded cluster complexes.

Figure 2. Correlation between d-electron number and the structureof some typical Re-Re bonded cluster complexes.

ples of Re cluster complexes. It is seen that the general viewof the basic structural cores is successfully applied to Re
chemistry. For example, a trivalent Re ion has four d electronsand can form dimers with a quadruple bond, or octahedral
hexarhenium clusters where each Re ion forms four Re-Re single bonds with four neighboring Re atoms. Other type of clusters like triangular trimers with three Re-Re double bonds isalso known. Tetravalent Re with d 3 electronic configuration
may form Re-Re triple bonded dimer or tetrahedral tetramer
with six Re-Re single bonds. It should be noted that the bondorder of edge-shared dimers of a d3 metal is complicated as theorder of δ and δ* orbitals are often in the reverse order (δ >δ*) depending on the interaction with the bridging ligands.6, 12
Pentavalent Re with d2 electonic configuration prefers to takemonomeric oxo complexes, but Re-Re double bonded dimer isalso known.13 Triangular trinulcear complex with two Re-Re
single bonds is another choice of cluster structure. The Re-Re
single bonded dimer is the only structure for d1 Re(VI).14 An
example of the dimer given in Figure 2 adopted the extreme
expression that assumes highly basic ligand CH22− coordinates
to Re(VI). The alternative way of considering covalent bond
between Re and CH2 is of course possible.
For the metal ions with the number of d electrons larger
than 4, dimers of reduced bond order may be formed, since
additional d electrons would occupy anti-bonding orbitals of
metal-metal bonds.
It should be noted that overall structure is completed by thecooperation of appropriate ligands that stabilized the given
oxidation state. For the higher oxidation states, highly basic
ligands such as O2− and N3− should be required. Chemistry of
tri-, tetra-, and hexanuclear calcogenido-bridged clusters with
Re-Re single bonds have developed considerably in the lastdecate.15−17 Mixed valent cluster complexes are often foundwhere the bond-order as summarized above have to be modified adequately.
3. Structures of Technetium-Technetium Bonded Cluster
Complexes
Figure 3 shows some technetium cluster complexes which
are summarized based on the general scheme given in Figure
1. It is obvious that Tc-Tc bonded cluster chemistry is so far
dominated by only dimeric complexes. Thus, Tc-Tc dimers
with single to quadruple bonds are known. The bond-orderdepends on the number of d electrons. The hexavalent Tc
forms Tc-Tc single bonded dimers that are stabilized by thecooperation of highly negative nitride ligands (Tc ≡ N).18 So
far, tri-, tetra- and hexamers with metal-metal single bonds are
not very common for Tc.19 This is unlikely to be the reflectionof intrinsic nature of technetium, but rather such compounds
have not yet been the targets of extensive study. Namely, sulfur bridged cluster complexes that characterize the tri-, tetra-

Figure 4. Simplified MO characteristics of hexarhenium(III) core.

and hexarhenium cluster chemistry, have not yet been studied
extensively for technetium.
4. Some Recent Developments in Octahedral Rhenium Cluster
Complexes
Among various Re cluster complexes, hexarhenium(III)complexes have been studied most extensively in these years.17
The complexes with an octahedral Re6 (µ3-E) 8 (E = S, Se, Te)core show various interesting properties which would likely beobserved for yet unexplored analogous hexatechnetium complexes. The structure of the hexarhenium core is seen in the d4part of Figure 2, in which six ligands oriented outward arecalled as terminal ones, and the eight µ3-ligands occupy capping sites. Since various characteristic properties of hexarhenium complexes described herein are a reflection of theelectronic structure of the hexameric structural core, simplified views of the molecular orbital studies as illustrated in
Figure 4 are considered first.20−23 Octa(calcogenido)hexarhenium(III) core is featured by strong bonding orbitals of the ReRe σ bond which are in deep bonding oribital level. The
HOMO region is mainly composed of capping and terminal
ligand filled orbitals. Orbitals in the LUMO region are mainlycomposed of empty metal orbitals which are basically of πcharacter with regard to the metal-metal bond. Therefore thehexarhenium core is considered as a poor π-donating andrather good π-accepting moiety.
The hexarhenium(III) complexes may be regarded as a giant
octahedron, by considering the Re6 (µ3-E) 8 core as a metal ion
and each terminal site (gray circles in the Figure) as a ligandof simple mononuclear metal complexes. By referring various
aspects of mononuclea r met a l complexes, some new
approaches to the hexarhenium cluster chemistry have beenmade. The hexarhenium core is extremely stable. The ligand
substitution reactions of the terminal sites are very slow andrequire higher temperature (> 100 ˚C) to cause the substitution
efficiently. Terminal sites can be substituted in a stepwisemanner by various ligands such as halide ions, CN−, NCS −,pyridyl ligands, phosphine ligands, CH3CH. DMSO, and soon. A series of the mixed terminal ligand complexes such as[Re6 (µ3-E) 8 (X) 6-n (L) n](4−n) − (L = P(C2H5)3, E = S, n = 2–6; E =Se, n = 3–6;24, 25 L = pyridine (py) and some pyridine deriva-

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 147

Recent Progress in the Chemistry of Rhenium Cluster Complexes
26, 27

tives, E = S, n = 2–4)
were prepared. Geometrical isomers
were also isolated for the complexes with n = 2–4. By using along chain chelating ligand (C6H 5) 2P(CH 2) 6P(C6H 5) 2 (dpph),
the formation of a novel giant chelate ring was found in a seriesof the complexes, [Re6 (µ3-Se) 8I6-2n (µ-dpph)n](2n−4)+ (n = 2–4).28
A bulky porphyrin ligand 5-(4-pyridyl)-10,15,20-tritolylporphyrin can be successfully introduced into all of the six terminal sites of the hexarhenium(III) core.29 A kinetic study ofligand exchange reactions of the mixed ligand system, cis- andtrans-[Re6 (µ3-S) 8Cl4(py) 2] 2−, with py-d5, showed selective substitution of py ligands but not Cl−.30 The results are in sharp
contrast to some mononuclear complexes with dπ rich metal
ions like Co(III) where less basic Cl− should be the ligands thatare preferentially substituted. The poor π-donating and rather
good π-accepting character of the hexarhenium(III) coremakes stronger coordination with Cl− as compared with py.
The hexarhenium(III) clusters undergo one-electron oxidation to the Re6 (III,III,III,III,III,IV) oxidation state where thehalf-filled orbital is considered to be delocalized over theentire hexarhenium core. X-ray structural analyses of oneelectron oxidized species have revealed Jahn-Teller distortionof the hexarhenium core.31 The one-electron oxidation potentials shift to positive direction with an increase in the numberof pyridine ligands; the redox potentials of [Re6 (µ3-S) 8Cl6] 4−,[Re6 (µ3-S) 8Cl4 (py) 2] 2− (both cis and trans), and mer-[Re6 (µ3S) 8Cl3 (py) 3] −, being +0.31, +0.77, and +0.97 V vs Ag/AgCl,
respectively.26 Thus the coordination of pyridine makes thehexarhenium clusters more difficult to oxidize. This fact may
be explained by considering that pyridine is a less effective πdonating ligand than Cl− is and reduce the electron density of
the core. Another interesting feature is the splitting of theligand reduction waves of [Re6 (µ3-S) 8Cl4 (L) 2] 2− (both cis andtrans) (L = pyrazine, 4-cyanopyridine). The splitting indicatesthe ligand-ligand electronic interaction through the giant
hexarhenium(III) core. This is remarkable since simple mononuclear metal center is not as effective as in this case for the
splitting of ligand redox waves.23 When six redox active ligands
are introduced into the six terminal sites of the hexarhenium(III),the ligand redox waves split into six in some cases. The ligandligand electronic interaction may be mediated through π-type
LUMO of the hexarhenium(III) core.
The most significant aspect of the hexarhenium(III) complexes is their strong luminescence with relatively long lifetime(in the order of µs).32 The emissive properties are reserved bychanging the terminal and capping ligands. Emissions areobserved also for other d4 octahedral complexes such as Mo(II)and W(II).33 This fact indicates that corresponding hexanulcear Tc(III) clusters would also be strongly luminescent .
All the interesting features of the hexarhenium cluster complexes mentioned here could be inherently observed for analogous hexatechnetium complexes in somewhat modified way,
and indicate potential versatility of technetium cluster chemistry. It should be noted that the MO characteristics (poor πdonor and good π-acceptor) as mentioned above could be acommon feature of metal cluster complexes particularly thosewith metal-metal single bonded ones. Therefore the knowledge of various characteristic properties of hexarhenium complexes is useful for the understanding of the properties of othertype of cluster complexes.
5. Similarity and Differences between Rhenium and Technetium Complexes
In general, second transition metals and their congener ofthird transition series give structurally similar complexes, andthe basic structures found in rhenium are very likely seen also
in technetium. The most significant difference in the structurally analogous rhenium and technetium complexes would befound in their redox potentials. From the general trends foundin the pairs of complexes in other groups such as Mo-W, RuOs, and Rh-Ir, it is expected that a technetium complex would
show redox potentials more positive by ca. 0.5 V than its rhenium analogue. Thus lower oxidation state is more stabilized intechnetium. Ligand substitution lability would be expected tobe somewhat higher in Tc than corresponding Re compounds.
In summary, rhenium cluster chemistry is now expanding
significantly and its versatility and significance have increasingly verified. There should be a wide variety of technetium
cluster chemistry to be explored, significantly contributing not
only to basic transition metal chemistry, but also to wider
potential applications. Technetium cluster chemistry should
certainly be a highly promising area.
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