Electronic properties of organometallic metal-benzene complexes [Mn(benzene)m (M = Sc-Cu)]

Article abstract of DOI:10.1021/om9807349

Neutral metal-benzene complexes, M_n(benzene)_m (M = Sc to Cu), are produced for all of the 3d transition metals in the gas phase by using the laser vaporization method. These species are characterized by mass spectrometry, photoioniza

Full text of DOI:10.1021/om9807349

1430
Organometallics 1999, 18, 1430-1438
Electr on ic P r op er ties of Or ga n om eta llic Meta l-Ben zen e
Com p lexes [Mn (ben zen e)m (M ) Sc-Cu )]
Tsuyoshi Kurikawa, Hiroaki Takeda, Masaaki Hirano, Ken J udai, Tadashi Arita,
Satoshi Nagao, Atsushi Nakajima, and Koji Kaya*
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi,
Kohoku-ku, Yokohama 223-8522, J apan
Received August 31, 1998
Neutral metal-benzene complexes, Mn(benzene)m (M ) Sc to Cu), are produced for all of
the 3d transition metals in the gas phase by using the laser vaporization method. These
species are characterized by mass spectrometry, photoionization spectroscopy, and chemical
probe experiments. Depending on the metal, there are two types of structures for
Mn(benzene)m: multiple-decker sandwich structures and metal clusters fully covered with
benzene molecules (rice-ball structures). The former sandwich structure is characteristic of
the complexes for early transition metals (Sc-V), whereas the latter is formed for late
transition metals (Fe-Ni). Electronic structures of M1(benzene)x (x ) 1, 2) complexes are
investigated through systematic measurements of ionization energies (Ei’s).
1. In tr od u ction
performed the CID experiments on the thermochemistry
of transition metal benzene complexes. Duncan and co-
workers have investigated dissociation processes of
metal ion-benzene complexes by laser photodissociation
spectroscopy.14-16 In their experiments, dissociative
charge-transfer processes are discussed in detail. Theo-
retical calculations have also been carried out. Langhoff
and co-workers17 have calculated binding energies for
all the 3d transition metal ions (M+) with benzene (Bz)
and have accounted for the effect of the electron cor-
relation between the metal atoms and the ligand.
However, almost all these studies have been restricted
to small cationic complexes denoted as M1(Bz)x+ (x ) 1,
2) due to the necessity for mass selection and to simplify
calculations.
In recent work, we have successfully synthesized
neutral 3d transition metal-benzene multinuclear com-
plexes by using the molecular beam method for vana-
dium (V)18 and cobalt (Co).19 In this article, through a
study of all of the 3d transition metal elements from Sc
to Cu, we will present evidence for two different
structures of (1) sandwich clusters and (2) rice-ball
clusters and discuss their formation mechanism. In
addition, we will systematically discuss the periodic
trends of ionization energies for the M1(Bz)x (x ) 1, 2)
Metal-ligand molecules have been a subject of many
studies in the past decade.1-6 Especially, complexes of
transition metal atoms and benzene molecules have
been the focus of this research as basic models for d-π
bonding interactions. Although many synthetic experi-
ments have been conducted on novel organometallics
in the condensed phase, various environmental factors
such as oxidation or reduction of the products make
these approaches problematic. Recently, the advent of
the laser vaporization method has caused rapid progress
in the study of the organometallic clusters in the gas
phase, because the gas-phase conditions can overcome
the environmental problems mentioned above. The
absence of solvent effects makes it possible to obtain
fundamental information on the chemical and physical
properties of the metal-molecule complexes, and sev-
eral groups have reported gas-phase studies of metal-
benzene complexes. Armentrout and co-workers7-9 have
reported extensive collision-induced dissociation (CID)
experiments which have revealed thermochemistry on
MLn+ complexes, where M and L are a metal atom and
a ligand molecule, respectively. By applying the laser
vaporization method, Freiser and co-workers10-13 have
(1) Kealy, T. J .; Pauson, P. L. Nature (London) 1951, 168, 1039.
(2) Miller, S. A.; Tebboth, J . A.; Tremaine, J . F. J . Chem. Soc. 1952,
632.
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A. Chem. Rev. 1982, 82, 499.
(4) Comprehensive Organometallic Chemistry; Wilkinson, G., Stone,
F. G. A., Abel, E. W., Eds.; Pergamon: New York, 1982; Vols. 3-6.
(5) Wadepohl, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 247.
(6) Braga, D.; Dyson, P. J .; Grepioni, F.; J ohnson, F. G. Chem. Rev.
1994, 94, 1585.
(11) Hettich, R. L.; J ackson, T. C.; Stanko, E. M.; Freiser, B. S. J .
Am. Chem. Soc. 1986, 108, 5086.
(12) Hettich, R. L.; Freiser, B. S. J . Am. Chem. Soc. 1987, 109, 3537.
(13) Afzaal, S.; Freiser, B. S. Chem. Phys. Lett. 1994, 218, 254.
(14) Willy, K. F.; Cheng, P. Y.; Bishop, M. B.; Duncan, M. A. J . Am.
Chem. Soc. 1991, 113, 4721.
(15) Willy, K. F.; Yeh, C. S.; Robbins, D. L.; Duncan, M. A. J . Am.
Chem. Soc. 1992, 96, 9106.
(16) Yeh, C. S.; Willy, K. F.; Robbins, D. L.; Duncan, M. A. Int. J .
Mass Spectrom. Ion Processes 1994, 131, 307.
(7) Ervin, K. M.; Armentrout, P. B. J . Chem. Phys. 1985, 83, 166.
(8) Chen, Y.-M.; Armentrout, P. B. Chem. Phys. Lett. 1993, 210, 213.
(9) Meyer, F.; Khan, F. A.; Armentrout, P. B. J . Am. Chem. Soc.
1995, 117, 9740.
(10) J acobson, D. B.; Freiser, B. S. J . Am. Chem. Soc. 1984, 106,
3900.
(17) Bauschlicher, C. W., J r.; Partridge, H.; Langhoff, S. R. J . Phys.
Chem. 1992, 96, 3273.
(18) Hoshino, K.; Kurikawa, T.; Takeda, H.; Nakajima, A.; Kaya,
K. J . Phys. Chem. 1995, 99, 3053.
(19) Kurikawa, T.; Hirano, M.; Takeda, H.; Yagi K.; Hoshino, K.;
Nakajima, A.; Kaya, K. J . Phys. Chem. 1995, 99, 16429.
10.1021/om9807349 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 03/17/1999

Metal-Benzene Complexes
Organometallics, Vol. 18, No. 8, 1999 1431
normalized ion intensity, I(ν), is obtained at each photon
energy as follows:
Idye FArF
I(ν) )
(1)
Fdye IArF
PIE curves were obtained by changing the photon energy
in the range 5.99-4.20 eV, and Ei’s the clusters were deter-
mined from the linear extrapolation of the final decline of PIE
curves. The typical uncertainty of the Ei’s was estimated to
be (0.05 eV. Some of the PIE curves have been already
reported for M1(Bz)2 (M ) Ti, V, and Cr).21 Dissociation
energies (D0’s) were evaluated by using these Ei values, along
with known D0 values for the corresponding cationic com-
plexes.9,17
F igu r e 1. Schematic of an experimental setup. Port 1:
benzene diluted with He. Port 2: reaction gas (CO or NH3)
diluted with He.
3. Resu lts a n d Discu ssion
complexes. Some of these results have been reported
previously.20 This work in the gas phase demonstrates
the potential for the synthesis of these novel network
organometallic clusters.
3.1. Geom etr ica l Str u ctu r es a n d Ion iza tion En -
er gies. Figures 2a-i show typical mass spectra result-
ing from ArF laser ionization of the Mn(Bz)m (M ) 3d
transition metals of Sc-Cu) clusters produced by the
foregoing procedure. The features of mass spectra will
be discussed in the following four parts: (a) Sc-V, (b)
Cr-Mn, (c) Fe-Ni, and (d) Cu.
2. Exp er im en ta l Section
Transition metal-benzene clusters, Mn(Bz)m (M ) 3d tran-
sition metal atoms from Sc to Cu), were produced by the
combination of the laser-vaporization method and a flow tube
reactor (FTR). A schematic diagram of an experimental setup
used in this experiment is shown in Figure 1, which is modified
from one reported previously.18-21 First, metal atoms (M) were
vaporized using the frequency-doubled output from a Q-
switched Nd3+:YAG laser (532 nm, ∼10 mJ /pulse) and were
cooled to around room temperature with an expansion of He
carrier gas (10 atm stagnation pressure). The laser light was
focused with a lens having f ) 60 cm onto a rotating and
translating metal rod which was purchased except for a
manganese (Mn) rod. The Mn rod was prepared electrochemi-
cally by the deposition onto a stainless steel rod. After the
growth of the clusters in a channel (2 mm diameter and 4 cm
length), benzene vapor (∼70 Torr) diluted with He carrier gas
(1-2 atm) was injected into the FTR. Metal-molecule binary
clusters thus generated were sent into the ionization chamber
through a skimmer (3 mm diameter). The clusters were ionized
by either an ArF excimer laser (193 nm; 6.42 eV) or the
frequency-doubled output of a tunable dye laser pumped by a
XeCl excimer laser (308 nm) in a static electric field. The
photoions were mass-analyzed by a reflectron time-of-flight
(TOF) mass spectrometer. To get information about the
structures of the clusters, chemical probe and photoionization
spectroscopic methods were employed. The Mn(Bz)m clusters
were further reacted with CO or NH3 gas inside the second
FTR which was mounted downstream from the benzene
addition port, and the adduct products were mass-analyzed.
In the measurement of the ionization energy (Ei) of the
complexes the photon energy was changed at 0.01-0.05 eV
intervals in the range 5.99-4.20 eV, while the abundance and
composition of Mn(Bz)m clusters were monitored by the ioniza-
tion of the ArF laser. Fluences of both the dye laser and the
ArF laser were monitored by a pyroelectric detector (Molectron
J -3) and were kept at ∼200 µJ /cm2 to avoid multiphoton
processes. To obtain photoionization efficiency (PIE) curves,
the ion intensities by the second harmonic of the tunable UV
dye laser (Idye) were plotted as a function of photon energy (ν)
3.1.a . Sc, Ti, a n d V. The mass spectra of early
transition metal-benzene complexes (Sc,20 Ti,21 and V18)
are shown in Figure 2a-c. Prominent peaks in each
spectrum occur at almost the same compositions, de-
noted as Mn(Bz)n+1 [henceforth (n, n+1)], although the
production efficiency for larger complexes depends on
the metal elements. The prominent peaks of (n, n+1)
remained unchanged in the mass spectrum, even if (1)
the concentration of benzene vapor was increased and
(2) CO reactant was also introduced. These results
indicate that all the metal atoms of (n, n+1) are
contained within the interior of the complexes, because
exterior metal atoms should react with CO. Further-
more, when the concentration of metal vapor was
increased, minor peaks denoted as (n, n) and (n+1, n)
appeared in the spectrum, but there were no peaks for
(n+2, n). These two kinds of clusters of (n, n) and (n+1,
n) showed adsorption reactions with CO, resulting in
(n, n) + 3CO and (n+1, n) + 6CO for V-Bz. These
results from the chemical probe experiments indicate
that the Mn(Bz)n+1 clusters (M ) Sc, Ti, and V) are
sandwich clusters in which metal atoms and benzene
molecules alternate in position. Namely, the superfluous
metal atom(s) over the (n, n+1) composition are exterior
atoms located at the end of the cluster. These (n, n) and
(n+1, n) clusters can reasonably be attributed to sand-
wich structures having one and two exterior metal
atom(s), respectively. Because there are two sites for the
exterior atoms, the (n+2, n) cluster is missing. Likewise
in the Vn(Bz)m experiment,18 the chemical probe experi-
ment leads us to the conclusion that the most likely
structure for Scn(Bz)m and Tin(Bz)m clusters is also a
multiple-decker sandwich (Figure 3a).22-26 Very re-
cently, Bowers and co-workers performed ion mobility
(22) Salzar, A.; Werner, H. Angew. Chem., Int. Ed. Engl. 1972, 11,
930.
(23) Duff, A. W.; J onas, K.; Goddard, R.; Kraus, H.-J .; Kru¨ger, C. J .
Am. Chem. Soc. 1983, 105, 5479.
with normalization to both the laser fluence (Fdye and FArF
)
and the ion intensities by the ArF ionization (IArF). Then, the
(20) Kurikawa, T.; Takeda, H.; Nakajima, A.; Kaya, K. Z. Phys. D.
1997, 40, 65.
(21) Yasuike, T.; Nakajima, A.; Yabushita, S.; Kaya, K. J . Phys.
Chem. A 1997, 101, 5360.
(24) Lamanna, W. M. J . Am. Chem. Soc. 1986, 108, 2096.
(25) Li, Q.-S.; Tang, A.-Q. Sci. China (B) 1989, 32, 897.
(26) J emmis, E. D.; Reddy, A. C. Proc. Indian Acad. Sci. (Chem. Sci.)
1990, 102, 379.

1432 Organometallics, Vol. 18, No. 8, 1999
Kurikawa et al.
F igu r e 2. Typical examples of mass spectra of the Mn(Bz)m complexes (M ) 3d transition metals: (a) Sc, (b) Ti, (c) V, (d)
Cr, (e) Mn, (f) Fe, (g) Co, (h) Ni, and (i) Cu). Mn(Bz)m is expressed as (n, m) in the figures, and asterisks (*) denote the
contamination peak of oxide.
Ta ble 1. Ion iza tion En er gies of Scn (Bz)m , Tin (Bz)m ,
a n d Vn (Bz)m Com p lexes in eV
composition
a
complexes
Scn(Bz)m
n
m
Ei
1
2
3
1
2
3
1
2
3
2
3
4
2
3
4
2
3
4
5.05(5)
4.30(5)
3.83(5)
5.68(4)
4.53(5)
4.26(5)
5.75(3)
4.70(4)
4.14(5)
Tin(Bz)m
Vn(Bz)m
a
The values in parentheses indicate an experimental uncer-
tainty; 5.05(5) represents 5.05 ( 0.05.
F igu r e 3. (a) Proposed structures for early transition
metals for Sc, Ti, and V; multiple sandwich. (b) Proposed
structures for late transition metals from Fe to Ni; rice-
ball.
feature for the multiple-decker sandwich structure. This
phenomenon has been theoretically elucidated29 by
DFT(B3LYP) calculations, showing that the ionization
occurs from a delocalized molecular orbital of metal-
metal character interposed by the π* orbitals of benzene.
Along the molecular axis of the sandwich cluster, one-
dimensional delocalization of the metal dδ(e2g) orbitals
occurs through the LUMO (e2g) of C6H6, giving a drastic
decrease in Ei.
To investigate the effect of other aromatic molecules
having a η6 ring, naphthalene (Np) and anthracene
(Ant) were reacted with the V vapor instead of Bz, where
the symbol η is conventionally used to signify how many
carbon atoms of the ring are bonded to the metal atom.
A typical mass spectrum of Vn(Np)m is shown in Figure
5. As found for Vn(Bz)m, prominent peaks in the spec-
trum are (n, n+1) for n ) 1-4, which seems to indicate
a series of multiple-decker clusters. In the mass spec-
trum of Vn(Np)m, however, many minor peaks not
experiments for the Vn(Bz)m clusters and reported that
they actually exist in the multiple sandwich structure.27
The Ei’s of Scn(Bz)m, Tin(Bz)m, and Vn(Bz)m clusters
were measured by the photoionization method, and the
values are tabulated in Table 1. Figure 4 shows the Ei
values plotted against the number of metal atoms. The
Ei values drastically decrease with cluster size n,
although the clusters consist of metal atoms and ben-
zenes having relatively high Ei’s: Ei(Sc) ) 6.54 eV, Ei-
(Ti) ) 6.82 eV, Ei(V) ) 6.74 eV, and Ei(Bz) ) 9.24 eV.28
The trend of decreasing Ei is understood as a common
(27) Weis, P.; Kemper, P. R.; Bowers, M. T. J . Phys. Chem. A 1997,
101, 8207.
(28) (a) Moore, C. E. Analysis of Optical Spectra, NSRDS-NBS 34;
National Bureau of Standards, 1971. (b) Weast, R. C. Handbook of
Chemistry and Physics; CRC Press: Boca Raton, 1980, Vol. 61, p E-69.
(c) Robinson, J . W. Handbook of Spectroscopy; CRC Press: Boca Raton,
1974; Vol. 1, p 257.
(29) Yasuike, T.; Yabushita, S. Submitted to J . Phys. Chem. A.

Metal-Benzene Complexes
Organometallics, Vol. 18, No. 8, 1999 1433
in the bulk synthesis of Cr1(Bz)2. It should be noted that
the peak intensity of Cr1(Bz)2 and Mn1(Bz)2 was rela-
tively small and was estimated to be about 1/10 and
1/200 of V1(Bz)2, respectively, under similar production
conditions. In contrast to the mass spectra of Sc-, Ti-,
and V-benzene, the characteristic distribution indicat-
ing the multidecker sandwich structure was not seen
in the mass spectra of Cr- and Mn-benzene. Although
mass spectra were obtained by photoionization of 6.42
eV photon, the absence of the larger clusters is not
caused by their high Ei’s. There were also no larger
clusters in the cations which could be observed with
direct extraction into the TOF mass spectrometer
without photoionization. As reported previously,21 the
absence of larger clusters can be explained by electron
spin restrictions on the growth process. Using the
assumption of electron spin conservation, multiple-step
nonadiabatic transitions are required to produce the
Cr- and Mn-benzene complexes, because the metal-
benzene complexes generally prefer to occupy lower
electron spin states. This is in contrast to the high
electron spin states of the ground state of Cr and Mn
atoms. The large difference in electron spin between the
product complexes and the reactant metal atoms limits
the production of larger clusters. In other words, orga-
nometallic complexes of Cr and Mn might be produced
efficiently if the electronic excited metal atom having a
lower electron spin could be prepared. From the view-
point of preparing excited-state atoms, the laser vapor-
ization method is promising. The laser plasma makes
it possible to prepare various excited metal states
without fast relaxation by any environmental factors.
F igu r e 4. Ionization energies of sandwich complexes of
Scn(Bz)n+1, Tin(Bz)n+1, and Vn(Bz)n+1 plotted against the
number of metal atoms from n ) 1 to 3. (0, Sc-Bz; b, Ti-
Bz; 9, V-Bz).
F igu r e 5. Typical examples of the TOF mass spectra of
the Vn(Np)m clusters. Vn(Np)m is expressed as (n, m) in the
figure.
The Ei’s of Cr1(Bz)1, Cr1(Bz)2, and Mn1(Bz)2 were
obtained as 5.13, 5.43, and 4.28 eV from the measure-
ment of the ionization thresholds. For Mn1(Bz)1, the Ei
could not be measured due to the poor abundance of this
complex. The details of these values are discussed later
in section 3.2.
Ta ble 2. Ion iza tion En er gies of V1(Bz)2, V1(Np )2,
a n d V1(An t)2 Com p lexes in eV
ligand
Ei[ligand]a
Ei[V1(molecule)2]b
benzene
naphthalene
anthracene
9.24
8.13
7.41
5.75(3)c
5.40(5)d
5.37(5)d
3.1.c. F e, Co, a n d Ni. Late transition metal-benzene
complexes of Fe, Co,19 and Ni were also produced. Figure
2f-h shows the photoionization mass spectra of Mn(Bz)m
complexes under relatively high concentration of ben-
zene vapor. Even if the concentration of benzene was
increased, the mass spectrum remained unchanged. As
shown in Figures 2f-h, Mn(Bz)m clusters exhibit the
specific maximum m value (mmax) for each n. For Co-
Bz, the prominent peaks of (1, 2) and (2, 3) are followed
by those of (3, 3), (4, 4), (5, 4), (6, 4), and so on. This
behavior in the mass spectrum is completely different
from that of the multiple-decker sandwich complexes
seen for early transition metals. The Mn(Bz)m complexes
at n ) 3 and 4, for example, take mmax ) 3 and 4,
respectively, while the sandwich complexes take m ) 4
and 5.
a
b
References 28b and 30. The values in parentheses indicate
an experimental uncertainty; 5.75(3) represents 5.75 ( 0.03.
d
c Reference 18. This work.
corresponding to the (n, n+1) series also appear in the
spectrum. Since a naphthalene molecule itself provides
four sites on both sides of two η6 rings, these minor
peaks presumably correspond to clusters having exterior
metal atoms. For naphthalene and anthracene com-
plexes, the Ei’s of V1(Np)2 and V1(Ant)2 were determined
to be 5.40 ( 0.05 eV [henceforth 5.40(5) eV] and 5.37(5)
eV, respectively, as listed in Table 2. Although the Ei’s
of benzene, naphthalene, and anthracene are 9.24, 8.13,
and 7.41 eV, respectively,28,30 the difference among Ei’s
of V1(Bz)2, V1(Np)2, and V1(Ant)2 are rather small (Ei-
[V1(Bz)2] ) 5.75(3) eV). Thus, it is reasonably presumed
that their Ei’s are predominantly determined by a local
interaction between the d electrons and the π electrons
in the sandwich complexes.
When the magic-number complexes having mmax were
exposed to the reactant gas NH3, all of them were
unreactive for Fe-Bz, Co-Bz, and Ni-Bz. In contrast
to this, other small mass peaks of Co-Bz, such as (n,
m) ) (1, 1), (2, 2), (3, 2), and (4, 3), are depleted
completely by the reaction with NH3, and instead mass
peaks corresponding to the adduct of Con(Bz)mNH3 are
formed. This behavior of Con(Bz)m cannot be explained
by sandwich structures, because the sandwich com-
plexes with formulas of (3, 3), (4, 4), and so on should
3.1.b. Cr a n d Mn . For Cr and Mn, only the (1, 2)
complex was produced (Figure 2d,e). The structure of
these clusters is reasonably the sandwich, as reported
(30) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata,
S. Handbook of HeI Photoelectron Spectra of Fundamental Organic
Molecules; J apan Scientific Societies Press/Halsted Press, Tokyo, 1981.

1434 Organometallics, Vol. 18, No. 8, 1999
Kurikawa et al.
Ta ble 3. Com p a r ison of Tota l Va len ce Electr on s
(VEs) of Con (CO)k a n d Con (Bz)m Com p lexes
have an exterior Co atom which is expected to react with
NH3. Considering that (1) every number of Co atoms
(n) has a specific mmax and that (2) they have no exterior
Co atoms, the most plausible structure of Con(Bz)m is
one with Con clusters covered with benzene molecules
(m): a rice-ball structure with laver as shown in Figure
3b.19
For Ni-Bz, the mass spectrum of Nin(Bz)m complexes
was shown in Figure 2h. Although the behavior in the
mass spectrum is essentially the same for Co-Bz and
Ni-Bz, the Ni-Bz complexes occasionally bind two
benzene ligands even under saturation conditions at n
) 2, 3, 6, and 10. At n ) 2, for example, the two peaks
of (2, 2) and (2, 3) were observed, and their intensity
ratio was almost constant with higher concentration of
benzene. Although there is ambiguity of mmax, the mmax
numbers for each n are very similar to those of Con(Bz)m.
Thus, this behavior seems to indicate that these Nin-
(Bz)m complexes also take the rice-ball structure in
which the Nin clusters are fully surrounded by benzene
molecules.
+
+
+
+
Con(CO)k
(n, k)+ a
Con(Bz)m
expected
number mE
b
+ c
total VEs
(n, mmax
)
total VEs
(2, 8)+
33
46
59
72
85
2(3)
3(4)
4
4(5)
5(6)
6(7)
6(7)
(2, 3)+
(3, 3)+
(4, 4)+
(5, 4)+
(6, 4)+
(7, 4)+
(8, 5)+
36
45
60
69
78
(3, 10)+
(4, 12)+
(5, 14)+
(6, 16)+
(7, 19)+
(8, 20)+
100
111
87
102
a
b
Reference 31. Expected number of benzene molecules, mE,
+
is obtained from the optimum number of VEs in Con(CO)k
c Observed complexes having the maximum number of benzene,
mmax
.
.
Ta ble 4. Com p a r ison of Tota l Va len ce Electr on s
+
+
(VEs) of Nin (CO)k a n d Nin (Bz)m Com p lexes
+
+
Nin(CO)k
(n, k)+ a
Nin(Bz)m
expected
number mE
b
+ c
total VEs
(n, mmax
)
total VEs
(2, 9)+
37
45
59
73
85
3
(2, 3)+
(3, 3)+
(4, 3)+
(5, 3)+
(6, 4)+
(7, 4)+
(8, 4)+
(9, 4)+
(10, 5)+
37
47
57
67
83
(3, 8)+
2(3)
3(4)
4
4(5)
5
5(6)
5(6)
6
(4, 10)+
(5, 12)+
(6, 13)+
(7, 15)+
(8, 16)+
(9, 17)+
(10, 18)+
For Fe-Bz, peaks denoted as (1, 2) and (2, 3) were
mainly observed together with weaker peaks of (3, 3),
(4, 3), (5, 3), (6, 4), and (7, 4). As in the cases of Co-Bz
and Ni-Bz, it seems reasonable that the peaks of n g
3 represent a structure fully covered by benzene mol-
ecules, i.e., the rice-ball structure, because the patterns
in their mass spectra are almost the same.
99
93
111
123
135
103
113
129
a
b
References 32 and 33. Expected number of benzene mol-
ecules, mE, is obtained from the optimum number of VEs in
+
.
c Observed complexes having the maximum number of
In this rice-ball structure, mmax should be governed
by electronic and/or geometric factors. As is well-known
in organometallic compounds, an electron counting rule
can predict the optimum composition for metal-
molecule complexes. Ferrocene [Fe1(cyclopentadiene)2]
and bis(benzene) chromium [Cr1(Bz)2] are well-known
to satisfy an 18-valence-electron (VE) rule. For multi-
nuclear systems, reactions of metal clusters with CO
provide useful information on the electronic stability of
the clusters.31-33 In these experiments, a mass-selected
metal cluster is reacted with excess CO to determine
the saturation coverage. The number of adsorbed CO
molecules thus obtained directly reflects the maximum
number of VEs that can fill in the valence orbitals of
the complex, because CO molecules have essentially no
steric interactions with each other. By comparing the
number of VEs of carbonyl and benzene clusters, the
stability of Mn(Bz)m clusters can be discussed from an
electronic point of view. According to Castleman and co-
Nin(CO)k
benzene, mmax
.
observed maximum number of added benzenes, mmax
,
starts to become less than the expected one, mE. Above
n ) 6, the value of mmax is 1 or 2 less than mE. That is
+
to say, the observed Con(Bz)m clusters do not neces-
sarily follow the electronic requirement of the metal
clusters.
For Ni clusters,32,33 a similar deviation from the
expected number of benzene molecules is also seen at
large n, which is summarized in Table 4. For Ni-Bz,
the value of mmax becomes less than that of mE at n )
5. These results indicate that, as cluster size increases,
mmax is governed not only by electronic structure but
also by the geometry of the corresponding metal cluster
(Mn). It is reasonable that the number of benzene
molecules is restricted by geometrical factors because
steric hindrance between benzene molecules becomes
crucial at large n. From the atomic radius of the dimer
and the bulk metal,28b the radii of Nin clusters are
presumed to be smaller than those of Con clusters. The
smaller core of the metal clusters induces the benzene
complex to be less stable due to the overlap of benzene
molecules at smaller n. This reasonably explains that
the deviation for Ni-Bz between mmax and mE is
observed at smaller n, compared to Co-Bz.
workers,31 each Con cluster has a maximum number
+
of CO molecules for adsorption. By considering CO and
benzene ligands as two- and six-electron donors together
with the nine d electrons of the Co atom, the valence
+
+
electrons of Con(CO)k and Con(Bz)m cations can be
counted as listed in Table 3. In the table, the expected
number of benzene ligands, mE, is also shown, which is
estimated from the optimum number of VEs in Con-
(CO)k+. Without geometrical restrictions, the Con+ clus-
ter would take the corresponding expected number, mE,
of benzene molecules on the basis of the electronic
requirement. As cluster size n increases, however, the
Ei’s of Con(Bz)m and Nin(Bz)m were also measured by
the photoionization method, and the values are shown
in Figure 6. The Ei’s of Nin(Bz)m decrease monotonically
as the cluster size n increases, while the Ei’s of Con(Bz)m
increase at n g 4. However, both Ei dependences of the
cluster size are less sensitive to the cluster size than
Vn(Bz)m. This result is reasonably attributed to the
differences in their structures; the sandwich structure
is prominent for V and the rice-ball structure dominates
for Co and Ni. With increasing the cluster size, the
(31) Guo, B. C.; Kerns, K. P.; Castleman, A. W., J r. J . Chem. Phys.
1992, 96, 8177.
(32) Fayet, P.; McGlinchey, M. J .; Wo¨ste, L. H. J . Am. Chem. Soc.
1987, 109, 1733.
(33) Vajda, S.; Wolf, S.; Leisner, T.; Busolt, U.; Wo¨ste, L. H.; Wales,
D. J . J . Chem. Phys. 1997, 107, 3492.

Metal-Benzene Complexes
Organometallics, Vol. 18, No. 8, 1999 1435
Ta ble 5. Ion iza tion En er gies of Meta l Atom s a n d
M1(Bz)x (x ) 1, 2) Com p lexes in eV
Ei[M1(Bz)1]b
this work
Ei[M1(Bz)2]b
this work
M
Ei[M1]a
refc
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
6.54
6.82
6.74
6.77
7.44
7.87
7.86
7.64
7.73
5.07(4)
5.10(4)
5.11(4)
5.13(4)
5.05(5)
5.68(4)
5.75(3)
5.43(2)
4.28(3)
5.18(5)
5.53(3)
5.86(3)
4.30(5)
5.50
5.45
>6.42
5.55(4)
5.99-6.42
5.63(3)
a
b
Reference 28. The value in parentheses indicates an experi-
mental uncertainty; 5.07(4) represents 5.07 ( 0.04. c Reference 35.
F igu r e 6. Ionization energies of Con(Bz)m and Nin(Bz)m
clusters measured by the photoionization method. The Ei’s
of Nin(Bz)m decrease monotonically with increasing cluster
size n, while the Ei’s of Con(Bz)m increase at n g 4. This
result is attributed to the difference in their electronic
structures.
sandwich structure makes the 3dδ electrons delocalized
along the molecular axis, whereas the rice-ball structure
forms only localized d-π interactions at the adsorption
sites of benzene. Indeed, the Ei difference between (6,
4) and Co6 or between (7, 4) and Co7 is less than 1 eV,34
which suggests that they have a common framework of
the Con cluster inside the rice-ball structure.
As for the different Ei’s of Con(Bz)m and Nin(Bz)m, the
energy difference in stabilization between neutrals and
cations should be taken into account. The Ei decrease
of Nin(Bz)m indicates that the amount of stabilization
of neutral Nin(Bz)m clusters is smaller than that of the
cationic clusters. On the contrary, the amount of sta-
bilization of neutral Con(Bz)m clusters is larger than that
of the cationic clusters. Since the Ni atom has one more
VE than Co, the interaction between neutral Ni and
benzene should be weaker than that in neutral Co-Bz
due to the vacancy for π-electron donation. For their
cations, on the other hand, a relatively similar interac-
tion through the positive charge could be expected for
both Co-Bz and Ni-Bz cations. Therefore, increasing
cluster size n can presumably give less stabilization to
Nin(Bz)m clusters than Con(Bz)m clusters. At present,
however, a precise description of the molecular orbitals
for the rice-ball Mn(Bz)m is not clear, and careful
estimations of molecular orbitals are essential for more
detailed discussion.
3.1.d . Cu . For Cu, only the (1, 2) complex was
produced, as shown in Figure 2i, and the production
efficiency is low. The same pattern is seen for Cr-Bz
and Mn-Bz. As shown in the photoionized mass spec-
trum by the ArF laser, the intensity of (1, 2) is inevitably
less than that of (1, 1), although the ionization energies
of both species are well below the photon energy, which
will be described in the following section. Since there is
no electron spin restriction in a growth process for Cu-
Bz, the low production efficiency can be ascribed simply
to the low binding energy between Cu atoms and
benzene molecules. As discussed later, indeed, the
binding energy is very low compared to those of other
transition metal-benzene complexes. The filled d orbit-
F igu r e 7. Ionization energies of M1(Bz)x (x ) 1, 2; M )
Sc-Cu) complexes as well as those of metal atoms. (b, Ei’s
of M1 [ref 28]; 2, Ei’s of M1(Bz)1; 0, Ei’s of M1(Bz)2). An
upward arrow shows that the value shown is the lower
limit.
als in the Cu atom should result in a low binding energy
toward benzene molecules because there are no d-π
interactions but repulsive interactions between the 4s
orbital and the benzene ligand. Then, there is no
possibility for multi-Cu metal complexes to grow into
the multiple sandwich complexes because the formation
of the multiple sandwich structure requires the d-π
interaction along the molecular axis. They could, how-
ever, grow in the rice-ball structures.
3.2. Ion iza tion En er gies a n d Bin d in g En er gies
of M1(Bz)x (x ) 1, 2; M ) Sc-Cu ). To investigate the
electronic properties of M1(Bz)x (x ) 1 and 2), ionization
energies (Ei’s) of these complexes were determined by
the photoionization method except for that of Mn1(Bz)1.
The results are summarized in Table 5 and are il-
lustrated in Figure 7, including a part of the results that
have been reported elsewhere.20 In some species, the
dissociation energies (D0’s) of these neutral complexes
could be evaluated, which will be described in section
3.2.c.
3.2.a . Ei’s of M1(Bz)1. From Sc to Cr, an insignificant
change is seen in the Ei’s of M1(Bz)1, and this trend
shows a striking similarity to the Ei’s of the metal
atoms, which vary within a narrow range of 0.22 eV.
Therefore, it can be expected that the Ei’s of M1(Bz)1
are determined by the energy levels of the corresponding
metal atoms. In the ionization of 3d transition metal
atoms of Sc-Cr, the ionizing state is the 4s orbital. On
the other hand, the highest occupied molecular orbitals
(HOMOs) of (1, 1) are metal-originated nonbonding
(34) Yang, S.; Knickelbein, M. B. J . Chem. Phys. 1990, 93, 1533.

1436 Organometallics, Vol. 18, No. 8, 1999
Kurikawa et al.
orbitals described as 3da1 under C6v symmetry.36 Then,
it can reasonably be presumed that the ionizing state
of M1(Bz)1 for Sc-Cr is the nonbonding 3da1 orbital,
which results in the parallelism between Ei’s of atom
and M1(Bz)1. The energy level of the nonbonding orbital
should be closely related to the atomic energy level.
same with 18 VEs. Therefore, we can regard Mn1(Bz)2
+
with 18 VEs as a stable complex. On the other hand,
neutral Mn1(Bz)2 has 19 VEs. Since the 3de1 HOMO of
the neutral Mn1(Bz)2 is expected to be antibonding,
occupation of this orbital by one electron offers little
energetic advantage. Thus, the Ei of Mn1(Bz)2 is ex-
tremely small compared to values for the other com-
plexes.
We could not measure the Ei of Mn1(Bz)1 because of
its poor intensity. Assuming that the HOMO of Mn1-
(Bz)1 is described as 3de1, which has antibonding
character, the generation of Mn1(Bz)1 produces little
bonding stabilization. This is consistent with the fact
that the Mn2 dimer is difficult to produce due to its
exceptionally small binding energy (∼200 meV).37
In the late transition metals, the Ei of the (1, 2)
complexes increases slightly from Fe to Ni, although the
Ei’s of the corresponding metal atoms are almost the
same. Moreover, the differences between the Ei values
of the (1, 2) complexes and those of the atoms are rather
drastic compared to those for the early transition metal
complexes. Although this trend cannot be explained only
on the basis of the atomic energy levels, the Ei change
is seemingly related to both/either the d-π interaction
and/or further structural changes in the (1, 2) com-
plexes. Considering each C6H6 ligand as a six-electron
donor, together with the valence electrons of the metal
atom, the total number of valence electrons in Fe1(Bz)2,
Co1(Bz)2, and Ni1(Bz) are 20, 21, and 22, respectively.
These (1, 2) complexes are reasonably expected to have
sandwich-like structures. As pointed out by Lauher et
al.,39 the HOMO of sandwich complexes having 19-22
valence electrons corresponds to an MO with e1 sym-
metry, and this orbital is energetically stabilized by
bending the relative orientation of the two benzenes.
Since the lower MOs become unstable with the bending
angle, the bending configuration is likely to be prefer-
able where the stabilization of the HOMO is compen-
sated by the destabilization of the lower MOs. As the
number of electrons increases in the e1 HOMO, that is
from Fe to Ni, the bending angle is expected to become
larger. Then, the ionization energy should follow the
change in the bending angle as well as the change in
the strength of the d-π interaction. In fact, the onset
of the photoionization efficiency curves is very gradual
compared to those for the early transition metal-
benzene complexes, which may indicate that the low-
frequency vibration of a bending mode is simultaneously
excited upon the ionization of (1, 2).
Among the benzene complexes for late transition
metals Fe, Co, and Ni, only the Co1(Bz)1 complex has a
quite low Ei value. According to the MO diagram for
M1(Bz)1 under C6v symmetry, the HOMO of these
complexes is expected to be the same 3de1 orbital, which
cannot explain this Ei trend. As pointed out by Muet-
terties et al.,3 however, partial occupation of electrons
in the antibonding 3de1 orbital should result in a J ahn-
Teller effect, leading to a distortion of the M-(η6-C6H6)
fragment to M-η4 bonding. The reduced symmetry due
to this distortion makes the 3de1 orbitals separate into
stable a′ and unstable a′′ orbitals, and thus, the original
HOMO of Co1(Bz)1 with a 3de13 configuration under C6v
symmetry becomes the (a′)2(a′′)1 configuration. Then, we
can consequently deduce that the electron removal from
the (a′′)1 configuration will considerably stabilize the
+
Co1(Bz)1 cation, leading to a low Ei of Co1(Bz)1 com-
pared to the neighboring Fe1(Bz)1 and Ni1(Bz)1.
For Cu1(Bz)1, the Ei value is much less than that of
Ni1(Bz)1. This is probably because the ionization from
the HOMO having a (4sa1)1 configuration gives stability
+
to the corresponding Cu1(Bz)1 cation.
3.2.b. Ei’s of M1(Bz)2. For the (1, 2) complexes from
Sc to Cr, theoretical calculations29 by the DFT(B3LYP)
method predict that the ionizing state is the bonding
3de2 orbital for Sc, Ti, and V, while it is the nonbonding
3da1 orbital for Cr. As the number of bonding electrons
in the 3de2 orbital increases, the neutral becomes
stabilized with respect to the corresponding cation,
resulting in the gradual increase of Ei for the (1, 2)
complex from Sc to V. When the photoionization ef-
ficiency curves of the (1, 2) complex are compared for
Sc, Ti, V, and Cr,29,38 the onset for Cr1(Bz)2 is the
sharpest. This is reasonable because there is little
geometric change between the neutral and the cation
caused by ionization from the nonbonding 3da1 orbital,
which is consistent with the theoretical prediction. In
case of Mn-Bz, however, the Ei of Mn1(Bz)2 decreases
drastically. This Ei drop can be qualitatively explained
by the 18-VE rule. As is well-known for the 18-VE rule,
Cr1(Bz)2 is a very stable complex due to a closed shell
electronic configuration. Similar to Cr1(Bz)2, the elec-
tronic configuration of cationic Mn1(Bz)2+ is almost the
The Ei of Cu1(Bz)2 is comparatively small among the
late transition metal complexes. This can reasonably be
attributed to the highly lying 4sa1 HOMO, which
originates from the diffuse 4s orbital.
3.2.c. D0 of M1(Bz)1 a n d M1(Bz)2. To investigate
dissociation energies (D0’s), we have to discuss the
electron multiplicity of the transition metal-benzene
complexes. Although a transition metal atom itself
prefers a high electron-spin configuration due to spin-
spin exchange interactions, organometallic transition
metal complexes, such as metal-benzene, generally
take a low electron-spin configuration. This is because
a ligand field removes the degeneracy of the d orbitals
of the organometallic complexes. There have been no
experimental reports on the electron spin of metal atom
complexes in the gas phase, but we can deduce their
electron spin in some cases, by comparing the experi-
mental results in the gas phase to those in bulk.
Ionization energies are a good indicator of the electron
spin. For (1, 2) complexes such as Ti1(Bz)2 and Cr1(Bz)2,
when we compare the Ei value in this work with those
(35) Mingos, D. M. P. In Comprehensive Organometallic Chemistry;
The Synthesis, Reactions and Structures of Organometallic Compounds;
Sir Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
New York, 1982; Vol. 3, p 29.
(36) Elian, M.; Chen, M. M. L.; Mingos, D. M. P.; Hoffman, R. Inorg.
Chem. 1976, 15, 1148.
(37) Kant, E. A.; Lin, S.-S.; Strauss, B. J . Chem. Phys. 1968, 49,
1983.
(38) Kurikawa, T.; Nakajima, A.; Kaya, K. Unpublished results.
(39) Lauher, J . W.; Hoffman, R. J . Am. Chem. Soc. 1976, 98, 1729.

Metal-Benzene Complexes
Organometallics, Vol. 18, No. 8, 1999 1437
Ta ble 6. Dissocia tion En er gies of M1(Bz)x (x ) 1, 2)
of vapor obtained from bulk complexes, they are in
agreement with each other as listed in Table 5. This
means that the products of the gas-phase reactions take
the same spin state as the bulk complexes. Electron spin
of the complexes in the electronic ground state has been
Com p lexesa in eV
electron spin multiplicity
b
species
neutral
cation
D0
Sc1(Bz)1
Ti1(Bz)1
Cu1(Bz)1
Cu1(Bz)2
2
3c
4c
1
0.63(26)
1.76(13)e
0.17(13)
0.28(20)
investigated by EPR or ESR measurements40-43 for
5d
2
+
several species of V1(Bz)1, V1(Bz)2, Cr1(Bz)2, Ti1(Bz)2
,
V1(Bz)2+, and Cr1(Bz)2+; their electron spin multiplicities
are 2, 2, 1, 2, 3, and 2, respectively. These complexes
actually prefer lower electron-spin state as the ground
state, but they are not necessarily the lowest multiplic-
ity, and there have been no reports on the electron spin
of the other species.
2
1
a
Dissociation energies were determined from the observed Ei
values of M1(Bz)x (x ) 1, 2) combined with the known dissociation
energies of M1(Bz)x+ (x ) 1, 2) which were determined by the CID
b
experiments; ref 9. The value in parentheses indicates an
experimental uncertainty; 0.64(26) represents 0.64 ( 0.26. c Cal-
d
culation; ref 17. Calculation; ref 21. e Since a Ti atom takes a
ground state of 3F, the energy difference between Ti(3F) and Ti(5F)
was added; ∆E[3F-5F] ) 0.81 eV. Then, D0 was evaluated by the
formula D0[Ti-Bz] ) Ei[TiBz] + D0[Ti-Bz+] - Ei[Ti] + ∆E[3F-
5F].
According to quantum chemical calculations for
neutral and cationic transition metal-benzene com-
plexes,17,44-46 many of the (1, 1) and (1, 1)+ complexes
still prefer higher electron-spin state. Bauschlicher and
co-workers have calculated that the spin multiplicity for
cationic D0[ScBz+-Bz]. The D0 values for cationic M1-
+
the ground state of V1(Bz)1 cation is 5, although that
+
+
(Bz)1 and M1(Bz)2 have been measured by collision-
induced dissociation (CID) and are distributed from 1.5
to 3 eV.9 Since interactions induced by the positive
charge can generally contribute to the binding energy
between the atom and benzene molecule, the D0 value
of the cationic complex is larger than that of the neutral.
Indeed, D0 for neutral Ti1(Bz)1 is 1.76 eV, while that
for Ti1(Bz)1+ is 2.68 eV.9 This difference becomes much
of the corresponding neutral is determined as 2 experi-
+
mentally. In this case, the ground sate of the V1(Bz)1
cation would be inaccessible by the photoionization of
V1(Bz)1 due to the selection rule for the electron
multiplicity of ∆S ) 1.
Dissociation energies (D0’s) of the neutral M1(Bz)x (x
) 1 and 2) complexes were evaluated, from the observed
Ei values of M1(Bz)x (x ) 1, 2; M ) Sc-Cu) combined
+
+
larger for Cu-Bz. The D0’s of Cu1(Bz)1 and Cu1(Bz)2
+
have been reported as 2.26 and 1.61 eV,9 whereas those
for Cu1(Bz)1 and Cu1(Bz)2 neutrals are only 0.17 and
0.28 eV, as listed in Table 6. The small dissociation
energy for neutral Cu-Bz is attributed to the filled d
orbitals in the Cu atom, which is consistent with the
low production efficiency of the neutral Cu-Bz com-
plexes.
with the known dissociation energies of M1(Bz)x (x )
1, 2)9,17 and the Ei’s of the metal atoms and neutral M1-
(Bz)x (x ) 1 and 2). The evaluation procedure for D0-
[M-Bz]and D0[MBz-Bz] is expressed by following
thermochemical cycles:
D0[M-Bz] ) Ei[MBz] + D0[M-Bz+] - Ei [M] (2)
D0[MBz-Bz] )
4. Con clu sion
Ei[MBz2] + D0[MBz+ -Bz] - Ei[MBz] (3)
Organometallic clusters of Mn(Bz)m (M ) 3d transition
metals) were synthesized by the reaction between laser-
vaporized metal atoms and benzene molecules. Different
reactivities of metal clusters Mn selectively produce the
multiple-decker sandwich clusters for early transition
metals of Sc, Ti, and V and the rice-ball structures for
late transition metals from Fe to Ni. The sandwich
structure complexes exhibit a rapid Ei decrease with
cluster size, which can be rationalized by the delocal-
ization of 3dδ electrons along the molecular axis. In the
case of the rice-ball structure complexes, the maximum
number of benzene molecules adsorped is determined
by the steric hindrance of benzene molecules as well as
the number of valence electrons. The Ei’s of M1(Bz)x (x
) 1, 2) are sensitive to the electronic configuration of
the complex. Especially for Mn and Cu, the (1, 2)
complexes exhibit comparatively low Ei values, which
can be explained by the instability of the corresponding
neutral complexes. The information on the electronic
structures of the organometallic complexes strongly
suggests the possibility of a synthetic approach to the
novel structures by controlling the reaction dynamics
through electron-spin selected chemistry.
where D0[M-Bz] (D0[MBz-Bz]) and D0[M-Bz+] (D0-
[MBz+ -Bz]) are the sequential dissociation energy of
the M-Bz (MBz-Bz) neutral and the MBz+ (MBz+-
Bz) cation, respectively. However, it is impossible to
evaluate the D0 value reasonably when the ground state
of the cation is inaccessible from the neutral, because
the Ei value never corresponds to the energy difference
between the ground states of the neutral and the cation.
V1(Bz)1 is an example of this case, as mentioned above.
Among all of M1(Bz)1 and M1(Bz)2, we can conclude that
the neutral complexes of Sc1(Bz)1, Ti1(Bz)1, Cu1(Bz)1, and
Cu1(Bz)2 are feasible for the application of this evalu-
ation procedure without ambiguity for electron spin.
Their D0’s are tabulated in Table 6. D0[Sc1Bz2] cannot
be determined at the present stage, not because of
ambiguity for electron spin, but because of the unknown
(40) Andrews, M. P.; Mattar, S. M.; Ozin, G. A. J . Phys. Chem. 1986,
90, 744.
(41) Mattar, S. M.; Hamilton, W. J . Phys. Chem. 1989, 93, 2997.
(42) Andrews, M. P.; Mattar, S. M.; Ozin, G. A. J . Phys. Chem. 1986,
90, 1037.
(43) Cloke, F. G. N.; Dix, A. N.; Green, J . C.; Perutz, R. N.; Deddon,
E. A. Organometallics 1983, 2, 1150.
(44) Bauschlicher, C. W., J r. Advances in Metal and Semiconductor
Clusters; J AI Press Inc.: London, 1994; Vol. 2, p 115.
(45) Yasuike, T.; Yabushita. S. Private communication.
(46) Ouhlal, A.; Selmani, A.; Yelon, A. Chem. Phys. Lett. 1995, 243,
269.
Ack n ow led gm en t. The authors are very grateful to
Prof. S. Yabushita and Mr. T. Yasuike for stimulating
discussion on the electronic states of the M-Bz com-
plexes. They are also very grateful to Prof. M. A. Duncan

1438 Organometallics, Vol. 18, No. 8, 1999
Kurikawa et al.
(the university of Georgia) for the correction of the
English language. This work is supported by a program
entitled “Research for the Future (RFTF)” of the J apan
Society for the Promotion of Science (98P01203) and by
a Grant-in-Aid for Scientific Research on Priority Areas
from the Ministry of Education, Science, Sports, and
Culture. One of the authors (TK) expresses his gratitude
to the Research Fellowships of the J apan Society for the
Promotion of Science for Young Scientists.
OM9807349