J. Phys. Chem. 1995, 99, 16248-16252
16248
Structures and Ionization Energies of Cobalt-Benzene Clusters (Co,(benzene),) Tsuyoshi Kurikawa, Masaaki Hirano, Hiroaki Takeda, Keiichi Yagi, Kuniyoshi Hoshino, Atsushi Nakajima, and Koji Kaya* Department of Chemistry, Faculty of Science and Technology Keio UniversiQ, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan Received: June 22, 1995; In Final Form: August 23, 1995'
Cobalt-benzene clusters, Co,,(benzene),,,, were synthesized by the reaction of laser-vaporized C o atoms with benzene vapor. The clusters exhibit magic-number behavior not only at m = n 1 ( n = 1-3) but also at (n-m) = (3-3), (4-4), (5-4), (6-4), (7-4), (8-5), and (9-6). Observed magic-number behavior and ionization energies are consistently explained by assuming the structure of the clusters to be multiple-decker sandwiches for n-m (m = n 1, n = 1 - 3) and to be an n atom metal cluster coated by m benzene molecules for iz-m ( n 2 4). In this paper, we will present the properties of Co,,(benzene),,, clusters from the viewpoints of mass distribution, reactivity, and ionization energies. The structure and formation mechanism of the Co,,(benzene),,, clusters will also be discussed.
+
+
1. Introduction Since the discovery of ferrocene, ( C ~ H ~ ) F ~ ( C ~ Hthe S),I chemistry of organometallic compounds has proved to be a subject of considerable interest both experimentally and the~retically.*-~The sandwich structure was demonstrated for a variety of metal and ring combinations, and many theoretical studies have revealed their bonding scheme. As well as ferrocene, chromium bis(benzene), (C6H,j)Cr(C&,), is also known to be a stable sandwich compound.6 Moreover, tripledecker sandwiches, first pointed out by Wilkinson,' were synthesized: for example, tris(7-cyclopentadieny1)dinickel cation, [Ni~(CsH5)3)]+,~ and (C5H5)V(benzene)V(C~Hs).~ With the combined efforts of synthetic and spectroscopic studies, these compounds have been experimentally characterized in solution and the solid state. However, the synthesis of metal clusters by laser vaporization of a metal rod enables us to investigate various kinds of transition metal-molecule complexes in the gas phase. Several groups independently have succeeded in the synthesis of metal-molecule complexes in the gas phase by modifying the laser vaporization method.I0-l6 Metalmolecule complexes in the gas phase have been recently studied in order to investigate the interaction between the metal atom and ligand molecules without the influence of the solvent. From an experimental point of view, metal ions offer many advantages over neutrals, because charged species can easily be selected by a mass-spectrometric technique. Duncan et al." and Freiser et a1.I8 have reported the photofragmentation processes of metal ion-benzene complexes, and Armentrout et al.I9 determined the binding energy of a metal ion-benzene complex in their collision-induced fragmentation experiments. Furthermore, Bauschlicher et al.*O have extensively calculated the binding energies of metal-ligand complexes with ab initio SCF methods. However, most of these studies have been concerned with the mononuclear complexes for two reasons: experimental configuration for laser ablation and simplification of the system. Very recently, vanadium-benzene clusters, V,,(benzene),,,, were synthesized by the reaction of laser-vaporized vanadium atoms with benzene vapor in our group." From the mass spectrum exhibiting magic-number behavior, reactivity toward CO and NH3, and their ionization energies, it was concluded that the V,(benzene), clusters take multiple-decker sandwich @
Abstract published in Advance ACS Ahsrmcts. October 1. 1995.
structures of altemating vanadium and benzene molecules. Since it is expected that the substitution of the V atoms by another transition metal element may reveal the bonding character of the metal-benzene complexes, we synthesized cobalt-benzene clusters.
2. Experimental Section Details of the experimental setup have been provided elsewhere.?' Co,,(benzene),,, clusters were synthesized by the combination of a laser-vaporization method and a flow-tube reactor. First, Co atoms were vaporized by the irradiation of the second harmonic of a pulsed Nd'+:YAG laser (532 nm), and vaporized hot Co atoms were cooled to room temperature by a pulsed He carrier gas (10 atm). Then, the Co atoms were sent into the flow-tube reactor, where benzene vapor seeded in He gas was injected in synchronization with the flowing of the Co atoms. After the cluster beam was skimmed, the Co,,(benzene),,, clusters were ionized by an ArF excimer laser (6.42 eV) or the second harmonic of a dye laser pumped by an XeCl excimer laser. The photoions were mass-analyzed by a reflectron time-of-flight (TOF) mass spectrometer. In order to obtain information on the structure of Co,,(benzene),,,, the Co,,(benzene),,, clusters were further reacted with NH' gas inside the second flow-tube reactor, which is added downstream of the benzene addition, and their adducts produced were also massanalyzed. The laser power of vaporization was 5-15 d l p u l s e , in which higher laser power is relatively preferable to produce large Co,,(benzene),,, clusters. In the E, measurement, the second harmonic of the dye laser was used as the ionization laser. The photon energy was changed at a 0.01-0.03 eV interval in the range of 5.9-3.5 eV, while the abundance and composition of the Co,,(benzene),,, clusters were monitored by the ionization of an ArF laser. The fluences of both the tunable ultraviolet (UV) laser and the ArF laser were monitored by a pyroelectric detector (Molectron 5-3) and were kept at -200 pJlcm' to avoid multiphoton processes. To obtain photoionization efficiency curves, the ion intensities of the mass spectra ionized by the tunable UV laser were plotted as a function of photon energy with normalization to both the laser fluence and the ion intensities of ArF ionization mass spectra. The E,'s of the Co,,(benzene),,,clusters were determined
0022-3654/95/2099-16248309.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 44,1995 16249
Structures of Cobalt-Benzene Clusters 1-2
Con (benzene), ~
4-4
2-3 3-3
8-5
I
I
I
I
100
300
500
700
-
1-2
(b) Co,(benzene), 100
200
300
400
500
600
700
800
900
1000
c
?
Mass Number (miz)
4-4
Figure 1. Time-of-flight mass spectra of Co,(benzene), clusters. Peaks
of the clusters are labeled according to the notation (n-m),denoting the number of Co atoms (n) and benzene molecules (m). from the final decline of the photoionization efficiency curves. The uncertainty of the El’s is estimated to be typically h0.05 eV.
3. Results and Discussion 3.1. Mass Spectrum and Structure of the CoJbenzene), Clusters. Figure 1 shows a typical mass spectrum of Co,(benzene), clusters produced by the above-described procedure. The peaks of the clusters are labeled according to the notation (nm), denoting the number of Co atoms ( n ) and benzene molecules (m). It should be noted that no dehydrogenated species were observed in the mass spectra. In the photoionization mass spectra, the ion intensities would reflect the abundance of clusters when their E,’s are lower than the photon energy of the ionization laser. Mass spectra of (n-m) clusters were measured under the condition of low fluence of the ionization lasers to prevent photofragmentation. The mass distribution was not influenced by the change of the photon energy of the ionization laser from 6.42 to 5.8 eV, which seems to guarantee that the mass intensities reflect the abundance of the neutral (n-m)clusters. As will be discussed later Ei’s of Co,(benzene), clusters are well below the ionization photon energy of 6.42 eV. Therefore, prominent clusters in Figure 1 are the abundant and stable clusters formed in the beam. The main products in the figures are (n-m) = (1-2), (2-3), (3-3), (4-4), (5-4), (6-4), (7-4), (8-5), and (9-6). These stable compositions were unchanged even when the amount of benzene vapor was increased. In the series of the abundant clusters, the number of Co atoms increases one by one, whereas the number of benzene molecules does not necessarily increase with the cluster size. Namely, every number of Co atoms ( n ) has a specific maximum number of benzene molecules adsorbed (mmax). Parts a and b of Figure 2 show the mass spectra of Co,(benzene), clusters before and after the reaction with NH3 gas, respectively. When the magic-number clusters were exposed to the NH3 reactant gas, all of them were unreactive. In contrast to this, other small mass peaks, such as (n-m) = (1-l), (2-2), (3-2), and (4-3), are depleted completely by the reaction with NH3, and instead mass peaks corresponding to Co,(benzene),NH3 newly appear in the spectrum, as shown in Figure 2b. Since it is known that one NH3 molecule is adsorbed onto one Co atom,22this result implies that all of the magicnumber clusters have no exterior Co atoms and that the less abundant clusters, (1-1), (2-2), (3-2), and (4-3),have one exterior Co atom. Namely, the magic-number clusters are saturatedly covered with benzene molecules, and no N H 3 molecule interacts with Co atoms in the clusters. As reported previously,21 V,(benzene), clusters, synthesized by the same method, exhibit
5-4 6-4 7-4
I
I
I
I
100
300
500
700
Mass Number (m/z) Figure 2. TOF mass spectra of Co,(benzene), clusters (a) before and
(b) after reaction with NH3 gas, respectively. Through the reaction with NH3, magic-numbered clusters are unchanged, but small mass peaks, such as (n-m) = (1-1), (2-2), (3-2), and (4-3), are depleted completely, and instead the adduct Co,(benzene),NH3 newly appears, as shown in part b.
Co, (benzene)
Co,(benzene)
Co,(benzene),
Co, (benzene),
Co,(benzene),
Figure 3. Proposed structures of Co,(benzene), clusters.
+
different magic-number behavior at m = n - 1, n, and n 1 (n = 1-5), which is explicable in terms of multiple-decker sandwich structure: the V,(benzene), clusters having the compositions of m = n 1 take a sandwich structure, and one and two V atoms should be on the exterior sides of the sandwiches for V,(benzene), and V,(benzene),-I clusters, respectively. However, the magic-number behavior of Co,(benzene), cannot be explained only by the sandwich structure, because the sandwich clusters having formulas of (3-3), (4-4), and so on should have an exterior Co atom which is expected to be a reactive site toward NH3. Taking into account the two facts that every number of Co atoms ( n ) has a specific mmaxand that they have no exterior Co atoms, the most plausible structure of Co,(benzene), is the structure of Con clusters covered with benzene molecules ( m ) , as shown in Figure 3. In this structure, the mmaxshould be governed by an electronic factor andor a geometric factor. As is well-known in organometallic compounds, the electron-counting rule can predict the optimum composition of metal-molecule complexes: 18 electron rule for one metal atom compounds and 30 and 34 electron rule for two metal atoms compounds.23 These rules are based upon bonding character of molecular orbitals (MOs) resulting from the interaction between d orbitals of metal atoms and MOs of ligands. For example, the 18 electron configuration corresponds to filling precisely the nonbonding eZg and alg levels ~ ~the metallocene having 19 and 20 for a D5h m e t a l l o ~ e n e .In electrons, an excess number of electrons goes into the higher
+
Kurikawa et al.
16250 J. Phys. Chem., Vol. 99, No. 44, 1995
TABLE 1: Ionization Energies of Co,,(benzene), Clusters (eV)
Co, (Benzene)2~
$ 1
e 2.
-
20
~
5jTpi
0 0. 0 1 0
5 4400
composition structure"
sandwich cluster
5 53(31 eV
0
~~~
.Z
550
1
5.53(3) 4.85(4)
3 2 3
4
580
5 6 7 I 2 3 3 4
Photon Energy eV I
lying e l gorbitals, and such a metallocene is unstable and is easily oxidized due to the antibonding character of the elg orbital. For cobalt compounds, cobalt carbonyl is of major economic importance in hydroformylation synthesis. CO?(CO)K,COJ(C0),2, and C06(C0)16 are well characterized stable compounds,?j and their valence electrons are 34, 60. and 86 electrons, respectively, considering each CO ligand as a twoelectron donor, together with the nine d electrons of the Co atom. Stable cobalt-benzene clusters, Coz(benzene)3 COS(benzene)j, and Cos(benzene)?, have 36, 60, and 78 valence electrons, considering each benzene as a six-electron donor. On comparison of the number of electrons between Co,,(CO),, and Co,(benzene),,, the numbers are almost in agreement for Co? and COScompounds but are different for Co6. This suggests that the number of ligands (mmax)is also ruled by the other factor of geometry. On the other hand, the mmaxshould be geometrically restricted by steric hindrance. The interaction between benzene molecules seems to be repulsive, though that between the metal atom and the benzene molecule is expected to be attractive through donation and back-donation of d electrons and i~ electrons. Therefore, the Co,, clusters are covered with benzene molecules, until no benzene molecules are overlapped on the cluster. Indeed, the expected maximum number of benzene molecules is almost in agreement with the observed mmax,assuming the predicted structure of the Con cluster^.?^.^' Both factors can contribute to settle the structure of Co,,(benzene),,, since they are not mutually exclusive. As for the structure of Col(benzene)2, the sandwich cluster is the most likely. Considering each C6H6 ligand as a sixelectron donor, together with the nine electrons of the Co atom, the total number of valence electrons in Col (benzene)? comes to be 21. As pointed out by Lauher et a1.,28the HOMO of the sandwich complex having 19-22 valence electrons corresponds to a MO having el, symmetry, and it is energetically stabilized by the bending of the relative orientation of two benzenes. Since the lower MOs become unstable with the bending angle, the bending configuration is likely to be preferable. as shown in Figure 3. In Figure 3, furthermore, two structures are proposed for Col(benzene)3: one is the sandwich structure, and the other is Co? coated by three benzenes. As described in section 3.4, two isomers reasonably coexist in this experiment. 3.2. Ionization Energies of Co,(benzene), Clusters. In order to obtain information about the electronic properties of the Co,l(benzene),, clusters, their ionization energies, El's. were measured using a photoionization spectroscopic method. Figure 4 shows a photoionization efficiency (PIE) curve of Col(benzene)*. As shown in the figure, the E, of Col(benzene)z was determined to be 5.53 rt 0.03 eV from the final decline of
E,)]
1
I
,
Figure 4. Photoionization efficiency curve of the Col(benzene)! cluster normalized to the power of the ionization laser. The photon energy was changed at an 0.015 eV interval in the range 5.75-5.45 eV. From the final decline of the cur\'e. the E, of the cluster was determined to be 5.53 i 0.03 eV.
I71
-
saturatedly. covered
560 5 7 0
I1
uncompleted cluster
4 4
5 5 5
3 3 3 3
3.64(5)
5.00(5) 5.16(6)
4.92(5) 4.98(5) 5.10(5j 5.18(5) 5.55(4) 4.94(5)
4 4 4 4 1
2 1 2
5.67(6)
5.44(5) 5.39(5)
1 2
5.27(5) -16i i.5 )
3
I 2 3
5.72t5) 5.32(5) 5.1 Si5 J
'I In the Co,,(benzene),,,clusters. the clusters of m = n f 1 take the sandwich structures. whereas other magic-numbered clusters are Co,, clusters saturatedly covered with benzene molecules. Cncompleted cluster means that the number of benzene molecules is less than that in the magic-numbered cluster (see text). " Values in parentheses indicate experimental uncertainty: 5.53(3) represents 5.53 5 0.03.
6.0
._ E 4.0
4
1
3-4 4-5
35
'
1
2
3
4
5
6
Number of Co atoms
7
1 2 3 4 Number of V atoms
Figure 5. Dependences of E,'s of magic-numbered Co,,(benzene),,,and V,,(benzene),,,against the number of metal atoms. While the E , of V,,(benzene 1 decreases drastically with the cluster size. the E, change of Co,,(benzene),,,is rather gradual. The difference in the E, changes strongly suggests that the structure of the magic-numbered CoJbenrene),,. is different from the sandwich structure of V,,(benzene),,,, especially at n 2 3. The E, value ofV~(benzenej4was lately determined to be 4.15 i 0.02 eV after the previous report." ))3+
the ion intensities as a function of the photon energy. Compared to the PIE curves of other Ml(benzene)z complexes (M = Ti, V. Cr, and so the final decline of Col(benzene)2 is rather gradual. This is somewhat because geometrical reorganization from neutral to cation results in the gradual onset in the PIE curve. As mentioned above. Col (benzene)? presumably holds a bending configuration, and therefore the gradual onset seemingly reflects the change in the angle of the bending configuration between the neutral and the cation. Similar to the case of Col (benzene)., the El's of the Co,,(benzene),,, clusters were determined as listed in Table 1 . When the vapor pressure of benzene injected was reduced, Co,,(benzene),,, clusters with m smaller than the saturation number mmaxappear in the mass spectrum. In the table. the values of E , for the unsaturated (a-m) clusters are also tabulated. Figure 5a and b shows the dependences of El's of magicnumbered Co,,(benzene),,, and V,(benzene),, clusters on the number of metal atoms. In Figure 5b, the Ei values of V3(benzene)i and V4(benzene)A are also presented to compare E, values for Co,,(benzene),,,and V,,(benzene),,.
J. Phys. Chem., Vol. 99, No. 44, 1995 16251
Structures of Cobalt-Benzene Clusters
As shown in Figure 5b, the E, of V,(benzene),+l decreases drastically with the cluster size, which is attributed to the interaction between d electrons of V atoms and n electrons of benzene in the sandwich structure. In contrast to this, the E, change of Co,(benzene), is rather gradual: On comparison of E, values for the same compositions of Co,(benzene), and V,(benzene),, the E, values of (n-m) = (1-2) and (2-3) are almost in agreement within 0.2 eV, whereas those of (3-3) and (4-4) are largely different by 0.7-0.9 eV. This E, change strongly suggests that the structure of magic-numbered Con(benzene), is different from the sandwich structure, especially at n 2 3. 3.3. Formation Mechanism. Two mechanisms are possible for the production of Co,(benzene), clusters. They are (a) benzene molecules reacted with Con clusters and (b) benzene molecules sequentially reacted with Co atoms. Through mechanism a, the formed cluster is presumably the Con cluster covered with benzene molecules. In order to produce the sandwich cluster along mechanism a, the fragmentation of the Con clusters into Co atoms is inevitable, but it is energetically unfavorable. As reported by Armentrout et aL30 the binding energy of the Co dimer is larger than 1.32 eV, and moreover, that of larger Con clusters is 2-4 eV. Since these binding energies exceed the heat of formation of the metal-benzene complex (0.5-1.0 eV),21.29mechanism a should result in the formation of the benzene-coated Con clusters. In contrast, mechanism b implements the formation of the sandwich cluster as well as the benzene-coated cluster. When the benzene molecule and the Co atom are altematively piled up, it is possible to form the layered clusters where metal atoms are situated between benzene molecules. It is meaningful to discuss the difference between the formation mechanisms of Co,(benzene), and V,(benzene), from the viewpoint of the reactivity of cationic Con+and V,+ clusters. As studied by Zakin et sequential dehydrogenation channels for V,C&Ik+ ( k 5 5) have been observed in the reaction of the V,+ clusters with C6D6; the reaction of the cluster proceeds via facile initial chemisorption followed by activated C-D cleavage and D2 elimination. As reported previously, therefore, mechanism b should be the main route of the sandwich clusters of V,(benzene),, because no dehydrogenated species were observed in the sandwich cluster. The products of reaction of the V, clusters with benzene could not be observed in the mass spectrum, probably because the products were distributed over the wide range of the compositions of V,C,Hk under high concentrations of benzene. On the other hand, the reactivity study of Conf clusters with benzene has been reported by Irion et al.32 According to their result, the Con+cluster reacts with benzene, forming their adduct without dehydrogenation. Then, it is deduced that the reacted Con clusters with benzene could be observed clearly, because the products were stacked at the specific compositions of Co,(benzene),,, under high concentrations of benzene. In fact, the cluster-rich condition was essential to produce the large Co,(benzene), clusters. Thus, mechanism a can contribute to form the Co,(benzene), cluster. As discussed in the preceding section, the structure of Col(benzene)z is seemingly the sandwich cluster with the bent configuration. For the structure of Co?;(benzene)smoreover, the triple-decker structure of benzene-Co-benzene-Cobenzene should be elucidated similarly to the structures of Vz(benzene)3 and (CsHs)V(benzene)V(CsH~),because mechanism b can also provide Co,(benzene), together with mechanism a. When Co2(benzene)3 is produced under both mechanisms a and b, therefore, two isomers of Coz(benzene)3 should exist:
8,
1
Co (benzene)
4.8- 4.9 5.0
5.1 5.2 5.3 5.4 Photon Energy I eV Figure 6. Photoionization efficiency (PIE)curves of the Coz(benzene), cluster normalized to the power of the ionization laser under two different source conditions shown in Figure 7. The photon energy was changed at an 0.015 eV interval in the range 5.3-4.8 eV. Solid circles and open squares show PIE curves under the cluster-rich (Figure 7a) and cluster-poor (Figure 7b) conditions, respectively. From the final decline of the curve, the sandwich Co~(benzene)3has an Ei of 4.85(4) eV and the benzene-coated Coz cluster has one of 5.00(5) eV. n=l
(a) C Iuster- ric h
CO" I
2
4
I
100
200
7 300
400
500
Mass Number ( m i z ) n=l
(b) Cluster-poor
L 100
200
300
400
500
Mass Number (m/z) Figure 7. TOF mass spectra of Con+ cluster cations without the injection of benzene molecules: (a) cluster-rich and (b) cluster-poor conditions.
When Coz reacts with benzene along mechanism a, then the dimer results in the structure of C02 coated by three benzenes. However, the triple-decker structure of Coz(benzene)s is also formed when two Co atoms react with benzenes sequentially via mechanism b. 3.4. Isomers of Coz(benzene)~. Figure 6 shows two PIE curves of Coz(benzene)3 obtained under different laser-vaporization conditions: cluster-rich (Figure 7a) and cluster-poor (Figure 7b) conditions. Since the Ei's o f bare Con clusters (2 5 n 5 6 ) are higher than 6.2 eV,33the abundance of the neutral Con cluster cannot be measured with a commercial W laser. Then, the abundances of the clusters were surveyed by the mass spectrum of cationic Con+ clusters instead of the photoionization of the neutrals. Figure 7 shows the TOF mass spectra of cationic Conf
Kurikawa et al.
16252 J. Phys. Chem., Vol. 99, No. 44, I995 clusters, obtained by direct extraction of cations in the beam. Indeed, the abundance observed qualitatively reflects that of the neutral clusters, because the abundances of neutrals and cations are similar at n 2 7, where the neutral Co,, clusters can easily be ionized with the commercial UV laser of an ArF excimer laser. When the laser power for the vaporization was set to be around 5 mJ/pulse, the intensities of the Co,,' cluster (n L. 2 ) were very weak compared to that of atomic Co', as shown in Figure 7b. When the laser power was increased up to around 15 mJ/pulse, the cluster became more abundant relative to the atomic Co+, as shown in Figure 7a. It is reasonable that the abundance of the clusters depends on the concentration of Co vapor. When benzene molecules were injected under these different conditions, Co:(benzene)7+ was produced under both conditions. As pointed out in the preceding section, it is presumed that two isomers for Co?(benzene)3 should exist when both formation mechanisms a and b work. Under the cluster-poor conditions, the sandwich structures of Co,,(benzene),, are mainly produced, whereas the benzene-coated Co,, clusters are also produced in addition to the sandwich clusters under the cluster-rich conditions. In fact. the PIE curves for the photoionization of the neutral Col(benzene)3 produced under these two conditions are distinctly different, as shown in Figure 6. In both PIE curves, onsets were observed around 4.9 eV. and the E, of Col(benzene)j was determined to be 4.85 f 0.04 eV from the final decline of the ion intensities. However, the second onset was apparently observed around 5.0 eV in the PIE curve of Co.(benzene)3 produced under the cluster-rich conditions. This result suggests that there indeed exist two isomers of Cor(benzene)3, that the sandwich Co?(benzene)i has an E, of 4.85(4) eV, and that the Co? cluster covered with three benzenes has that of 5.00(5) eV. These structures are schematically shown in Figure 3. It should be noted that Coi(benzene)q was observed although the peak intensity was weak. As listed in Table 1. the E, is determined to be 4.64(5) eV, which is more than O S eV lower than that of Co?(benzene)j. Since the E, values of the unsaturated clusters show a decrease of 0.1-0.2 eV with the number of benzene molecules, the large decrement of O S eV suggests the structure of Co3(benzene)J is also the sandwich one. The extrapolation of the El's of sandwich Col(benzene)z and Co?(benzene)3 offers rational agreement with that of the sandwich Coj(benzene)J. 3.5. Dissociation Energy of Col(benzene)l. From the observed E, value of Col(benzene)l ( E , = 5.55 eV) combined with the known binding energy of Col(benzene)l+ (DO= 2.95 eV)34and the ionization energy of the Co atom ( E , = 6.74 eV). the binding energies of the neutral Col(benzene)l cluster were evaluated by the following formula: D,(Co-Bz)
= E,(CoBz)
+ D,,(Co--Bz)
- E,(Co) ( I )
The binding energy obtained is 0.64 eV for Col(benzene)l. The value for Col(benzene)I is much smaller than that for V I (benzene), (1.06 eV)" but is comparable to that for Crl(benzene)l or Til(benzene)l (0.5-0.7 eV).lY At the present stage, we cannot explain why the dissociation energy for V I (benzene)l is large compared to others, but it seems that the MOs concemed with the bond between the V atom and benzene play an important role in generating the multiple-decker sandwich cluster. The calculation of the bonding scheme of the metal-benzene clusters is in progress in our group. 4. Conclusions Organometallic clusters of Co,,(benzene),,, were synthesized with the reaction between laser-vaporized Co atoms and benzene
molecules. From the analyses of the magic-number behavior in the mass spectrum, their reactivity toward NH3, and their ionization energies, it was deduced that there are two types of Co,,(benzene),,, clusters: the sandwich structure and the benzenecoated Co,, cluster. It was also suggested that the former, the sandwich cluster, was produced via a sequential reaction between Co atoms and benzenes, while the latter was produced via a reaction between Co,, clusters and benzenes.
Acknowledgment. This work is supported by a Grant-inAid for Scientific Research on Priority Areas from the Ministry of Education. Science and Culture. References and Notes ( I ) !a) Ked]. T. J.: Pauaon. P. L. Noture ( L C J ~ ~ 1951. O F I )168. 1039. ( b ) Miller. S. A , : Tebboth, J. A,: Tremaine. J. F. J . Clieni. Soc. 1952, 632. ( 2 ) Muettei-ties. E. L.: Bleeke. J. R.: LVucherer. E. J.: Albright. T. A . C/i?/)i.RcL'.1982. 82. 499. t3) Compreiieiisiw Or,qmoriietnllic Chemisrn; Wilkinson, G.. Stone. F. G . A,. Abel. E. L'v., Eds.: Pergamon: New York. 1982: Vols. 3-6. (4)Wadepohl. H. Ai7jiew C h e m . In?. Ed. Enji/. 1992, 31. 247. t5) Braga. D.: Dy\on. P. J.: Grepioni. F.: Johnson. F. G. Ciiein. R ~ L , . 1994. 94. 1585. ( 6 ) Haaland. A. A(,rci. Chem. Sccind. 1965. 19. 41. ( 7 ) Wilkinson. G . J . A M . C11em.Soc. 1954. 76. 209. ( 8 ) Salzer. A,: Werner. H. A ~ I W WCliem., Int. Ed. Engl. 1972. 11. 930. (9 Duff. A . W.: Jonah. K.: Goddard. R.: Kraus. H.: Kriiger. C. J . Am. Clie~n.Sot.. 1983. 105. 5479. (10) Yeh. C. S.: h'ille>, K. F.: Robbins. D. L.: Pilgrim. J. S.: Duncan. M . A. C h e m P / i j c . Let/. 1992. 196. 233. ( 1 I ) Higaaide. H.: Ka)a. T.: Kobalashi. M.: Shinohara. H.: Sato. H. C h i f i . Ph) s. Latt. 1990. 171. 297. ( 1 2 ) Holland. P. bl,: Castleman. A. W.. Jr. J . C/7eni, Phys. 1982. 76. 4195. (13 I Robel
