Ni+(H2)n: Ligand bond energies for ground state ions

Article abstract of DOI:10.1016/S0009-2614(98)00831-8

The H₂ ligand binding energies in Ni⁺(H₂)_n were measured to be 17.3, 17.6, 11.3, 7.1 and 4.2 kcal/mol, for n=1-5, respectively. The very weakly bound sixth ligand begins a new solvation sphere. Association entropies were also measured. MP2 calculations were done to determine geometries and vibration frequencies as well as the origin of the bonding. The observed changes in BDE with ligation are due to electronic rather than steric effects. Comparisons are made to the Fe⁺(H₂)_n, Co⁺(H₂)_n, Cu⁺(H₂)_n and Ni⁺(CO)_n systems. A highly symmetric D_3h planar structure is found in Ni⁺(H₂)₃ and forms the core of the Ni⁺(H₂)_{4, 5} ions.

Full text of DOI:10.1016/S0009-2614(98)00831-8

4 September 1998
Ž
.
Chemical Physics Letters 293 1998 503–510
qž /
Ni H2 n: Ligand bond energies for ground state ions
)
Paul R. Kemper, Patrick Weis, Michael T. Bowers
Department of Chemistry, The UniÕersity of California at Santa Barbara, Santa Barbara, CA 93106-9510, USA
Received 12 May 1998; in final form 13 July 1998
Abstract
qŽ
.
The H2 ligand binding energies in Ni H2 n were measured to be 17.3, 17.6, 11.3, 7.1 and 4.2 kcalrmol, for ns1–5,
respectively. The very weakly bound sixth ligand begins a new solvation sphere. Association entropies were also measured.
MP2 calculations were done to determine geometries and vibration frequencies as well as the origin of the bonding. The
observed changes in BDE with ligation are due to electronic rather than steric effects. Comparisons are made to the
qŽ
.
qŽ
.
qŽ
.
qŽ
.
qŽ
.
Fe H2 n, Co H2 n, Cu H2 and Ni CO systems. A highly symmetric D3h planar structure is found in Ni H2
n
n
3
qŽ
.
and forms the core of the Ni H2
ions. q 1998 Published by Elsevier Science B.V. All rights reserved.
4, 5
w
x
1. Introduction
orbitals 16,15,17,18 . This promotes s donation to
the ion by returning electron density to the H2
ligands, and increases the 3d–3d exchange stabiliza-
tion on the metal. Third, in ions with half filled 3ds
orbitals, the 3dz2 and the 4s orbitals hybridize to
Over the past ten years systematic experiments
have examined a large number of gas-phase transi-
tion metal ion MqPXn clusters. Relatively simple
w
x
w
x
systems with XsH2 1–9 , CO 10,11 , and the rare
w
x
reduce on-axis Pauli repulsion 16,15,17–19 . Fourth,
the 4p orbitals, while significantly higher in energy,
may play a significant role 16–18 . Finally, the
non-covalent electrostatic interactions charge-in-
duced-dipole and charge quadrupole are present, but
w
x
gases 12 , have been examined, as well as saturated
and unsaturated hydrocarbons and other species for
Ž
w
x
w
x.
a general discussion, see Ref. 13 . The strong influ-
ence of the metal ion species on both bond energies
and structures in these clusters clearly shows the
presence of covalent forces in the bonding. Experi-
mental and theoretical investigations have identified
many of these. First, electron donation from the H2
s orbital to the metal stabilizes the ion charge
Ž
.
usually comprise a small fraction of the total bond
w
x
strength 1,16,19 . The relative importance of these
five factors depends strongly on the valence configu-
ration of the metal ion. The 4s orbital is especially
critical and its occupation is always severely destabi-
Žw
x
w x.
14,15 ; for a general discussion, see Ref. 16 .
w
x
lizing 6,12,16 .
Most of this donation is to the metal 4s orbital with a
minor amount to the 3ds orbital. Second, metal ions
with filled 3dp orbitals donate into the H2 s)
In addition to experimental investigations a large
number of theoretical calculations have been done on
these MqPX systems. The M H2 clusters with
qŽ
.
w x
w
x
nw x
MsFe 7 , Co 15,17,18 and Cu 9 have been
)
especially well characterized. These late transition
Corresponding author. Fax: q1-805-893-4120.
0009-2614r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 98 00831-8

(
)
504
P.R. Kemper et al.rChemical Physics Letters 293 1998 503–510
w
x.
metal ions all have relatively strong bond dissocia-
75% excited state 22 . The excited states were
resistant to collisional deactivation by H2 and, be-
cause the 4s configuration binds only very weakly to
Ž
.
tion energies BDEs and similar structures for the
first and second clusters. Very different structures
are found for larger numbers of ligands, however. In
the third cluster, both Fe and Co assume a ‘T’ shape
w x
H2 6 , caused a significant perturbation in the data
for the first association. This was ultimately solved
by using surface ionization which only forms ;1%
excited Niq. After formation, the isotope of interest
w
x
w x
7,17,18 while Cu has a planar D3h geometry 9 .
Both Fe and Co have six H2 ligands in their first
58
60
either Niq or Niq is mass selected in the first
w
x
Ž
.
solvation spheres while Cu has four 7,9,17,18 .
w
x
qŽ
.
Nui et al. 20 have examined the Ni H2
1, 2, 4, 6
quadrupole and injected into a driftrreaction cell
filled with typically 10 Torr of H2. An equilibrium
between the various Ni H2 cluster products ns
0–6 is quickly established as the ions are moved
through the 4 cm long cell with a small electric field.
The field is small enough that no measurable pertur-
bation of the thermodynamic temperature occurs.
The ions then exit the cell and are mass analyzed in
a second quadrupole. The resulting mass peaks are
recorded and integrated and, together with the pres-
ions using an unrestricted Hartree–Fock approach
qŽ
.
Ž
Ž
.
with correlated valence electrons . Their results for
n
.
the first two clusters are in general agreement with
our findings. Nui et al. predict, however, that Niq
will add between six and ten H2 ligands — a
prediction at odds with previous theoretical and ex-
qŽ
.
qŽ
.
perimental studies of Fe H2 n, Co H2
n
and
qŽ
.
Cu H2 n, and, as will be seen, with the present
results. The geometries and bond energies for the
larger clusters also disagree with the present results
due to the assumption of D4h and Oh symmetries for
Ž
.
Ž .
.
sure of H2 pH and the temperature T are used to
calculate the equilibrium constant Kp and standard
2
Ž
qŽ
.
o
Ž
.
Ni H2
and possibly to the rather small basis
free energy DGT
4, 6
sets used.
Niq
H
)n 760
Ž
.
In this Letter we present experimental association
enthalpies and entropies for the sequential clustering
of H2 ligands to ground state Niq ions. These results
are complimented by high-level ab initio calculations
that give insight into the bonding mechanisms pre-
2
Kpo s
,
1
Ž .
)
Niq
H
ny1 pH
Ž
.
2
2
DGTo syRT ln Kpo .
2
Ž .
qŽ
.
sent. The Ni H2 clusters will be compared with
other late transition metal clusters to illustrate the
Measurements were taken as a function of tempera-
ture from 77 to 780 K. The resulting plots of DGTo
vs. T were linear over the experimental temperature
range, yielding D HTo and DSTo as the intercept and
slope, respectively. To determine the heat of reaction
at 0 K D H0 , the experimental DGT vs. T curve is
matched with a corresponding theoretical curve cal-
culated using the theoretically determined structures
and vibrational frequencies. The low vibrational fre-
quencies and possible mass discrimination are varied
n
evolution of the bonding in the series. In addition,
qŽ
.
n
the Ni H2 clusters will be compared with previ-
qŽ
.
w x
ous results on the Ni CO ions 11 to evaluate
how well bonding concepts transfer from the H2 to
the CO systems.
n
o
o
Ž
.
2. Experimental
to determine the value and uncertainty in the bond
dissociation energy BDE'yD H0 sD0 23 .
o
Ž
. w x
Experimental details have been given previously
q
w
x
2,5,21 . Briefly, the Ni ions were formed either by
Ž
.
glow discharge using an Ar bath gas , by electron
3. Theoretical methods
Ž
.
impact on Ni CO or by surface ionization of
4
Ž
.
Ni CO . Electronic state chromatography experi-
w4
x
Ž2
.
ments 22 showed that both ground state D, 3d9
The product ions discussed here were all exam-
ined theoretically both to determine the molecular
parameters needed to analyze the experimental data
and to identify factors important in the bonding.
Ž2, 4
and electronically excited Niq)
F, 4s1 3d8 were
.
present when the discharge or electron impact sources
were used electron impact ionization produced )
Ž

(
)
P.R. Kemper et al.rChemical Physics Letters 293 1998 503–510
505
w
x
qŽ
3dn complexes of Fe H2 and Co H2 17.8 and
.
qŽ . Ž
Calculations were carried out at the MP2 level 24
w
x
.
18.2 kcalrmol, respectively but somewhat greater
using the TURBOMOLE package 25 . The hydrogen
w
x
qŽ . Ž
.
basis set was the ‘SVP’ set 26 . For the nickel basis
set we used the ‘ECP-10-MDF’ which includes a
pseudo-potential to describe the core electrons 27 .
than that of Cu H2 15.4 kcalrmol, see Table 3 .
Our calculated BDE is 15.6 kcalrmol, or ;90% of
the experimental value. The cluster structure is nearly
w
x
identical to that of the other late metals: all have C2v
q
˚
symmetry and an M –H2 bond length of ;1.65 A.
4. Results and discussion
We might expect that both the electrostatic and
qŽ
.
covalent attractions in Cu H2 would be larger due
Fig. 1 shows the experimental DGTo vs. T data.
The experimental enthalpies and entropies for the
association reactions are summarized in Table 1
along with theoretical binding energies for both
ground and excited state cluster ions. Calculated
vibrational frequencies are listed in Table 2. Struc-
to the slightly smaller ionic radius 28,29 ; however,
w
x
qŽ
.
the Ni H2 BDE is actually ;15% greater. The
10
qŽ
origin of the lower bond strength in Cu H2 , a 3d
.
ion, is the filled 3dz2 ds orbital on Cuq which
qŽ
Ž
.
.
destabilizes the complex. In both Co H2 and
qŽ
.
Ni H2 the H2 approaches a half-filled 3dz2 orbital
and calculations indicate 3dsr4s hybridization re-
duces the Mq–H2 repulsion by moving the z axis
qŽ
.
tures of the various Ni H2 ions are shown in Fig.
n
2. Finally, Table 3 compares BDEs for the first-row
qŽ
.
w
x
transition metal ion M H2 complexes.
electron density to the xy plane 16–19 . This hy-
bridization is a higher-energy process in Cuq due to
the necessity of moving two electrons and the result-
n
4.1. Niq H2
(
)
w x
ing bond is weaker 9 .
The NiqH 2 complex has an experimental BDE of
17.3 kcalrmol. This is similar to that found with the
Binding energies were also calculated for the
qŽ
.
excited states of Ni H2 which correspond to mov-
qŽ
Fig. 1. Plot of experimental DGTo vs. temperature for the association reaction Ni H2 ny1 qH2 ™ Ni H2 n. Open symbols refer to
.
qŽ
.
q)
q
Ž
.
Ž .
electron impact data ;75% electronically excited Ni
, closed symbols refer to surface ionization data 99% ground state Ni . The
excited Niq) only affects the data for the first association.

(
)
506
P.R. Kemper et al.rChemical Physics Letters 293 1998 503–510
Ž2
ing the electron hole in ground state Niq D, 3d9
to one of the other five 3d orbitals Table 1 . When
.
qŽ
.
qŽ
.
instead of one. Both M CO and M H2 show
similar trends as the Mq ion is changed from Coq to
Niq to Cuq, but the Mq–H2 bond strengths are
;40–45% of the MqCO values.
Ž
.
Ž
.
the 3dx y 3dd is half filled, the BDE is reduced by
;1.5 kcalrmol. As expected, this corresponds
qŽ
.
closely to the BDE for Cu H2 since the Mq–H2
4.2. Niq H2
(
)
2
interactions are now very similar. A major reduction
Ž
.
in BDE 7.7 kcalrmol occurs when the back bond-
2
Ž
.
ing 3dp is half filled the B1 state , showing the
The second H2 ligand binds opposite the first in
D2h symmetry with a measured BDE of 17.6
kcalrmol. The calculated bond lengths are nearly
importance of this interaction. As expected, the BDE
Ž
.
is now similar to the early metals Table 3 .
qŽ
.
qŽ
.
identical to those in Ni H2 , reflecting the similar-
The Ni CO ions have been investigated both
n
Ž
.
experimentally and theoretically and exhibit bonding
ity in bonding Fig. 2 . The second BDE is 0.3
kcalrmol larger than that of the first cluster, how-
ever, due to the benefit of the symmetric 4sr3ds
hybridization which occurred with the first H2 addi-
qŽ . w
x
very similar to those in Ni H2
11,30 . The car-
n
bon s orbital donates into the Niq 4s orbital while
the filled Niq dp orbitals 3dx z and 3d y z donate
into the CO pp) orbitals. Hybridization of the Niq
4s and 3ds orbitals is used to reduce Pauli repulsion.
Again, the s and p dative interactions are synergis-
tic: the p donation makes the CO more negative and
thus better able to donate into the Mq 4s orbital. The
strength of the Niq–CO bond is much greater than
Ž
.
w
x
tion 19 . The D2d structure is calculated to be 0.8
kcalrmol higher in energy than the D2h despite the
fact that in D2h symmetry back donation to both
Ž
.
ligands is from a single p-type orbital d y z . In both
qŽ
.
qŽ
.
Co H2 and Cu H2 the D2h states are higher
in energy ;0.7 and ;0.3 kcalrmol, respectively .
This reverse preference is also present in V H2
2
2Ž
.
that of NiqH2 41.711 vs. 17.3 kcalrmol at least
partly due to the presence of two p interactions
Ž
.
qŽ
.
2
4
Ž
.
3d , D2h and may be due to the availability of a
Table 1
qŽ
.
n
Data summary for Ni H2 clusters
Ion
Experiment
D H0o a
17.3"0.3e
Theory
symmetryd
C2v 2A1
ex. 2A2
ex. 2 B1
ex. 2 B2
yD HTo a
yDSTo b
20.7"2
Tc
De
D0
d
d
qŽ
Ni H2
.
18.7"0.5
650"90
17.9
16.5
10.2
13.1
15.6
qŽ
Ni H2
.
17.6"0.3e
18.0"0.4
24.1"2
575"175
D2h
D2d
19.4
18.6
15.5
2
qŽ
Ni H2
.
.
11.3"0.3e
7.1"0.3e
11.7"0.4
8.7"0.6
23.5"2
24.5"2
425"175
300"120
D3h plan.f
14.1
9.6
7.1
3
4
qŽ
Ni H2
Cs pyr.
10.7
4.8
D4h vert.f
qŽ
Ni H2
.
.
4.2"0.2e
0.8"0.2g
4.9"0.4
0.9"0.2
22.5"2
10.0"2
180"70
120"40
Cs bipyd.
D2h
5.3
3.2
–
5
6
qŽ
Ni H2
y8.5e, h
a In kcal moly1
.
b In cal moly1 Ky1
.
c
Ž
.
In kelvin " refers to temperature range, not uncertainty .
d MP2 geometries.
e Fitting with theoretical frequencies and geometries.
f
Ž
.
Ž
.
The H–H bonds are all either perpendicular vert. or parallel plan. to the sh plane.
g Fitting with DCp correction error small at low temperature .
Ž
.
hSixth H2 is unbound in first solvation shell.

(
)
P.R. Kemper et al.rChemical Physics Letters 293 1998 503–510
507
2
second A g state to mix with the ground state in D2h
Ž
.
symmetry absent in D2d symmetry .
qŽ
.
The Ni CO ion is also linear but, unlike the in
2
H2 case, the second CO BDE is slightly weaker than
Ž
w x.
the first 40.1 vs. 41.7 kcalrmol 11 . In both the
first and second clusters, however, the Niq–H2 bond
strength is 42"2% of the corresponding CO BDE.
4.3. Niq H2
(
)
3
The observed bond dissociation energy for the
Ž
.
third H2 ligand is 11.3 kcalrmol Table 1 . This
represents a decrease in average BDE from 17.4 to
15.4 kcalrmol. The optimized MP2 structure for the
qŽ
.
Ni H2 ion has D3h symmetry with all atoms in
the xy plane Fig. 2 . This same structure is found in
Cu H2 and as discussed previously, both the
3
Ž
.
qŽ
.
3
highly symmetric structure and the marked decrease
in bond strength are mainly due to the repulsive
interaction of the H2 s orbitals with the 3d shell on
Cuq and Niq. In the planar D3h structure, the three
H2 ligands have a repulsive s interaction with three
of the N iq 3d orbitals
3d z 2 a 1
and
Ž
Ž
.
X
Ž .
.
3dx2yy2r3dx y e , the z axis is the symmetry axis .
The ion reduces this repulsion in two ways. First, the
4s and 3dz2 orbitals hybridize to move electron
density from the xy plane to the "z lobes. Second,
and more important, the Niq ion donates electron
X
e
Ž .
density from the repulsive 3dx2yy2r3dx y
pair of
orbitals to the three H2 s) orbitals. This pair of
orbitals is symmetry adapted to produce both two
repulsive s interactions and two attractive p interac-
tions with the three ligands. The resulting p back
donation is similar to that in the first two clusters,
but in the third cluster the back donation also re-
duces the s repulsion. This ‘push–pull’ mechanism
has the effect of greatly increasing the extent of back
donation while at the same time reducing s repul-
sion and is the primary driving force for the D3h
structure. The large increase in H–H bond length
Ž
.
Fig. 2 and reduction in H2 vibrational frequency
Ž
.
Table 2 show the presence of increased back dona-
tion in the third cluster.
Ž
.
The second vertical D3h structure, with the H2
ligands perpendicular to the xy plane, is higher in
energy and is actually a transition state. This might
seem surprising since the vertical and planar D3h
forms have nearly identical s repulsion and each has

(
)
508
P.R. Kemper et al.rChemical Physics Letters 293 1998 503–510
a degenerate pair of d orbitals available for back
A very similar reduction in BDE is found between
. x
qŽ qŽ . w
the Ni CO and Ni CO 11 . Although no cal-
2 3
Y
X
Ž
.
bonding either the e or e set . Further, the vertical
Ž
qŽ
culations have been done on Ni CO , a D3h ground
.
3
isomer has lower ligand–ligand repulsion although
this is negligible . The destabilization of the vertical
.
yŽ
.
state is found in the iso-electronic Fe CO ion
3
w
x
D3h cluster is due to the loss of the very beneficial
‘push–pull’ mechanism present in the planar geome-
try since back donation from the eY pair of orbitals
does not lead to any reduction in s repulsion. The
relative stabilities of the planar D3h geometries
31 .
4.4. Niq H2
(
)
4
The fourth H2 ‘caps’ the planar third cluster to
form a distorted trigonal pyramid. The symmetry is
Cs, and Fig. 2 shows that the three equatorial ligands
are strongly bound while the polar ligand is much
more weakly attached. There is little change in the
Ž
qŽ qŽ .3.
found in Ni H2 and Cu H2 and the asym-
.
3
Ž
qŽ
.
qŽ .3.
metric ‘T’ structures in Fe H2 and Co H2
have been discussed previously 9 .
w x3
qŽ
Fig. 2. Theoretical geometries of the Ni H2
.
1 – 5
ions calculated at the MP2 level. All distances are in angstrom.
˚
¨

(
)
P.R. Kemper et al.rChemical Physics Letters 293 1998 503–510
509
yŽ
been done. However, the isoelectronic Fe CO ion
.
4
Table 3
n
a
q Ž
.
q Ž
.
yŽ
is formed by capping the planar Fe CO ion 31 .
.
w x
M
3d : comparison of binding energies for M H2 ny1qH2
3
q Ž
.
™ M H2
n
n
Tiq
Vq
Crq
Feq
Coq
Niq
Cuq
4.5. Niq H2
(
)
5
1
2
3
4
5
6
10.0b
9.7
9.3
8.5
8.2
10.2
10.7
8.8
7.6
9.0
4.7
3.4
1.4
1.1
16.5c
15.7
7.5
8.6
2.2
18.2
17.0
9.6
9.6
4.3
17.3
17.6
11.3
7.1
15.4
16.7
8.8
qŽ
.
4
Our MP2 calculations show the Ni H2 ion
Ž
.
ground state to be a trigonal bipyramid Fig. 2 with
9.0
5.1
4.2
4.2
1.0d
1.0
two weakly bound polar H2 ligands capping the
qŽ
.
3
8.7
9.6e
2.3
4.0
0.8d
planar Ni H2 core group. Both the bond lengths
Ž
.
and the H2 vibrational frequencies Table 2 show
a In units of kcal moly1
.
qŽ
.
that the Ni H2 core is largely unchanged from
b Binding energy with respect to lowest d3 configuration.
c Binding energy with respect to lowest d7 configuration.
dStart of next solvation sphere.
3
the third cluster, indicating again that this structure is
extremely stable. The BDE of the fifth H2 is 4.2
kcalrmol, down form the 7.1 kcalrmol in the fourth
e The increase in yD H0o for ns6 is due to a spin change from
quintet to triplet on the core Vq ion see Refs. 4,16 .
Ž
w
x.
cluster. This is partly due to destabilization of the
q
qŽ
.
Ž
Ni H2 core the Ni –H2 bond lengths increase
3
Niq–H2 or H–H bond lengths of the equatorial H2
ligands between the third and fourth clusters and the
very stable D3h core is largely preserved. The experi-
mental BDE is 7.1 kcalrmol, which is 8.3 kcalrmol
less than the three equal 15.4 kcalrmol bonds in the
third cluster. The decrease is due primarily to in-
creased s repulsion from the 3dz2 orbital. Remem-
ber that in the third cluster 4sr3d hybridization was
used to reduce s repulsion by moving the 3dz2
equatorial electron density to the "z poles. This
same hybridization now works to greatly increase
repulsion toward the fourth H2 ligand and a lower
BDE results.
very slightly, ;0.01 A but it is the fourth H2
˚
.
Ž
ligand which seems to be most affected here the
NiqH2 bond length increases 0.13 A . We speculate
that the 4pz orbital is used in the fourth cluster to
polarize the z axis electron density away from the
˚
.
yŽ . w
x
polar H2 ligand as in Fe CO 31 . This mecha-
4
nism is counter-productive when the fifth H2 ligand
is added opposite the fourth, and a reduction in BDE
occurs.
The fifth addition closes the first solvation sphere
qŽ
.
Ž
for the Ni H2 clusters. Both Feq and Coq as
n
.
well as all the early transition metal ions add six H2
ligands in their first solvation shell while Cuq adds
four. The determining factor is the occupation of the
This structure is unique among the transition
metal–H2 complexes. The Cu H2
quasi-tetrahedral geometry with four nearly equal
qŽ
.
Ž
.
ion has a
Cartesian orbital set the 3dx2yy2 and 3dz2 orbitals
w x
4
9 . If these orbitals are each empty or half filled,
octahedral occupation occurs; as the set is filled, s
repulsion increases and the first shell occupation is
qŽ
.
Ž .
geometry capped D3h is
bonds. The Ni H2
4
unstable in Cuq due to the increased repulsion from
qŽ qŽ
.
.
Ž
qŽ . .
reduced to five Ni H2
Ž
qŽ . .
or four Cu H2
the filled 3dz2 orbital. Both Fe H2 and Co H2
.
4
4
5
4
have ‘butterfly’ structures with two strongly bound
ligands on the z axis and two more weakly bound on
4.6. Niq H2
(
)
6
w
x
the x and y axes 7,17,18 . This is due to the larger
number of half-filled orbitals which stabilize the
Ž .
Both the extremely weak BDE 0.8 kcalrmol
side-on ‘T’ geometry relative to the planar D3h in
the third and fourth clusters. The Ni H2 D3h
and, especially, the very small DSTo show that the
sixth ligand is added in the second solvation shell.
The very low vibrational frequencies, free H2 rota-
tions and large rotational moments which result all
Ž
.
qŽ
.
3
qŽ
.
geometry is actually unstable in Fe H2
and
3
Co H2 due to an unequal occupation of the eX
qŽ
.
3
orbital set which causes a Jahn–Teller distortion of
the D3h symmetry. These effects have been dis-
give rise to more product entropy and make the DSTo
y1
of association more positive y10.5 cal mol Ky1
Ž
cussed in detail previously 9 .
vs. y24 to y21 cal moly1 Ky1 for the first five
association reactions, see Table 1 . The lowest en-
ergy first solvation shell structure for Ni H2 is
w x
qŽ
The BDE of Ni CO is also much lower than
.
.
4
qŽ . w
Ni CO 11 but no calculations on these ions have
x
qŽ
.
3
6

(
)
510
P.R. Kemper et al.rChemical Physics Letters 293 1998 503–510
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1991 10600.
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13 B.S. Freiser Ed. , Organometallic Ion Chemistry, Kluwer,
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The support of the National Science Foundation
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Akademischer Austauschdienst’’ for financial sup-
port.
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