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Zero-field splitting in the triplet ground state of nickelocene
CHr;MICAL
PHYSICS
LETTERS
1
(1967)
54-55.
NORTH-HOLLAND
ZERO-FIELD IN THE TRIPLET GROUND
PVHLISHING
COMPANY,
SPLITTING STATE OF NICKELOCENE
AMSTERDAM
r
R. PRINS and J. D. W. VAN VOORST Laboratoq
for
Physicai
Chemists,
University
of Amsteniam,
Amsterdam,
The Netherlands
and C. J. SCHINKEL Physical
Laboratory,
University
of Amsterdam,
Received
8 March
possibility
we measured
the magnetic
r2 ’
=
sN&
!_%
the formula
exp(-D/kT) 1+2 exp(-D/kT) +21
g2 D
+ 1-exp(-D/kT)
1+2
exp(-D/kT
(1)
in which it is assumed that the nickelocene molecule has axial symmetry [5]. At sufficiently high temperatures this formula reduces to
suscepii-
and at very low temperature
From the sclsceptibility was calculated that D/g!
= + 6.05
to
extrapolated f 0.12
to OoK it
cm-l.
Owing to the positive zero-field splitting, at low temperature the highest magnetic levels of the ground state will become depopulated and due to the lack of angular momentum of the lowest state (ms = 0) no ferromagnetic ordering will occur. There is general agreement that, from the twenty electrons available (ten n electrons from the ligands and ten electrons from the metal), the first eighteen electrons occupy the bonding orbitals. The rema$ning$mp~ orbjtals are the antibonding a$g, a2u, elg, eIu, e2g orbitals and the
t The investigations were supported (in part) by the Net’nerlands Foundation for Chcmicrl Research (S.Q.N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.) 1967
from 6.5O - 300°K. The measured curve can = + 25.6 cm-l. The ground state configuration
must then confirm
bility as a function of temperature in the range 6.5o to 3000K by means of a pondero-motoric method [S]. The measured susceptibilities were corrected for the temperature independent susceptibility. We estimated the diamagnetic contribution to be Xm = - I.2 X 10-4 cgs units [4] and for the high frequency contribution we calculatedxm = + 0.4 X 10-4 cgs units. Beyond 700K the results can be described by the Curie-Weiss law x = Np2/3k (T-Q) with 0 = 6oK and j.~= 2.89 f 0.15 pg. Below 70oK however, there is a departure from this law. Although the positi,ve 8 points to ferromagnetic interaction, ferromagnetism cannot be the reason for the departure from the Curie-Weiss law, as the magnetisation varies linearly with the magnetic field, also at 6.5oK. For %is departuro only a large zero-field splitting of the spin levels can be responsible. The susceptibility
March/Apr.il
The Netherlands
1967
The magnetic susceptibility of nickelocene was measured be fitted with the parameters: g/ = 2.0023, g, = 2.06 andD of nickelocene is presumably 3A2 (“ii) or 3A2 (e;;).
Although static magnetic susceptibility measurements have shown [ 1] that the ground state of bis -cyclopentadienyl-nickel (nickelocene) is a spin triplet, up to now one has not been able to obtain auy ESR signal [2]. If relaxation broadening can be Gmitted, a zero-field splitting larger than 7 cm -1 might be seen as the origin for the absence of any ESR signal, To check this
Amsterdam,
54
TRIPLET
nonbonding ligand ezu orbital. t!xt the remaining
two unpaired
GROUND
STATE
It is very likely electrons in
nickelocene are not in an orbitally degenerate corfiguration. For a degenerate configuraiion one calculates that, because of spin-orbit coupling, the effective moment M is strongly temperature dependent. On the other hand it is expected that the zero-field splitting for the configuration a*.ezu, for which p is temperature independent, is much smaller than the observed splitting. The possible orbitally nonde enerate configurations are e&, egg, em;{, el*B (all 3A2). For all these configurations 1sg/=%OO23. Then it follows from the high temperaiure susceptibility that g, = 2.06 f 0.10, which in turn leads to D = + 25.6 f 3.0 cm-l. A large zero-field splitting however, is only to be expected when the two unpaired electrons are in molecular orbitals which contain a large metal contribution. For t-his reason the e!ju configuration can be ruled out. Furthermore, for reasons of overlap, the eig orbital is expected to have a higher energy than the e2u orbital. No decision can be made whether the ey{ or the e;& configuration is the right one. Although a qualita lve MO scheme for ferrocene predicted the e$ orbital to be the lowest antibonding [6], SC3 calculations gave either the aIg [7] or the e2u orbital [8] as the lowest antibonding, while semi-empirical calculations gave the eig orbital as the lowest antibonding [9]. As a check the whole x2 -T carve was calculated from formula (1) with the parameters given above. As shown in fig. 1 the measured points nicely fit the calculated curve. Like in vanadocene [lo], the zero-field splitting in nickelocene is very large as compared to the difference of theg values. A calculation of spin-spin coupling and a configurational interaction treatment within D5d symmetry with Spinorbit coupling is in progress. Thzzks are due to Prof. G. J. Hoijtink for his stimulating interest in this work and to Prof. F. Jellinek and Dr. H. J. de Liefde Meijer for providing the nickelocene. One of us (R. P.) thanks the Koninklijke/Shell Laboratorium {Shell Research N. V.) for financial support.
OF NICKELOCENE
55
L I?
200
-
100
-
I
I
100
1
I
205
I
-
Fig. 1. Temperature dependence of the inverse moIar susceptibility. Fuil curve calculated with go = 2.00231 g1 = 2.06 and D = + 25.6 cm-l_ The circles represent the measured points.
REFERENCES [l]
[2] [3]
[4] I51 . .
E.O.Fischer and R. Jira, Z.Naturforsch. 86 (1953) 217; G. Wilkinson. P. L. Pauson and F.A. Cotton, 3.Am. Chem.Soc. 76 (1954) 1970. M. Nussbaum and J. VoitlPnder, Z. Naturforsch. 20a (1965) 1417. C. J.Schinkel, Thesis, University of Amsterdam. 1965: G. W.Rathenau and J.L.Snoek, Philips Res.Repts. 1 (1946) 239. L. N.Mulay and ICI.E. Fos, J. Chem.Phys. 3s (1963) 760. E-Weiss and E.O.Fischer. Z.Acors.Chem. 278 (1955) 219.
[6] J.D.Dunitz and L.E.Orgel. J.Chem.Phys. 23 (1954) 954. [7] E.M.Shustorovich and M. E.Dyat!iina, J.Struct. Chem.USSR l(l960) 98. [S] J. P.Dahl and C. J. Ballhausen, Mat. Fys.Rfzdd.Dan. Vid.Selsk. 33. No. 5 (1961). [9] R.D.Fischer,. Theor&.Chim.Acta l(l963) 41s. [lo] R.Prins, P.Biloen and J.D.W.van Voorst. to be published in J. Chem. Phys. 46 (1967) 1216.
f
PHYSICS
LETTERS
1
(1967)
54-55.
NORTH-HOLLAND
ZERO-FIELD IN THE TRIPLET GROUND
PVHLISHING
COMPANY,
SPLITTING STATE OF NICKELOCENE
AMSTERDAM
r
R. PRINS and J. D. W. VAN VOORST Laboratoq
for
Physicai
Chemists,
University
of Amsteniam,
Amsterdam,
The Netherlands
and C. J. SCHINKEL Physical
Laboratory,
University
of Amsterdam,
Received
8 March
possibility
we measured
the magnetic
r2 ’
=
sN&
!_%
the formula
exp(-D/kT) 1+2 exp(-D/kT) +21
g2 D
+ 1-exp(-D/kT)
1+2
exp(-D/kT
(1)
in which it is assumed that the nickelocene molecule has axial symmetry [5]. At sufficiently high temperatures this formula reduces to
suscepii-
and at very low temperature
From the sclsceptibility was calculated that D/g!
= + 6.05
to
extrapolated f 0.12
to OoK it
cm-l.
Owing to the positive zero-field splitting, at low temperature the highest magnetic levels of the ground state will become depopulated and due to the lack of angular momentum of the lowest state (ms = 0) no ferromagnetic ordering will occur. There is general agreement that, from the twenty electrons available (ten n electrons from the ligands and ten electrons from the metal), the first eighteen electrons occupy the bonding orbitals. The rema$ning$mp~ orbjtals are the antibonding a$g, a2u, elg, eIu, e2g orbitals and the
t The investigations were supported (in part) by the Net’nerlands Foundation for Chcmicrl Research (S.Q.N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.) 1967
from 6.5O - 300°K. The measured curve can = + 25.6 cm-l. The ground state configuration
must then confirm
bility as a function of temperature in the range 6.5o to 3000K by means of a pondero-motoric method [S]. The measured susceptibilities were corrected for the temperature independent susceptibility. We estimated the diamagnetic contribution to be Xm = - I.2 X 10-4 cgs units [4] and for the high frequency contribution we calculatedxm = + 0.4 X 10-4 cgs units. Beyond 700K the results can be described by the Curie-Weiss law x = Np2/3k (T-Q) with 0 = 6oK and j.~= 2.89 f 0.15 pg. Below 70oK however, there is a departure from this law. Although the positi,ve 8 points to ferromagnetic interaction, ferromagnetism cannot be the reason for the departure from the Curie-Weiss law, as the magnetisation varies linearly with the magnetic field, also at 6.5oK. For %is departuro only a large zero-field splitting of the spin levels can be responsible. The susceptibility
March/Apr.il
The Netherlands
1967
The magnetic susceptibility of nickelocene was measured be fitted with the parameters: g/ = 2.0023, g, = 2.06 andD of nickelocene is presumably 3A2 (“ii) or 3A2 (e;;).
Although static magnetic susceptibility measurements have shown [ 1] that the ground state of bis -cyclopentadienyl-nickel (nickelocene) is a spin triplet, up to now one has not been able to obtain auy ESR signal [2]. If relaxation broadening can be Gmitted, a zero-field splitting larger than 7 cm -1 might be seen as the origin for the absence of any ESR signal, To check this
Amsterdam,
54
TRIPLET
nonbonding ligand ezu orbital. t!xt the remaining
two unpaired
GROUND
STATE
It is very likely electrons in
nickelocene are not in an orbitally degenerate corfiguration. For a degenerate configuraiion one calculates that, because of spin-orbit coupling, the effective moment M is strongly temperature dependent. On the other hand it is expected that the zero-field splitting for the configuration a*.ezu, for which p is temperature independent, is much smaller than the observed splitting. The possible orbitally nonde enerate configurations are e&, egg, em;{, el*B (all 3A2). For all these configurations 1sg/=%OO23. Then it follows from the high temperaiure susceptibility that g, = 2.06 f 0.10, which in turn leads to D = + 25.6 f 3.0 cm-l. A large zero-field splitting however, is only to be expected when the two unpaired electrons are in molecular orbitals which contain a large metal contribution. For t-his reason the e!ju configuration can be ruled out. Furthermore, for reasons of overlap, the eig orbital is expected to have a higher energy than the e2u orbital. No decision can be made whether the ey{ or the e;& configuration is the right one. Although a qualita lve MO scheme for ferrocene predicted the e$ orbital to be the lowest antibonding [6], SC3 calculations gave either the aIg [7] or the e2u orbital [8] as the lowest antibonding, while semi-empirical calculations gave the eig orbital as the lowest antibonding [9]. As a check the whole x2 -T carve was calculated from formula (1) with the parameters given above. As shown in fig. 1 the measured points nicely fit the calculated curve. Like in vanadocene [lo], the zero-field splitting in nickelocene is very large as compared to the difference of theg values. A calculation of spin-spin coupling and a configurational interaction treatment within D5d symmetry with Spinorbit coupling is in progress. Thzzks are due to Prof. G. J. Hoijtink for his stimulating interest in this work and to Prof. F. Jellinek and Dr. H. J. de Liefde Meijer for providing the nickelocene. One of us (R. P.) thanks the Koninklijke/Shell Laboratorium {Shell Research N. V.) for financial support.
OF NICKELOCENE
55
L I?
200
-
100
-
I
I
100
1
I
205
I
-
Fig. 1. Temperature dependence of the inverse moIar susceptibility. Fuil curve calculated with go = 2.00231 g1 = 2.06 and D = + 25.6 cm-l_ The circles represent the measured points.
REFERENCES [l]
[2] [3]
[4] I51 . .
E.O.Fischer and R. Jira, Z.Naturforsch. 86 (1953) 217; G. Wilkinson. P. L. Pauson and F.A. Cotton, 3.Am. Chem.Soc. 76 (1954) 1970. M. Nussbaum and J. VoitlPnder, Z. Naturforsch. 20a (1965) 1417. C. J.Schinkel, Thesis, University of Amsterdam. 1965: G. W.Rathenau and J.L.Snoek, Philips Res.Repts. 1 (1946) 239. L. N.Mulay and ICI.E. Fos, J. Chem.Phys. 3s (1963) 760. E-Weiss and E.O.Fischer. Z.Acors.Chem. 278 (1955) 219.
[6] J.D.Dunitz and L.E.Orgel. J.Chem.Phys. 23 (1954) 954. [7] E.M.Shustorovich and M. E.Dyat!iina, J.Struct. Chem.USSR l(l960) 98. [S] J. P.Dahl and C. J. Ballhausen, Mat. Fys.Rfzdd.Dan. Vid.Selsk. 33. No. 5 (1961). [9] R.D.Fischer,. Theor&.Chim.Acta l(l963) 41s. [lo] R.Prins, P.Biloen and J.D.W.van Voorst. to be published in J. Chem. Phys. 46 (1967) 1216.
f