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Exchange interaction via hydrogen bridges in dimeric copper(II) complexes. A theoretical analysis
CHEMICAL PHYSICS LETTERS
Volume 128, number 3
EXCHANGE INTERACTION VIA HYDROGEN BRIDGES IN DIMERIC COPPER(H) COMPLEXES. A THEORETICAL F. NEPVEU,
S. GEHRING
25 July 1986
ANALYSIS
and L. WALZ
Institut fb Physikalische Chemie, Technische Hachschule Darmstadt, Petersenstrasse 20, D-6100 Darmstadt, West Germany
Received 22 April 1986
The singlet-triplet splitting (EST = 2512) is calculated in dimeric [Cu(H20)(NH3)2(OH)]* where the copper atoms are joined by two O-H- - -0 bridges. This molecule acts as a model for a well investigated copper(R) complex (0- - -0 2.32(2) R) which shows an antiferromagnetic spin coupling (2112 = -94 cm-l). Calculations with different oxygen-oxygen distances (2.34-2.S4 A) show a nearly linear dependence of the singlet-triplet splitting which is far from being negligible even for the greater O-O distances (2.64-2.84 A).
1. Introduction Many oligomeric and polymeric spincoupled copper(I1) complexes have beerrsynthesized within the last thirty years. The bridges which join the copper atoms are various and many of these compounds were structurally as well as magnetically investigated. In most cases the bridging units enable superexchange to take place between magnetic ions which are separated by distances of 2.6 to 6 A. The number of papers in this field is immense and a summary cannot be given here (see for instance refs. [l-4] ). Many results of investigations on copper(I1) complexes with the same bridging unit underlay the magneto-structural correlations [5-l l] . With the aid of these correlations the influence of geometrical parameters on the sign and the size of the exchange interaction can be seen. This phenomenological description was an important step towards a more theoretical description of the interaction between weakly coupled ions. Although no ab initio calculation of the singlettriplet separation was carried out before 1981 [ 121 , several qualitative and semiquantitative explanations for the spin coupling within these compounds were given. For instance Hay et al. [13] have given a convincing explanation of the angle dependence of the singlet-triplet splitting in hydroxo-bridged dimers. 300
Recently [14,15] some magneto-structural correlations were verified using a method proposed by de Loth et al. [12] . The agreement of these results with experiment encouraged us to calculate the singlet -triplet splitting in systems where the pathway for the exchange interaction contains hydrogen bridges. Magneto-structural correlations for such molecules are not known, since only a few examples exist. The reason for these calculations was our structural and magnetic investigations on two dimeric copper(I1) complexes which both had a very weak antiferromagnetic spin coupling; nevertheless their singlet ground states had been confirmed by magnetic susceptibility measurements at low temperatures (4.2-15 K) and by EPR spectra [16,17] . Fig. 1 shows the central cores in dichloro-bis [N-(2-hydroxy-2-phenylethyl)salicylaldiminatocopper(II)] (a, 2J12 = -7.1(4) cm-l [ 161) and in [bis(3-t-butyl-5-methylsalicylidene)-1,3-diamino-2hydroxypropane] copper(R) (b, 25,, = -2.8(4) cm-l [ 171). For a the question arises whether in addition to the superexchange through the chlorine bridges the pathway via the hydrogen bridges plays a role; in b superexchange through the saturated propan-2-01 bridge seemed unlikely and had not been previously observed. Except for these compounds only a few other examples of spin-coupled hydrogen-bridged copper(I1)
CHEMICAL PHYSICS LETTERS
Volume 128, number 3
28 July 1986
0 b
a Fig. 1. Schematic representation
of the central cores in a and b (see text). Hydrogen bridges (A) are represented with dashed lines.
complexes are known. Bertrand et al. [ 181 described in 1976 a copper(I1) complex in which the double Schiff base of 2,4-pentanedione and 2-aminoethanol coordinates the copper atom. Our model molecule was derived from this complex and is depicted in fig. 2. In dimeric aminoethanol-aminoethanolatocopper(I1) nitrate and its N,Ndimethylated monohydrate analogue the monomeric subunits are also connected by hydrogen bonds (2J12 = -56 and -70 cm-l, respectively [ 191). The mean value of the hydrogen-bridge bond lengths for these two compounds is 2.47 f 0.04 A. Hydrogen-bridged dimeric and trimeric copper(I1) triflate compounds with several aminopropanols were investigated by Nieuwpoort et al. [20]. They observed a very weak spin coupling for one complex (2J12 = -8.5 cm-l). In this paper we present the results of calculations on the singlet-triplet splitting as a function of the hy-
H3C )C-N’ HC
H$
\
CH,-CH,
‘CH
2-
CL
CH,
CH,
,I-C<
\C+_H/
C__N’ ’
CH,-
$bH----4,
‘“lN_
2. Method Our approach was similar to that of de Lath et al. [ 121. Several recent calculations have demonstrated the validity of this method [4,14,15,21]. Since all details have been reported, we recall only the basic principles. The first step of the method is to obtain the magnetic orbitals by an open-shell SCF calculation. The mean-type Fock operator used is defined as
where h is the oneelectron operator, j and i are the Coulomb and exchange operators, respectively, associated with the molecular orbital. For the above equationj runs over all the doubly occupied orbitals, i.e. all ligand orbitals and doubly occupied 3d orbitals of the copper atom. From the nearly degenerate singly occupied molecular orbitals (SOMOs) ug and uU we define two localized magnetic orbitals a = 2-1/2(cg
CH3
Fig. 2. Schematic representation of the Bertrand complex (point group Ci). OH---O 2.32(2) A, 2Jlz(exp) = -94 cm-l
1181.
bond length in a modeled copper(H)
C”tH \
2
drogen-bridge dimer .
t 0,)
b = 2-1/2(0g
As a first approximation are defined by l\ku = An(ii)(ab
- ou).
the singlet and triplet states
- bi) = 2-1/2(@,
- * 2) 9 301
Volume 128, number 3
CHEMICALPHYSICS LETTERS
3\k, = Arr(ii)(ab+ b@ = 2-1/2(ip1 + Q2). At this stage, the splitting is equal to 2KQb where Kab the exchange integral between the magnetic orbitals
is
Kab =
Expressions for the second- and fourth-order contributions to the singlet-triplet splitting are obtained by a perturbation configuration interaction treatment and can be found in ref. [21] . They will not be repeated here. The calculations were performed on the model system depicted in fig. 3, which is constructed from the complex described by Bertrand et al. [ 181 (see fig. 2). This complex was chosen since it has been structurally as well as magnetically well investigated and the copper atoms are only connected via the hydrogen bridges. Azomethin nitrogens are replaced by ammonia molecules, the alkoxo groups by hydroxy groups and the alcohol groups by water molecules. The bond lengths and angles between the Cu, N and 0 atoms are the same as within the real molecule. In the model complex the N-H and O-H bonds are of length 1.OOand 0.96 A, respectively. Tetrahedral symmetry was assumed for the NH, groups. The hydrogen atoms of the hydroxy groups are positioned on the O-C vector of the real molecule. The hydrogen atoms of the water molecules are placed in the 0 ---O-C plane assuming an angle of 104.45’, the bridging hydrogen atoms are located on the O---O hydrogen bridges. Then calculations were carried out on this model with successive elongations of the hydrogen bridges (O---O 2.342.84 A) in steps of 0.1 A. This modification to the real molecule was necessary due to program limitations on the basis set size. Recently we described the influence of simplification of real molecules by representation as models by performing calculations as well
W\ H, N’
/H ,OH----O,Cu,NH,
cu
’
‘NH,
O----HO’ H’
‘H
1
-
H\
2+
Fig. 3. Model system used for the calculations derived from the Bertrand complex (see fig. 2).
302
25 July 1986
as through the influence of the size of the atomic basis set on the final result [21] . Ab initio SCF calculations were carried out using an extended version of the PSHONDO program (a modified version of the HONDO package [22] ). Pseudopotentials as defined in ref. [23] were used for all atoms, parameters for the copper pseudopotential were taken from ref. [24] . In line with our previously reported work [21] , a split-valence type basis set was used for copper and oxygen and minimal basis sets were chosen for the other atoms.
3. Results and discussion All contributions entering into the singlet-triplet splitting are listed in table 1. Fig. 4 shows a plot EST versus ‘o___o calculated for our model complex. For all model molecules an antiferromagnetic coupling results which decreases with increasing oxygen-oxygen distance. From 2.5 A this decrease is almost linear. This agrees with experimental values for complexes exhibiting this type of hydrogen bridging and particularly with the complex of Bertrand which is the basis of our model. A difference of less than 35 cm-l between calculated and experimental values is not large and shows that it is possible to simplify the real complex into a model without losing crucial contributions studying the influence of one parameter in a series. If we use our experience, that the trend of certain magneto-structural correlations can be verified by calculation even if the numerical values do not agree exactly with experiment , we can point our from the present calculations that for longer hydrogen bridges a non-negligible spin coupling should be possible. This result is noticeable since the complex of Bertrand contains an exceptionally short hydrogen bond. Other exchange-coupled copper(I1) dimers with spin coupling through hydrogen bonding contain longer bridges. On the other hand it must be emphasized that the exchange interaction does not depend only on distances in the bridging unit; the influence of other parameters has also been extensively studied [5-l 1,1315] . Our attempt was to test, for distances near 3 A, whether an exchange coupling is possible at all; angle effects and influences arising from different donor properties of the ligands seem to be less decisive in this case in our opinion. Angle variations are important for
Table 1 Contributions to the sin@-triplet
25 July 1986
CHEMICALPHYSICS LETTERS
Volume 128, number 3
splitting (cm-‘) as a function of the O---O distance (A) ‘O-0
potential exchange kinetic exchange (KE) double spin polarization (DSP) charge transfer (l&and + copper) charge transfer (copper -, l&and) kinetic exchange polarization (KE, KE + P)2 (KE, KE + P)4 x 'EST
2.34
2.44
2.54
2.64
175.1 -139.9
139.5 -117.7
111.6 -98.1
-8.6
-10.0
-46.0 -4.8
-49.6
-55.2 -129.0
2.74
2.84
-83.5
71.7 -69.6
51.5 -56.8
-10.6
-10.3
-10.0
-9.2
-31.2
-30.3
-24.7
-20.2
-16.4
-3.9
-3.1
-2.6
-2.1
-1.7
-31.3 -46.3 -112.9
-28.0 -38.7 -97.8
-20.8 -32.6 -85.1
-15.4 -27.1 -12.7
-11.2 -22.0 -59.8
complexes in which the exchange pathway includes a strong contribution from p orbitals. This was an important result from the extensive calculations on oxygen-bridged copper(H) complexes [ 14,151. While the singlet-triplet splitting in hydroxo- and alkoxo-
E5T cm-’ -130
0 1
89.4
bridged complexes depends strongly on the size of the bridging angle, it was different for the pyridine-N-oxide-bridged dimers. Comparing the A0 coefficients of the bridging oxygen atoms considerable differences were observed for the s and p orbitals. High s orbital contributions to the magnetic orbitals in the pyridine-N-oxide-bridged complexes are responsible for a much weaker angle dependence than in hydroxo- and aIkoxobridged complexes. The same is observed for the hydrogen atoms involved in the bridge of the complex investigated here. Finally, if we transfer these results to the bridging units depicted in fig. 1, one can see that the long hydrogen bridges in both compounds should not be a reason to exclude a spin coupling through this pathway. The much smaller exchange coupling observed for these compounds may be due to different effects. The bridging units show in both iases a geometry different to that in the model and the effects of other geometrical parameters have not been taken into account in these investigations. In addition the influence of the remaining pathways may level the interaction by effects leading to ferromagnetic contributions.
Acknowledgement Fig. 4.Variation of EST with the oxygen-oxygen bridge distance in the model system.
FN is indebted to the Alexander von HumboldtStiftung, Bonn, for a scholarship. Financial support by 303
Volume 128, number 3
the Deutsche Forschungsgemeinschaft acknowledged.
CHEMICAL PHYSICS LETTERS
is gratefully
References 111 M. Kato, H.B. Jonassen and J.C. Fanning, Chem. Rev. 64 (1984) 99, and references therein.
121 M. Mehn’k, Coord. Chem. Rev. 36 (1981) 1, and references therein.
131 M. Melnik, Coord. Chem. Rev. 42 (1982) 259, and references therein.
141 L. Walz, Dissertation, Technische Hochschule Darmstadt (1984).
151 W.E. Hatfield, in: Extended Interactions between metal ions, ed. L.V. Interrante, ACS Symp. Ser., Vol. 5 (Am. Chem. Sot., Washington, 1974). 161 L. Merz and W. Haase, J. Chem. Sot. Dalton Trans. (1980) 875. 171 L. Merz, Dissertation, Technische Hochschule Darmstadt (1980). 181 W.E. Marsh, K.C. Patel, W.E. Hatfield and D.J. Hodgson, Inorg. Chem. 22 (1983) 511. PI K. Nieminen, Ann. Acad. Sci. Fennicae AII, 197 (1983) 1. 1101 L. WaIz, H. Paulus and W. Haase, J. Chem. Sot. Dalton Trans. (1985) 913.
304
28 July 1986
[ 111 L. Walz and W. Haase, J. Chem. Sot. Dalton Trans. (1985) 1243. [ 121 P. de Loth, P. Cassoux, J.P. Daudey and J.P. Mahieu, J. Am. Chem. Sot. 103 (1981) 4007. [ 131 P.J. Hay, J.C. Thibeault and R. Hoffmann, J. Am. Chem. Sot. 97 (1975) 4884. [ 141 F. Nepveu, W. Haase and H. Asthehner, J. Chem. Sot. Faraday Trans., to be published. [ 151 H. Astheimer, Dissertation, Technische Hochschule Darmstadt (1986). [16] F. Nepveu, F.-J. Bormuth and L. Walz, J. Chem. Sot. Dalton Trans., to be published. [17] E. Hilms, H. EIias, H. Paulus and L. Walz, J. Chem. Sot. Dalton Trans., to be published. [ 181 J.A. Bertrand, T.D. Black, PG. EIIer, F.T. Helm and R. Mahmood, Inorg. Chem. 15 (1976) 2965. [191J.A. Bertrand, E. Fujita and D.G. Vanderveer, Inorg. Chem. 19 (1980) 2022. PI G. Nieuwpoort, G.C. Verschoor and J. Reedijk, J. Chem. Sot. Dalton Trans. (1983) 531. 1211 P. de Loth, J.P. Daudey, H. Astheimer, L. Walz and W. Haase, J. Chem. Phys. 82 (1985) 5048. PI M. Dupuis, J. Rys and H.F. King, HONDO-76, QCPE No. 338. 1231 J.C. BartheIat and P. Durand, Gazz. Chhn. Ital. 108 (1978) 255. 1241 M. Pellssier, J. Chem. Phys. 75 (1981) 775.
Volume 128, number 3
EXCHANGE INTERACTION VIA HYDROGEN BRIDGES IN DIMERIC COPPER(H) COMPLEXES. A THEORETICAL F. NEPVEU,
S. GEHRING
25 July 1986
ANALYSIS
and L. WALZ
Institut fb Physikalische Chemie, Technische Hachschule Darmstadt, Petersenstrasse 20, D-6100 Darmstadt, West Germany
Received 22 April 1986
The singlet-triplet splitting (EST = 2512) is calculated in dimeric [Cu(H20)(NH3)2(OH)]* where the copper atoms are joined by two O-H- - -0 bridges. This molecule acts as a model for a well investigated copper(R) complex (0- - -0 2.32(2) R) which shows an antiferromagnetic spin coupling (2112 = -94 cm-l). Calculations with different oxygen-oxygen distances (2.34-2.S4 A) show a nearly linear dependence of the singlet-triplet splitting which is far from being negligible even for the greater O-O distances (2.64-2.84 A).
1. Introduction Many oligomeric and polymeric spincoupled copper(I1) complexes have beerrsynthesized within the last thirty years. The bridges which join the copper atoms are various and many of these compounds were structurally as well as magnetically investigated. In most cases the bridging units enable superexchange to take place between magnetic ions which are separated by distances of 2.6 to 6 A. The number of papers in this field is immense and a summary cannot be given here (see for instance refs. [l-4] ). Many results of investigations on copper(I1) complexes with the same bridging unit underlay the magneto-structural correlations [5-l l] . With the aid of these correlations the influence of geometrical parameters on the sign and the size of the exchange interaction can be seen. This phenomenological description was an important step towards a more theoretical description of the interaction between weakly coupled ions. Although no ab initio calculation of the singlettriplet separation was carried out before 1981 [ 121 , several qualitative and semiquantitative explanations for the spin coupling within these compounds were given. For instance Hay et al. [13] have given a convincing explanation of the angle dependence of the singlet-triplet splitting in hydroxo-bridged dimers. 300
Recently [14,15] some magneto-structural correlations were verified using a method proposed by de Loth et al. [12] . The agreement of these results with experiment encouraged us to calculate the singlet -triplet splitting in systems where the pathway for the exchange interaction contains hydrogen bridges. Magneto-structural correlations for such molecules are not known, since only a few examples exist. The reason for these calculations was our structural and magnetic investigations on two dimeric copper(I1) complexes which both had a very weak antiferromagnetic spin coupling; nevertheless their singlet ground states had been confirmed by magnetic susceptibility measurements at low temperatures (4.2-15 K) and by EPR spectra [16,17] . Fig. 1 shows the central cores in dichloro-bis [N-(2-hydroxy-2-phenylethyl)salicylaldiminatocopper(II)] (a, 2J12 = -7.1(4) cm-l [ 161) and in [bis(3-t-butyl-5-methylsalicylidene)-1,3-diamino-2hydroxypropane] copper(R) (b, 25,, = -2.8(4) cm-l [ 171). For a the question arises whether in addition to the superexchange through the chlorine bridges the pathway via the hydrogen bridges plays a role; in b superexchange through the saturated propan-2-01 bridge seemed unlikely and had not been previously observed. Except for these compounds only a few other examples of spin-coupled hydrogen-bridged copper(I1)
CHEMICAL PHYSICS LETTERS
Volume 128, number 3
28 July 1986
0 b
a Fig. 1. Schematic representation
of the central cores in a and b (see text). Hydrogen bridges (A) are represented with dashed lines.
complexes are known. Bertrand et al. [ 181 described in 1976 a copper(I1) complex in which the double Schiff base of 2,4-pentanedione and 2-aminoethanol coordinates the copper atom. Our model molecule was derived from this complex and is depicted in fig. 2. In dimeric aminoethanol-aminoethanolatocopper(I1) nitrate and its N,Ndimethylated monohydrate analogue the monomeric subunits are also connected by hydrogen bonds (2J12 = -56 and -70 cm-l, respectively [ 191). The mean value of the hydrogen-bridge bond lengths for these two compounds is 2.47 f 0.04 A. Hydrogen-bridged dimeric and trimeric copper(I1) triflate compounds with several aminopropanols were investigated by Nieuwpoort et al. [20]. They observed a very weak spin coupling for one complex (2J12 = -8.5 cm-l). In this paper we present the results of calculations on the singlet-triplet splitting as a function of the hy-
H3C )C-N’ HC
H$
\
CH,-CH,
‘CH
2-
CL
CH,
CH,
,I-C<
\C+_H/
C__N’ ’
CH,-
$bH----4,
‘“lN_
2. Method Our approach was similar to that of de Lath et al. [ 121. Several recent calculations have demonstrated the validity of this method [4,14,15,21]. Since all details have been reported, we recall only the basic principles. The first step of the method is to obtain the magnetic orbitals by an open-shell SCF calculation. The mean-type Fock operator used is defined as
where h is the oneelectron operator, j and i are the Coulomb and exchange operators, respectively, associated with the molecular orbital. For the above equationj runs over all the doubly occupied orbitals, i.e. all ligand orbitals and doubly occupied 3d orbitals of the copper atom. From the nearly degenerate singly occupied molecular orbitals (SOMOs) ug and uU we define two localized magnetic orbitals a = 2-1/2(cg
CH3
Fig. 2. Schematic representation of the Bertrand complex (point group Ci). OH---O 2.32(2) A, 2Jlz(exp) = -94 cm-l
1181.
bond length in a modeled copper(H)
C”tH \
2
drogen-bridge dimer .
t 0,)
b = 2-1/2(0g
As a first approximation are defined by l\ku = An(ii)(ab
- ou).
the singlet and triplet states
- bi) = 2-1/2(@,
- * 2) 9 301
Volume 128, number 3
CHEMICALPHYSICS LETTERS
3\k, = Arr(ii)(ab+ b@ = 2-1/2(ip1 + Q2). At this stage, the splitting is equal to 2KQb where Kab the exchange integral between the magnetic orbitals
is
Kab =
Expressions for the second- and fourth-order contributions to the singlet-triplet splitting are obtained by a perturbation configuration interaction treatment and can be found in ref. [21] . They will not be repeated here. The calculations were performed on the model system depicted in fig. 3, which is constructed from the complex described by Bertrand et al. [ 181 (see fig. 2). This complex was chosen since it has been structurally as well as magnetically well investigated and the copper atoms are only connected via the hydrogen bridges. Azomethin nitrogens are replaced by ammonia molecules, the alkoxo groups by hydroxy groups and the alcohol groups by water molecules. The bond lengths and angles between the Cu, N and 0 atoms are the same as within the real molecule. In the model complex the N-H and O-H bonds are of length 1.OOand 0.96 A, respectively. Tetrahedral symmetry was assumed for the NH, groups. The hydrogen atoms of the hydroxy groups are positioned on the O-C vector of the real molecule. The hydrogen atoms of the water molecules are placed in the 0 ---O-C plane assuming an angle of 104.45’, the bridging hydrogen atoms are located on the O---O hydrogen bridges. Then calculations were carried out on this model with successive elongations of the hydrogen bridges (O---O 2.342.84 A) in steps of 0.1 A. This modification to the real molecule was necessary due to program limitations on the basis set size. Recently we described the influence of simplification of real molecules by representation as models by performing calculations as well
W\ H, N’
/H ,OH----O,Cu,NH,
cu
’
‘NH,
O----HO’ H’
‘H
1
-
H\
2+
Fig. 3. Model system used for the calculations derived from the Bertrand complex (see fig. 2).
302
25 July 1986
as through the influence of the size of the atomic basis set on the final result [21] . Ab initio SCF calculations were carried out using an extended version of the PSHONDO program (a modified version of the HONDO package [22] ). Pseudopotentials as defined in ref. [23] were used for all atoms, parameters for the copper pseudopotential were taken from ref. [24] . In line with our previously reported work [21] , a split-valence type basis set was used for copper and oxygen and minimal basis sets were chosen for the other atoms.
3. Results and discussion All contributions entering into the singlet-triplet splitting are listed in table 1. Fig. 4 shows a plot EST versus ‘o___o calculated for our model complex. For all model molecules an antiferromagnetic coupling results which decreases with increasing oxygen-oxygen distance. From 2.5 A this decrease is almost linear. This agrees with experimental values for complexes exhibiting this type of hydrogen bridging and particularly with the complex of Bertrand which is the basis of our model. A difference of less than 35 cm-l between calculated and experimental values is not large and shows that it is possible to simplify the real complex into a model without losing crucial contributions studying the influence of one parameter in a series. If we use our experience, that the trend of certain magneto-structural correlations can be verified by calculation even if the numerical values do not agree exactly with experiment , we can point our from the present calculations that for longer hydrogen bridges a non-negligible spin coupling should be possible. This result is noticeable since the complex of Bertrand contains an exceptionally short hydrogen bond. Other exchange-coupled copper(I1) dimers with spin coupling through hydrogen bonding contain longer bridges. On the other hand it must be emphasized that the exchange interaction does not depend only on distances in the bridging unit; the influence of other parameters has also been extensively studied [5-l 1,1315] . Our attempt was to test, for distances near 3 A, whether an exchange coupling is possible at all; angle effects and influences arising from different donor properties of the ligands seem to be less decisive in this case in our opinion. Angle variations are important for
Table 1 Contributions to the sin@-triplet
25 July 1986
CHEMICALPHYSICS LETTERS
Volume 128, number 3
splitting (cm-‘) as a function of the O---O distance (A) ‘O-0
potential exchange kinetic exchange (KE) double spin polarization (DSP) charge transfer (l&and + copper) charge transfer (copper -, l&and) kinetic exchange polarization (KE, KE + P)2 (KE, KE + P)4 x 'EST
2.34
2.44
2.54
2.64
175.1 -139.9
139.5 -117.7
111.6 -98.1
-8.6
-10.0
-46.0 -4.8
-49.6
-55.2 -129.0
2.74
2.84
-83.5
71.7 -69.6
51.5 -56.8
-10.6
-10.3
-10.0
-9.2
-31.2
-30.3
-24.7
-20.2
-16.4
-3.9
-3.1
-2.6
-2.1
-1.7
-31.3 -46.3 -112.9
-28.0 -38.7 -97.8
-20.8 -32.6 -85.1
-15.4 -27.1 -12.7
-11.2 -22.0 -59.8
complexes in which the exchange pathway includes a strong contribution from p orbitals. This was an important result from the extensive calculations on oxygen-bridged copper(H) complexes [ 14,151. While the singlet-triplet splitting in hydroxo- and alkoxo-
E5T cm-’ -130
0 1
89.4
bridged complexes depends strongly on the size of the bridging angle, it was different for the pyridine-N-oxide-bridged dimers. Comparing the A0 coefficients of the bridging oxygen atoms considerable differences were observed for the s and p orbitals. High s orbital contributions to the magnetic orbitals in the pyridine-N-oxide-bridged complexes are responsible for a much weaker angle dependence than in hydroxo- and aIkoxobridged complexes. The same is observed for the hydrogen atoms involved in the bridge of the complex investigated here. Finally, if we transfer these results to the bridging units depicted in fig. 1, one can see that the long hydrogen bridges in both compounds should not be a reason to exclude a spin coupling through this pathway. The much smaller exchange coupling observed for these compounds may be due to different effects. The bridging units show in both iases a geometry different to that in the model and the effects of other geometrical parameters have not been taken into account in these investigations. In addition the influence of the remaining pathways may level the interaction by effects leading to ferromagnetic contributions.
Acknowledgement Fig. 4.Variation of EST with the oxygen-oxygen bridge distance in the model system.
FN is indebted to the Alexander von HumboldtStiftung, Bonn, for a scholarship. Financial support by 303
Volume 128, number 3
the Deutsche Forschungsgemeinschaft acknowledged.
CHEMICAL PHYSICS LETTERS
is gratefully
References 111 M. Kato, H.B. Jonassen and J.C. Fanning, Chem. Rev. 64 (1984) 99, and references therein.
121 M. Mehn’k, Coord. Chem. Rev. 36 (1981) 1, and references therein.
131 M. Melnik, Coord. Chem. Rev. 42 (1982) 259, and references therein.
141 L. Walz, Dissertation, Technische Hochschule Darmstadt (1984).
151 W.E. Hatfield, in: Extended Interactions between metal ions, ed. L.V. Interrante, ACS Symp. Ser., Vol. 5 (Am. Chem. Sot., Washington, 1974). 161 L. Merz and W. Haase, J. Chem. Sot. Dalton Trans. (1980) 875. 171 L. Merz, Dissertation, Technische Hochschule Darmstadt (1980). 181 W.E. Marsh, K.C. Patel, W.E. Hatfield and D.J. Hodgson, Inorg. Chem. 22 (1983) 511. PI K. Nieminen, Ann. Acad. Sci. Fennicae AII, 197 (1983) 1. 1101 L. WaIz, H. Paulus and W. Haase, J. Chem. Sot. Dalton Trans. (1985) 913.
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