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Distinctive features of the electronic structure of tetrachloride titanium complexes
MOLSTR 11166
Journal of Molecular Structure 522 (2000) 201–208 www.elsevier.nl/locate/molstruc
Distinctive features of the electronic structure of tetrachloride titanium complexes G.N. Dolenko a, O.Kh. Poleshchuk b,*, B.A. Gostewskii c, J.N. Latosinska d, M. Ostafin d a
Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences, 669033 Irkutsk, Russian Federation b Department of Inorganic Chemistry, Tomsk Pedagogical University, 634041 Tomsk, Russian Federation c Institute of Chemistry, Russian Academy of Sciences, 669033 Irkustk, Russian Federation d Institute of Physics, Adam Mickiewicz University, 61-614 Poznan, Poland Received 22 March 1999; received in revised form 2 August 1999; accepted 23 August 1999
Abstract The paper reports results of measurements of the ClKa shifts in the X-ray fluorescence spectra and 35Cl-NQR frequency for a group of tetrachloride titanium complexes. The experimental ClKa shifts were correlated with the parameters describing donor properties of ligands such as ligand strength (DN), ionisation potential (IP) or parameters of influence (P). Analysis of the correlation dependencies has indicated that changes in the electronic density on the chloride atom of the acceptor, taking place upon complexation, are determined by the polarisation effects. q 2000 Elsevier Science B.V. All rights reserved. Keywords: TiCl4 complexes; Electronic structure
1. Introduction As far as the elements of the third group of the periodic table are concerned, the shifts depend on the effective charge on these atoms [1,2], and the electron density on these atoms increases linearly with negative electric charge. X-ray fluorescence spectroscopy is the effective method for investigation of electron density distribution in compounds containing elements from the 3rd group of the Periodic Table. In earlier studies [3–5] of SnCl4 and SbCl5 complexes, we obtained very good correlations between the effective charge on the chloride atoms
* Corresponding author. Tel.: 148-61-827300. Ext. 277; fax: 148-61-8257758. E-mail address: [email protected] (J.N. Latosinska).
and donors number DN. These dependencies prompted us to conclude that with increasing DN the electron density on chloride atoms increases. We have been looking for a similar relationship for titanium complexes, however, unsuccessfully. Moreover, a strong correlation between the DClKa shifts and 35 Cl-NQR frequencies observed for chlorides of non-transition elements [3], was not noted for TiCl4L2 complexes. Taking into account a different behaviour of TiCl4 complexes when compared to that of the earlier studied complexes, we have undertaken a detailed study of the processes of complexation for the former. The ClKa shifts determined from the X-ray fluorescence spectra and 35Cl-NQR frequencies measured for a few TiCl4L2 complexes were correlated with the parameters describing the electron-donor properties of the ligands.
0022-2860/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00361-0
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2. Experimental and calculations TiCl4L2 complexes were crystallised from a mixture of the donor and acceptor under high vacuum [6]. The structure and purity of the compounds were checked by IR spectroscopy and elemental analysis. The ClKa shifts were measured by X-ray spectrometer ‘STEARAT’ by the method described in detail in Ref. [1]. 35Cl-NQR measurements were made by an FT-NQR spectrometer built at the Institute of Physics, AMU. Taking into regard the broad NQR lines (the half-width of the narrowest was of about 9 kHz), the classical Hahn sequence was applied with the pulse length of 2.5 ms and the repetition time 2 s. All measurements were performed at the liquid nitrogen temperature. The ab initio calculations were carried using the Gaussian 94 suite of programs [7] in the intermediate basis set 6-31G p on the MP2 level of the theory. The calculations were performed assuming the experimental geometry taken from Cambridge database [8,9], or when experimental data were not available, the optimised geometry.
strengths [10], parameters of influence and ionisation potentials of donors [11] are given in Table 1 and Fig. 1. As follows from the experimental results, as far as TiCl4 complexes are concerned, we cannot talk about a correlation between the DClKa shifts and the other parameters, i.e. ligand strength DN, parameters of influence, ionisation potential and 35Cl-NQR frequencies, in contrast to the situation for SbCl5 and SnCl4 complexes [3]. The donor properties of ligands can be determined on the basis of the ionisation potentials of molecular orbitals, degree of localisation MO for heteroatoms, effective charges localised on these heteroatoms and steric factors [12]. However, in the case when none of the parameters has a decisive influence, interpretation of the data from Fig. 1 can lead to false conclusions. In such a situation we decided to use a multiparameter linear regression assuming as variables DN and IP. For TiCl4L2 complexes we obtained the following relation (Fig. 2a): 2DClKa 3:7 8DN 1 10 2IP 1 1 15 r 0:969; s 22; n 10
1
3. Results and discussion The experimental results of DClKa, ligand
Eq. (1) implies that the electron density on chloride atoms in TiCl4L2 complexes depends
Table 1 The experimental and calculated parameters of TiCl4L2 complexes No.
L
Complexes 2DClKa (eV 1000, relative to Cl2)
1 2 3 4 5 6 7 8 9 10
(Me2N)3PO (CH2)4O PPh3 Py Me2NCOH NBz3 Me2CO MeCN Me2S PhCN
271(10) 211(11) 210(12) 203(11) 195(13) 187(9) 175(14) 173(10) 154(9) 143(14)
a
Free ligands
n Cl (MHz) 35
9.49 8.59 8.64 8.37 9.04 – 8.59 8.40 8.65 9.27
Experimental data
Calculated data
DN[10] (kcal/mol)
P [11]
IP (eV) (Ref. no.)
2e (eV)
a
qX
38.6 20.0 26 b 33.1 26.5 32 b 25 14.1 23.5 11.9
25.96 – 10.00 0.00 25.96 – 29.75 0.51 1.76 25.52
10.44 [12] 9.43 [13] 7.92 [13] 9.65 [13] 10.46 [13] 8.0 [13] 9.71 [13] 12.18 [16] 8.65 [14] 9.73 [15]
12.17 10.82 – 11.05 11.73 – 11.09 9.04 9.04 12.58
0.417 0.447 – 0.493 0.487 – 0.446 0.548 0.720 0.259
20.597 20.520 – 20.423 20.422 – 20.393 20.166 0.096 20.353
a Here, and subsequently, the mean-square errors in the last significant digit, taken for the 98% confidence interval (Student’s criterion), are given in parentheses. b These DN values estimated from correlation analysis data for SbCl5L complexes [3].
G.N. Dolenko et al. / Journal of Molecular Structure 522 (2000) 201–208
203
Fig. 1. The dependence of ClKa shift on: (a) DN; (b) IP; (c) P and (d) n 35Cl for the studied TiCl4 complexes.
first of all on the energy of those MO donors, which interact with acceptor’s vacant orbitals. On the other hand, electron density on chloride atoms depends also on the ligand strength DN. The positive sign of the parameter before the IP variable indicates that electron density on chlorine atoms increases with increasing donor properties of the ligands, so the situation is the opposite to that for non-transitive element complexes. As follows from the above, the electron density on chlorine atoms in TiCl4L2 complexes depends mainly on the polarisation effects, which can be described by the formula: A ! Ti–Cl ) A ! Ti1d –Cl2d
2
where A is the coordinating atom from the ligand, able to donate n- or p-electron density. This effect can be illustrated by the resonance structures, with the
contribution of (III) being the most significant:
It is worth noting that with increasing polarity of the TiCl bond (the structure in Eq. (2)), the degree of the bond decreases. Let us analyse the electron density distribution in the molecules obtained by the ab initio method. The calculated parameters characterising the donor properties of free ligands (uncomplexed) i.e. single electron energy (e), the highest occupied MO, interaction with vacant acceptor orbitals, degree of delocalisation of the highest occupied MO (a) onto the coordinating atom of the ligand (for an individual MO a 1) and the charge on the coordinating atom of the ligand, are collected in Table 1. Analysis of the DClKa
204
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Fig. 2. The dependence of ClKa shift on (a) DN and IP (b) qA and a .
dependencies on the above parameters suggests that the coefficient at e is the least important, and DClKa depends mainly on the charge on the coordinating atom of the ligand and degree of delocalisation of
the molecular orbital (Fig. 2b): 2DClKa 2228 24qA 1 234 23a 2 0 10 r 0:987; s 17; n 8
3
G.N. Dolenko et al. / Journal of Molecular Structure 522 (2000) 201–208
205
This dependence supports the conclusion that electron density on chlorine atoms should increase much less with increasing donor properties of the ligand than with increasing polarisation effects. Relations between 35Cl NQR frequency and each of the parameters DN, IP and P, are shown in Fig. 3. Although no correlation between these can be noted, there is a certain characteristic tendency, that is the 35 Cl NQR frequency increases with increasing ligand strength. This tendency is the opposite to that observed for the non-transitive elements complexes [4]. A correlation analysis performed for n 35Cl frequencies (given in Table 1), leads to the conclusion that also in this case a multiparameter regression should be applied (Fig. 4a and b, respectively):
n35 Cl 0:014 7 2DClKa 1 0:6 1IP 1 0:2 5 r 0:981; s 0:8; n 9
n35 Cl 0:06 3DN 1 0:71 9IP 1 0:2 5 r 0:981; s 0:9; n 9
4
5
The above relations imply that n 35Cl depends first of all on IP of the relevant MO of the ligand. With increasing IP the donor properties of the ligand decrease, however, the NQR frequency does not decrease but it increases. A significant positive coefficient at DClKa in Eq. (4) suggests an increase in the electric field gradient on chlorine atom with increasing negative effective charge on these atoms. In the complexes of non-transitive elements SnCl4L2 and SbCl5L, the situation was the reverse, that is the 35 Cl NQR frequency decreased with increasing electron density on chlorine atoms [17]. The different behaviour of titanium complexes is a result of the influence of TiCl bond order on the NQR frequency described in terms of the Townes–Dailey approximation [18] by: e2 Qqzz =e2 Qqat 1=2 Nx 1 Ny 2 Nz 1 2 s2 1 2 i 2 3=2 2 s2 p;
Fig. 3. The dependence of 35Cl-NQR frequency on: (a) DN; (b) IP; (c) P for the studied TiCl4 complexes.
6
where e2 Qqzz is the quadrupole coupling constant, e2 Qqat is the quadrupole coupling constant for a single p electron, s 2 the degree of sp hybridisation, i and p are the ionicity and bond order, respectively.
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Fig. 4. The dependence of 35Cl-NQR frequencies on (a) 2DClKa and IP (b) DN and IP and (c) 21 and a .
Increasing 35Cl NQR frequency as a result of complexation was explained by the fact that in chloride complexes of transitive elements, the electron density transfer from pp orbitals of the chlorine
atom onto vacant dp orbitals of transition element atom plays a greater role [19]. The quantum chemistry calculations have proved [20,21] that upon complex formation, the electron density transfer pp ! dp
G.N. Dolenko et al. / Journal of Molecular Structure 522 (2000) 201–208
207
Fig. 4. (continued)
decreases (or the population of pp orbital of the chlorine atom increases) on the TiCl bond in the acceptor. Such a reduction of the bond order of the TiCl bond leads to the correlation dependence (6), so to an increase in 35Cl NQR frequency. Analysis of the n 35Cl frequency dependence on the parameters describing donor properties of the ligands suggests that the NQR frequencies do not depend on the charge on the coordinating atom of the ligand, but they do on the parameters describing this MO, which is responsible for the electron density transfer onto the acceptor. The relevant correlation dependence takes the form (Fig. 4c):
n35 Cl 0:63 3 21 1 4:0 7a 1 0:0 2 r 0:998; s 0:3; n 8
7
This dependence indicates the electron density transfer pp ! dp, and consequently an increase in 35 Cl NQR frequency as a result of increasing pp electron density on the chlorine atom. Strong correlation dependencies (4), (5), (7) against no correlation between n 35Cl and DClKa, lead to the conclusion that in tetrachlorine titanium complexes
the electron density gradient on the chlorine atoms is mainly determined by the bond order of TiCl bonds, which decreases with increasing polarisation effects. In the case of the complexes of non-transition elements, this gradient was determined only by the distribution of the total electron density on the chlorine atoms. The ab initio calculations performed for TiCl4, SnCl4 and SbCl5 with H2S, in Ref. [21], suggested that the electron density transfer ns ! dM dominates for TiCl4, whereas for SnCl4 and SbCl5 the dominant p . is the ns ! sM–Cl Analysis of the charge distribution in the complexes studied leads to the following conclusions. Upon complex formation, the electron density on the chlorine atoms of the acceptor may change as a result of the two processes: (a) direct transfer of electron density localised on the donor onto vacant 3d orbitals of the Ti atom, (b) an increase of the positive charge on the Ti atom and negative charge on Cl atoms as a consequence of polarisation effects. Moreover, the former process dominates in complexes of non-transition elements, while the latter in TiCl4 complexes.
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References [1] G.N. Dolenko, J. Mol. Struct. 291 (1993) 23. [2] G.N. Dolenko, Z. Latajka, H. Ratajczak, Heteroat. Chem. 6 (1995) 553. [3] G.N. Dolenko, O.Kh. Poleshchuk, V.P. Elin, J. Koput, Izv. AN SSSR, Ser. Khim. (1996) 1465. [4] O.Kh. Poleshchuk, B. Nogaj, G.N. Dolenko, V.P. Elin, J. Mol. Struct. 297 (1993) 295. [5] G.N. Dolenko, O.Kh. Poleshchuk, V.P. Elin, A.L. Ivanovskii, Izv. AN SSSR, Ser. Khim. (1989) 2522. [6] D.F. Shriver, The Manipulation of Air-Sensitive Compounds, McGraw Hill, New York, 1969. [7] M. Frisch, J. Trucks, G.W. Schlegel et al., gaussian94, Revision B. 1, Gaussian Inc., Pittsburgh, PA, 1995. [8] Cambridge Structural Database System, 1998 update. [9] O.Kh. Poleshchuk, B. Nogaj, J.N. Latosinska, A. Glaser, M. Ostafin, J. Mol. Struct. 415 (1997) 153. [10] V. Gutman, Coord. Chem. Rev. 15 (1975) 207.
[11] V.I. Nefedov, M.M. Gofman, Russ. Koord. Chem. 4 (1978) 820. [12] V.V. Zverev, Yu.P. Kitaev, Uspehi khimii 46 (1977) 1515. [13] V.I. Vovna, Electronnay structura organicheskih soedinenii po dannum fotoelectronnoj specroscopii, Nauka, Moscow, 1991, p. 248. [14] V.I. Vovna, F.I. Vilesov, Uspehi fotoniki (1975) 3. [15] J.W. Rabalais, R.J. Colton, J. Electron. Spectrosc. Relat. Phenom. 1 (1972) 83. [16] B.J.M. Neijzen, C.A. De Lange, J. Electron. Spectrosc. Relat. Phenom. 14 (1978) 187. [17] O.Kh. Poleshchuk, J.N. Latosinska, J. Koput, Russ. Koord. Chem. 23 (1997) 744. [18] C.H. Townes, B.P. Dailey, J. Chem. Phys. 17 (1949) 782. [19] A. Yu, E.A. Buslaev, L. Kravchenko, Kolditz, Coord. Chem. Rev. 82 (1987) 1. [20] O.Kh. Poleshchuk, J.N. Latosinska, J. Koput, Russ. Koord. Chem. 22 (1996) 33. [21] O.Kh. Poleshchuk, G. Dolenko, J. Koput, J.N. Latosinska, Russ. Koord. Chem. 43 (1997) 643.
Journal of Molecular Structure 522 (2000) 201–208 www.elsevier.nl/locate/molstruc
Distinctive features of the electronic structure of tetrachloride titanium complexes G.N. Dolenko a, O.Kh. Poleshchuk b,*, B.A. Gostewskii c, J.N. Latosinska d, M. Ostafin d a
Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences, 669033 Irkutsk, Russian Federation b Department of Inorganic Chemistry, Tomsk Pedagogical University, 634041 Tomsk, Russian Federation c Institute of Chemistry, Russian Academy of Sciences, 669033 Irkustk, Russian Federation d Institute of Physics, Adam Mickiewicz University, 61-614 Poznan, Poland Received 22 March 1999; received in revised form 2 August 1999; accepted 23 August 1999
Abstract The paper reports results of measurements of the ClKa shifts in the X-ray fluorescence spectra and 35Cl-NQR frequency for a group of tetrachloride titanium complexes. The experimental ClKa shifts were correlated with the parameters describing donor properties of ligands such as ligand strength (DN), ionisation potential (IP) or parameters of influence (P). Analysis of the correlation dependencies has indicated that changes in the electronic density on the chloride atom of the acceptor, taking place upon complexation, are determined by the polarisation effects. q 2000 Elsevier Science B.V. All rights reserved. Keywords: TiCl4 complexes; Electronic structure
1. Introduction As far as the elements of the third group of the periodic table are concerned, the shifts depend on the effective charge on these atoms [1,2], and the electron density on these atoms increases linearly with negative electric charge. X-ray fluorescence spectroscopy is the effective method for investigation of electron density distribution in compounds containing elements from the 3rd group of the Periodic Table. In earlier studies [3–5] of SnCl4 and SbCl5 complexes, we obtained very good correlations between the effective charge on the chloride atoms
* Corresponding author. Tel.: 148-61-827300. Ext. 277; fax: 148-61-8257758. E-mail address: [email protected] (J.N. Latosinska).
and donors number DN. These dependencies prompted us to conclude that with increasing DN the electron density on chloride atoms increases. We have been looking for a similar relationship for titanium complexes, however, unsuccessfully. Moreover, a strong correlation between the DClKa shifts and 35 Cl-NQR frequencies observed for chlorides of non-transition elements [3], was not noted for TiCl4L2 complexes. Taking into account a different behaviour of TiCl4 complexes when compared to that of the earlier studied complexes, we have undertaken a detailed study of the processes of complexation for the former. The ClKa shifts determined from the X-ray fluorescence spectra and 35Cl-NQR frequencies measured for a few TiCl4L2 complexes were correlated with the parameters describing the electron-donor properties of the ligands.
0022-2860/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00361-0
202
G.N. Dolenko et al. / Journal of Molecular Structure 522 (2000) 201–208
2. Experimental and calculations TiCl4L2 complexes were crystallised from a mixture of the donor and acceptor under high vacuum [6]. The structure and purity of the compounds were checked by IR spectroscopy and elemental analysis. The ClKa shifts were measured by X-ray spectrometer ‘STEARAT’ by the method described in detail in Ref. [1]. 35Cl-NQR measurements were made by an FT-NQR spectrometer built at the Institute of Physics, AMU. Taking into regard the broad NQR lines (the half-width of the narrowest was of about 9 kHz), the classical Hahn sequence was applied with the pulse length of 2.5 ms and the repetition time 2 s. All measurements were performed at the liquid nitrogen temperature. The ab initio calculations were carried using the Gaussian 94 suite of programs [7] in the intermediate basis set 6-31G p on the MP2 level of the theory. The calculations were performed assuming the experimental geometry taken from Cambridge database [8,9], or when experimental data were not available, the optimised geometry.
strengths [10], parameters of influence and ionisation potentials of donors [11] are given in Table 1 and Fig. 1. As follows from the experimental results, as far as TiCl4 complexes are concerned, we cannot talk about a correlation between the DClKa shifts and the other parameters, i.e. ligand strength DN, parameters of influence, ionisation potential and 35Cl-NQR frequencies, in contrast to the situation for SbCl5 and SnCl4 complexes [3]. The donor properties of ligands can be determined on the basis of the ionisation potentials of molecular orbitals, degree of localisation MO for heteroatoms, effective charges localised on these heteroatoms and steric factors [12]. However, in the case when none of the parameters has a decisive influence, interpretation of the data from Fig. 1 can lead to false conclusions. In such a situation we decided to use a multiparameter linear regression assuming as variables DN and IP. For TiCl4L2 complexes we obtained the following relation (Fig. 2a): 2DClKa 3:7 8DN 1 10 2IP 1 1 15 r 0:969; s 22; n 10
1
3. Results and discussion The experimental results of DClKa, ligand
Eq. (1) implies that the electron density on chloride atoms in TiCl4L2 complexes depends
Table 1 The experimental and calculated parameters of TiCl4L2 complexes No.
L
Complexes 2DClKa (eV 1000, relative to Cl2)
1 2 3 4 5 6 7 8 9 10
(Me2N)3PO (CH2)4O PPh3 Py Me2NCOH NBz3 Me2CO MeCN Me2S PhCN
271(10) 211(11) 210(12) 203(11) 195(13) 187(9) 175(14) 173(10) 154(9) 143(14)
a
Free ligands
n Cl (MHz) 35
9.49 8.59 8.64 8.37 9.04 – 8.59 8.40 8.65 9.27
Experimental data
Calculated data
DN[10] (kcal/mol)
P [11]
IP (eV) (Ref. no.)
2e (eV)
a
qX
38.6 20.0 26 b 33.1 26.5 32 b 25 14.1 23.5 11.9
25.96 – 10.00 0.00 25.96 – 29.75 0.51 1.76 25.52
10.44 [12] 9.43 [13] 7.92 [13] 9.65 [13] 10.46 [13] 8.0 [13] 9.71 [13] 12.18 [16] 8.65 [14] 9.73 [15]
12.17 10.82 – 11.05 11.73 – 11.09 9.04 9.04 12.58
0.417 0.447 – 0.493 0.487 – 0.446 0.548 0.720 0.259
20.597 20.520 – 20.423 20.422 – 20.393 20.166 0.096 20.353
a Here, and subsequently, the mean-square errors in the last significant digit, taken for the 98% confidence interval (Student’s criterion), are given in parentheses. b These DN values estimated from correlation analysis data for SbCl5L complexes [3].
G.N. Dolenko et al. / Journal of Molecular Structure 522 (2000) 201–208
203
Fig. 1. The dependence of ClKa shift on: (a) DN; (b) IP; (c) P and (d) n 35Cl for the studied TiCl4 complexes.
first of all on the energy of those MO donors, which interact with acceptor’s vacant orbitals. On the other hand, electron density on chloride atoms depends also on the ligand strength DN. The positive sign of the parameter before the IP variable indicates that electron density on chlorine atoms increases with increasing donor properties of the ligands, so the situation is the opposite to that for non-transitive element complexes. As follows from the above, the electron density on chlorine atoms in TiCl4L2 complexes depends mainly on the polarisation effects, which can be described by the formula: A ! Ti–Cl ) A ! Ti1d –Cl2d
2
where A is the coordinating atom from the ligand, able to donate n- or p-electron density. This effect can be illustrated by the resonance structures, with the
contribution of (III) being the most significant:
It is worth noting that with increasing polarity of the TiCl bond (the structure in Eq. (2)), the degree of the bond decreases. Let us analyse the electron density distribution in the molecules obtained by the ab initio method. The calculated parameters characterising the donor properties of free ligands (uncomplexed) i.e. single electron energy (e), the highest occupied MO, interaction with vacant acceptor orbitals, degree of delocalisation of the highest occupied MO (a) onto the coordinating atom of the ligand (for an individual MO a 1) and the charge on the coordinating atom of the ligand, are collected in Table 1. Analysis of the DClKa
204
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Fig. 2. The dependence of ClKa shift on (a) DN and IP (b) qA and a .
dependencies on the above parameters suggests that the coefficient at e is the least important, and DClKa depends mainly on the charge on the coordinating atom of the ligand and degree of delocalisation of
the molecular orbital (Fig. 2b): 2DClKa 2228 24qA 1 234 23a 2 0 10 r 0:987; s 17; n 8
3
G.N. Dolenko et al. / Journal of Molecular Structure 522 (2000) 201–208
205
This dependence supports the conclusion that electron density on chlorine atoms should increase much less with increasing donor properties of the ligand than with increasing polarisation effects. Relations between 35Cl NQR frequency and each of the parameters DN, IP and P, are shown in Fig. 3. Although no correlation between these can be noted, there is a certain characteristic tendency, that is the 35 Cl NQR frequency increases with increasing ligand strength. This tendency is the opposite to that observed for the non-transitive elements complexes [4]. A correlation analysis performed for n 35Cl frequencies (given in Table 1), leads to the conclusion that also in this case a multiparameter regression should be applied (Fig. 4a and b, respectively):
n35 Cl 0:014 7 2DClKa 1 0:6 1IP 1 0:2 5 r 0:981; s 0:8; n 9
n35 Cl 0:06 3DN 1 0:71 9IP 1 0:2 5 r 0:981; s 0:9; n 9
4
5
The above relations imply that n 35Cl depends first of all on IP of the relevant MO of the ligand. With increasing IP the donor properties of the ligand decrease, however, the NQR frequency does not decrease but it increases. A significant positive coefficient at DClKa in Eq. (4) suggests an increase in the electric field gradient on chlorine atom with increasing negative effective charge on these atoms. In the complexes of non-transitive elements SnCl4L2 and SbCl5L, the situation was the reverse, that is the 35 Cl NQR frequency decreased with increasing electron density on chlorine atoms [17]. The different behaviour of titanium complexes is a result of the influence of TiCl bond order on the NQR frequency described in terms of the Townes–Dailey approximation [18] by: e2 Qqzz =e2 Qqat 1=2 Nx 1 Ny 2 Nz 1 2 s2 1 2 i 2 3=2 2 s2 p;
Fig. 3. The dependence of 35Cl-NQR frequency on: (a) DN; (b) IP; (c) P for the studied TiCl4 complexes.
6
where e2 Qqzz is the quadrupole coupling constant, e2 Qqat is the quadrupole coupling constant for a single p electron, s 2 the degree of sp hybridisation, i and p are the ionicity and bond order, respectively.
206
G.N. Dolenko et al. / Journal of Molecular Structure 522 (2000) 201–208
Fig. 4. The dependence of 35Cl-NQR frequencies on (a) 2DClKa and IP (b) DN and IP and (c) 21 and a .
Increasing 35Cl NQR frequency as a result of complexation was explained by the fact that in chloride complexes of transitive elements, the electron density transfer from pp orbitals of the chlorine
atom onto vacant dp orbitals of transition element atom plays a greater role [19]. The quantum chemistry calculations have proved [20,21] that upon complex formation, the electron density transfer pp ! dp
G.N. Dolenko et al. / Journal of Molecular Structure 522 (2000) 201–208
207
Fig. 4. (continued)
decreases (or the population of pp orbital of the chlorine atom increases) on the TiCl bond in the acceptor. Such a reduction of the bond order of the TiCl bond leads to the correlation dependence (6), so to an increase in 35Cl NQR frequency. Analysis of the n 35Cl frequency dependence on the parameters describing donor properties of the ligands suggests that the NQR frequencies do not depend on the charge on the coordinating atom of the ligand, but they do on the parameters describing this MO, which is responsible for the electron density transfer onto the acceptor. The relevant correlation dependence takes the form (Fig. 4c):
n35 Cl 0:63 3 21 1 4:0 7a 1 0:0 2 r 0:998; s 0:3; n 8
7
This dependence indicates the electron density transfer pp ! dp, and consequently an increase in 35 Cl NQR frequency as a result of increasing pp electron density on the chlorine atom. Strong correlation dependencies (4), (5), (7) against no correlation between n 35Cl and DClKa, lead to the conclusion that in tetrachlorine titanium complexes
the electron density gradient on the chlorine atoms is mainly determined by the bond order of TiCl bonds, which decreases with increasing polarisation effects. In the case of the complexes of non-transition elements, this gradient was determined only by the distribution of the total electron density on the chlorine atoms. The ab initio calculations performed for TiCl4, SnCl4 and SbCl5 with H2S, in Ref. [21], suggested that the electron density transfer ns ! dM dominates for TiCl4, whereas for SnCl4 and SbCl5 the dominant p . is the ns ! sM–Cl Analysis of the charge distribution in the complexes studied leads to the following conclusions. Upon complex formation, the electron density on the chlorine atoms of the acceptor may change as a result of the two processes: (a) direct transfer of electron density localised on the donor onto vacant 3d orbitals of the Ti atom, (b) an increase of the positive charge on the Ti atom and negative charge on Cl atoms as a consequence of polarisation effects. Moreover, the former process dominates in complexes of non-transition elements, while the latter in TiCl4 complexes.
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References [1] G.N. Dolenko, J. Mol. Struct. 291 (1993) 23. [2] G.N. Dolenko, Z. Latajka, H. Ratajczak, Heteroat. Chem. 6 (1995) 553. [3] G.N. Dolenko, O.Kh. Poleshchuk, V.P. Elin, J. Koput, Izv. AN SSSR, Ser. Khim. (1996) 1465. [4] O.Kh. Poleshchuk, B. Nogaj, G.N. Dolenko, V.P. Elin, J. Mol. Struct. 297 (1993) 295. [5] G.N. Dolenko, O.Kh. Poleshchuk, V.P. Elin, A.L. Ivanovskii, Izv. AN SSSR, Ser. Khim. (1989) 2522. [6] D.F. Shriver, The Manipulation of Air-Sensitive Compounds, McGraw Hill, New York, 1969. [7] M. Frisch, J. Trucks, G.W. Schlegel et al., gaussian94, Revision B. 1, Gaussian Inc., Pittsburgh, PA, 1995. [8] Cambridge Structural Database System, 1998 update. [9] O.Kh. Poleshchuk, B. Nogaj, J.N. Latosinska, A. Glaser, M. Ostafin, J. Mol. Struct. 415 (1997) 153. [10] V. Gutman, Coord. Chem. Rev. 15 (1975) 207.
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