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The electronic structure of PtCl2(NH3)2 (cis and trans isomers): an SCFXα study
Journal of Molecular Structure, 57 (1979) 169-173 6 EBevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
THE ELECTRONIC STRUCTURE OF PtC12(NH& (CIS AND TRAM3 ISOMERS): AN SCF-Xo STUDY
M. BARBER, Chemistry
J. D. CLARK and A. HINCHLIFFE
Department,
UMIST, Manchester
M60 1QD (Gt. Britain)
(Received 11 April 1979)
ABSTRACT SCF-Xa calculations are reported for the cis- and frans- isomers of PtCl,(NH,),. The applicability of non-relativistic methods of calculation to platinum compounds is discussed. Ionization energies calculated by the transition-state method are compared with experimental ESCA values and differences in the electronic structure of the two isomers are discussed.
INTRODUCTION There is considerable current interest in the electronic structure of platinum complexes, partly because many such complexes are highly effective antitumour agents; cis-PtClz(NH& is indeed a potent anti-tumour agent [ 11 whilst the tians-isomer is quite inactive. Little work has been done on the electronic structure of such molecules beyond the level of Gray’s review [ 21 of 1965, however, for two reasons. Firstly, it is not clear whether ordinary non-relativistic quantum-mechanical methods are appropriate for molecules containing very heavy elements whose spin-orbit parameters are large, and secondly because the molecules are probably too large to be handled easily by conventional ab initio SCF-MO theory, the size of the atomic orbital basis set needed for reliable calculations being prohibitive. A small number of calculations have appeared on UF, [ 31, PtF6 and El,,,F6 [4]* using Slater’s SCF-Xor [5] method. Such calculations do not suffer from the well-known n 3*sdifficulty associated with conventional SCF-MO theory, but are really to be regarded as “parameterized ab initio” calculations: SCF-Xe theory fits neatly into the gap between the extremes of ab initio and semi-empirical theory. A recent review is given in ref. 6. The method is non-relativistic, but Moskowitz and co-workers [ 31 report calculations using either frozen core relativistic X& or non-relativistic XQ methods, and find negligible differences in the valence orbit&. Other workers have come to much the same conclusion [7] . For this paper, therefore, we have used the non-relativistic method.
*E,,0 refers to the unnamed element 110.
170 CALCULATIONS
Experimental geometries were used for both molecules. Sphere radii were chosen so as to be proportional to Slater’s atomic radii [ 51, but because the ratio of the radius of Pt to N and Cl gives different radii for Pt, the average was used. The spherical harmonics used for Pt, N and H were those corresponding to occupied orbit& on the neutral atoms, whilst for Cl in PtCl,(NH3) ‘2, 1,._ = 2 was used to allow for possible backdonation to the Cl 3d levels. The (Yvalues of Schwarz [7] were used in all cases except for Pt (e = 0.7). The parameters used are summarized in Table 1. The SCF calculations were repeated until the largest ‘fractional change in the potential was less than lo-‘, and the transition-state calculations were each continued until the eigenvalue had converged to four figures. RESULTS
AND DISCUSSION
Orbital energies calculated by the SCF-Xc method do not have the same significance as the orbital energies of canonical SCF MO’s in that there is no analogue of Koopmans’ theorem in SCF-Xa theory. We therefore record in TABLE
1
Geometry,
sphere radii and exchange factors
Cis-Pt(NHJ $1, planar C,, [8] z;TN1
: ;*;;
rN-M
= 1:03 A
LClPtCl = 91.9”
;
Pt Cl H N Outer sphere
LNPtN LHNH Sphere radius (a,,)
I,,,
cv
2.54665 1.85641 0.694766 1.25170 6.25947
3 2 0 1 4
0.70000 0.72277 0.97804 0.75118 0.865104
Tram-Pt(NH,) ,dl, planar Cl,, [8] rpt_cl = 2.32 A = 2.05 A rPt-N rN-H
= 1.03 A
Pt Cl N H Outer sphere
= 87.0” = 109.5”
LCl,PtN = 91.5” LCl,PtN = 88.5” LHNH = 109.5”
Sphere radius (a,)
1max
Q
2.56674
3
0.70000
1.81743 1.30720 0:639215 6.20159
2 1 0 4
0.72277 0.75118 0.97804 0.865104
171 TABLE 2 Xa transition state ionization energies (eV) for valence region !hm-Pt(NH,),Cl,
Cis-Pt(NH,),Cl,
7a2
28a, 19b, 27a, 12b, 26a, Mb, 25a, llb, 6a, 24a, 17b, 16b, 5a, lob, 230, 15b, 22a, 14b, 21a,
11.62 11.66 11.69 12.77 12.93 13.46 14.09 16.68 16.94 16.95 18.18 18.56 18.85 19.00 19.24 19.42 24.41 24.51 I ii*;:)
Cl Cl Cl c1 Cl NH, Cl Pt Pt Pt Pt NH, NH, NH, NH, NH,
3p 3p 3p 3p 3p 3a,, Cl 3p dx, dyz dxz dzt, NH, 30, le+ lele+ le-
Cl
3s
NH,
20,
943 2%
25b, 24b, 3P
lla, 24as 8bs 23ag 23b,
3a,
7b, 22a, 21ag 100, 22b, 6bg 20ag 21b, 19as 20b, 1%
11.21 11.21 11.81 11.97 12.14 12.36 13.21 13.33 13.56 14.39 14.62 15.96 15.96 15.98 16.25 16.37
Cl Cl Cl Cl Cl Cl Pt Pt NH, Pt Pt NH, NH, NH, NH, NH,
3p 3p 3p 3p 3p 3p dxz dxy 3a; dyt dzz 3a; le+ le+ lele-
;:*;;5)
Cl
3s
26:39 26.61
I NH3
3a1
Table 2 the valence electron ionization energies calculated according to the transition-state method. The transition-state method allows directly for orbital relaxation on ionization, and the energies produced can be used for comparison with photoelectron data. We also indicate in Table 2 the dominant A0 contributions to the MOs. The orbit& with large contributions from N and H have coefficients in similar ratios to those expected on the basis of local C3” symmetry and we have therefore used the IR labels of CJv accordingly, the local z axis being the N-Pt axis. In both molecules the lowest virtual orbital has the symmetry of the Pt 3d,zAyz AO, with a significant density in the Pt atomic sphere. ESCA results for the solid compounds are listed in Table 3. The most obvious difference in the spectra is in the region below about lO-eV binding energy. Three distinct peaks are resolved in the cis compound but for the tram compound there is a single peak with two shoulders. There are peaks at 17-eV binding energy which are probably due to Cl 3s orbitals, and peaks at 23 eV which are probably due to N 2s (NH3 2a,) orbitals. The higher binding energy orbitals have much the same energy in both complexes. In comparing theory with experiment, it should be remembered that the calculations are appropriate to gas-phase, isolated molecules whilst the experimental data refer to solids. The absolute values of the experimental ionization energies should therefore be assigned little importance, and the absolute values of the calculated ionization energies depend on the choice of sphere radii and
172 TABLE 3 ESCA ionization energies(eV) for solid compounds
Valence Cl 3s NH, 2% Pt 5P Pt 4f Cl 2p Cl 2s Pt4d Cl 1s
Cis-Pt(NH,),Cl,
!hns-Pt(NH,),Cl,
4.2 6.0 8.7 17.1 24.0 54.0 76.7 73.4 199.8 189.4 269.2 335. 315. 400.
3.6 5.6 7.8 17.0 22.5 55.0 76.6 73.3 200.2 198.8 268.5 333. 313. 400.
exchange factors, although in a linear, well-behaved manner [9]. Thus one should compare groupings and spacings rather than absolute numbers. Again, there are no formal selection rules for PES, and levels separated by less than 1 eV are unlikely to be resolved. Thus, comparing the calculated ionization energies with the ESCA results it can be seen that the general features agree qualitatively. The calculated splitting of the NH3 2aI and Cl 3s orbitals is larger in the cis complex than in the trans complex, and this is confirmed by the ESCA results. The NH3 le orbit& are hardly affected by the rest of the molecule and are nearly degenerate. The NH3 3al orbitals (the lone pairs) are markedly affected, however, and the molecular orbitals formed as their sum and difference are quite widely separated. The highest orbitals in both compounds are formed from chlorine 3p atomic orbitals and are quite close together. The Pt 5d orbit& are split into distinct groups but the splitting is different in the two compounds. A possible interpretation of the spectra is as follows. The peak at 8.7 eV in the cis-Pt(NH&C& spectrum is due to the NH%le orbit&, a NH3 2a, orbital and one composed mainly of Pt 5d,z but with a little NH3 3a,. The peak at 6.0 eV is made up of Pt 4d,,, 5d,,, 5d,, while the highest peak is mainly Cl 3p but with one orbital that is mainly NH3 3aI. The energy levels of the trans complex are split into four groups. The lowest consists of NH3 le and 3aI orbitals and corresponds to the peak at 7.8 eV. The next consists of Pt 5d,z and 5d,, and corresponds to the peak at 5.6 eV. The third group comprises NH, 3a1, Pt 5d,,, 5d,, and is not clearly resolved in the spectra. The highest group is again Cl 3p orbitals.
173 TABLE 4 Partitioning of electron density. OUT refers to the eIectron density outside the outer sphere
OUT H(average) N Cl Pt Intersphere NH,
Trans-PtCl,(NH,),
Cis-PtCl,(NH,),
0.7945 0.1979 5.9140 14.7881 7 6.9487 1.1665 6.5077
0.7088 0.2666 5.7115 14.7957 77.0025 1.1666 6.5113
CONCLUSIONS
Any discussion of differences in the electronic structure of the molecules based on the individual molecular orbit& is of necessity subjective: the total wavefunction is invariant to any unitary transformation, and any set of MOs could be used. It is probably safer to look at the electron density as a whole, this being the only invariant. In Table 4 we record the SCF-Xcz partitioning of total electron density into the intersphere and atomic sphere regions. There is little experience available as how to interpret the numbers rigorously, and there is almost certainly no relationship between them and the ordinary Mulliken gross atom populations produced by conventional SCF-MO theory. However, it is probable that comparisons of such numbers between rather similar molecules will be relevant. The electron density in the Pt and spheres is the same in either molecule, although the NH3 groups are a little different, but the total NH3 electron density is the same in either molecule. The implication therefore is that the different reactivities of the two isomers are due to steric rather than electronic effects. REFERENCES 1 B. Rosenberg, L. van Camp, J. E. Trusko and V. H. Mansour, Nature (London), 222 (1969) 385. 2 H. B. Gray, in R. L. Carlin (Ed.), Transition Metal Chemistry, Vol. 1, Arnold, London, 1965. 3 M. Boring, J. H. Wood and J. W. Moskowitz, J. Chem. Phys., 61 (1974) 3800. 4 J. T. Waber and F. W. Averill, J. Chem. Phys., 60 (1974) 4466. 5 J. C. Slater, Quantum Theory of Molecules and Solids, Vol. 4, McGraw-Hill, New York, 1974. 6 J. W. D. Connolly, in H. F. Schaefer III (Ed.), Modem Theoretical Chemistry, Vol. 7, Plenum Press, New York, 1977. 7 K. Schwarz, Phys. Rev. B,, 5 (1972) 2466. 8 M. R. Truter and C. W. Milburn, J. Chem. Sot. A, (1966) 1609. 9 Unpublished results,
THE ELECTRONIC STRUCTURE OF PtC12(NH& (CIS AND TRAM3 ISOMERS): AN SCF-Xo STUDY
M. BARBER, Chemistry
J. D. CLARK and A. HINCHLIFFE
Department,
UMIST, Manchester
M60 1QD (Gt. Britain)
(Received 11 April 1979)
ABSTRACT SCF-Xa calculations are reported for the cis- and frans- isomers of PtCl,(NH,),. The applicability of non-relativistic methods of calculation to platinum compounds is discussed. Ionization energies calculated by the transition-state method are compared with experimental ESCA values and differences in the electronic structure of the two isomers are discussed.
INTRODUCTION There is considerable current interest in the electronic structure of platinum complexes, partly because many such complexes are highly effective antitumour agents; cis-PtClz(NH& is indeed a potent anti-tumour agent [ 11 whilst the tians-isomer is quite inactive. Little work has been done on the electronic structure of such molecules beyond the level of Gray’s review [ 21 of 1965, however, for two reasons. Firstly, it is not clear whether ordinary non-relativistic quantum-mechanical methods are appropriate for molecules containing very heavy elements whose spin-orbit parameters are large, and secondly because the molecules are probably too large to be handled easily by conventional ab initio SCF-MO theory, the size of the atomic orbital basis set needed for reliable calculations being prohibitive. A small number of calculations have appeared on UF, [ 31, PtF6 and El,,,F6 [4]* using Slater’s SCF-Xor [5] method. Such calculations do not suffer from the well-known n 3*sdifficulty associated with conventional SCF-MO theory, but are really to be regarded as “parameterized ab initio” calculations: SCF-Xe theory fits neatly into the gap between the extremes of ab initio and semi-empirical theory. A recent review is given in ref. 6. The method is non-relativistic, but Moskowitz and co-workers [ 31 report calculations using either frozen core relativistic X& or non-relativistic XQ methods, and find negligible differences in the valence orbit&. Other workers have come to much the same conclusion [7] . For this paper, therefore, we have used the non-relativistic method.
*E,,0 refers to the unnamed element 110.
170 CALCULATIONS
Experimental geometries were used for both molecules. Sphere radii were chosen so as to be proportional to Slater’s atomic radii [ 51, but because the ratio of the radius of Pt to N and Cl gives different radii for Pt, the average was used. The spherical harmonics used for Pt, N and H were those corresponding to occupied orbit& on the neutral atoms, whilst for Cl in PtCl,(NH3) ‘2, 1,._ = 2 was used to allow for possible backdonation to the Cl 3d levels. The (Yvalues of Schwarz [7] were used in all cases except for Pt (e = 0.7). The parameters used are summarized in Table 1. The SCF calculations were repeated until the largest ‘fractional change in the potential was less than lo-‘, and the transition-state calculations were each continued until the eigenvalue had converged to four figures. RESULTS
AND DISCUSSION
Orbital energies calculated by the SCF-Xc method do not have the same significance as the orbital energies of canonical SCF MO’s in that there is no analogue of Koopmans’ theorem in SCF-Xa theory. We therefore record in TABLE
1
Geometry,
sphere radii and exchange factors
Cis-Pt(NHJ $1, planar C,, [8] z;TN1
: ;*;;
rN-M
= 1:03 A
LClPtCl = 91.9”
;
Pt Cl H N Outer sphere
LNPtN LHNH Sphere radius (a,,)
I,,,
cv
2.54665 1.85641 0.694766 1.25170 6.25947
3 2 0 1 4
0.70000 0.72277 0.97804 0.75118 0.865104
Tram-Pt(NH,) ,dl, planar Cl,, [8] rpt_cl = 2.32 A = 2.05 A rPt-N rN-H
= 1.03 A
Pt Cl N H Outer sphere
= 87.0” = 109.5”
LCl,PtN = 91.5” LCl,PtN = 88.5” LHNH = 109.5”
Sphere radius (a,)
1max
Q
2.56674
3
0.70000
1.81743 1.30720 0:639215 6.20159
2 1 0 4
0.72277 0.75118 0.97804 0.865104
171 TABLE 2 Xa transition state ionization energies (eV) for valence region !hm-Pt(NH,),Cl,
Cis-Pt(NH,),Cl,
7a2
28a, 19b, 27a, 12b, 26a, Mb, 25a, llb, 6a, 24a, 17b, 16b, 5a, lob, 230, 15b, 22a, 14b, 21a,
11.62 11.66 11.69 12.77 12.93 13.46 14.09 16.68 16.94 16.95 18.18 18.56 18.85 19.00 19.24 19.42 24.41 24.51 I ii*;:)
Cl Cl Cl c1 Cl NH, Cl Pt Pt Pt Pt NH, NH, NH, NH, NH,
3p 3p 3p 3p 3p 3a,, Cl 3p dx, dyz dxz dzt, NH, 30, le+ lele+ le-
Cl
3s
NH,
20,
943 2%
25b, 24b, 3P
lla, 24as 8bs 23ag 23b,
3a,
7b, 22a, 21ag 100, 22b, 6bg 20ag 21b, 19as 20b, 1%
11.21 11.21 11.81 11.97 12.14 12.36 13.21 13.33 13.56 14.39 14.62 15.96 15.96 15.98 16.25 16.37
Cl Cl Cl Cl Cl Cl Pt Pt NH, Pt Pt NH, NH, NH, NH, NH,
3p 3p 3p 3p 3p 3p dxz dxy 3a; dyt dzz 3a; le+ le+ lele-
;:*;;5)
Cl
3s
26:39 26.61
I NH3
3a1
Table 2 the valence electron ionization energies calculated according to the transition-state method. The transition-state method allows directly for orbital relaxation on ionization, and the energies produced can be used for comparison with photoelectron data. We also indicate in Table 2 the dominant A0 contributions to the MOs. The orbit& with large contributions from N and H have coefficients in similar ratios to those expected on the basis of local C3” symmetry and we have therefore used the IR labels of CJv accordingly, the local z axis being the N-Pt axis. In both molecules the lowest virtual orbital has the symmetry of the Pt 3d,zAyz AO, with a significant density in the Pt atomic sphere. ESCA results for the solid compounds are listed in Table 3. The most obvious difference in the spectra is in the region below about lO-eV binding energy. Three distinct peaks are resolved in the cis compound but for the tram compound there is a single peak with two shoulders. There are peaks at 17-eV binding energy which are probably due to Cl 3s orbitals, and peaks at 23 eV which are probably due to N 2s (NH3 2a,) orbitals. The higher binding energy orbitals have much the same energy in both complexes. In comparing theory with experiment, it should be remembered that the calculations are appropriate to gas-phase, isolated molecules whilst the experimental data refer to solids. The absolute values of the experimental ionization energies should therefore be assigned little importance, and the absolute values of the calculated ionization energies depend on the choice of sphere radii and
172 TABLE 3 ESCA ionization energies(eV) for solid compounds
Valence Cl 3s NH, 2% Pt 5P Pt 4f Cl 2p Cl 2s Pt4d Cl 1s
Cis-Pt(NH,),Cl,
!hns-Pt(NH,),Cl,
4.2 6.0 8.7 17.1 24.0 54.0 76.7 73.4 199.8 189.4 269.2 335. 315. 400.
3.6 5.6 7.8 17.0 22.5 55.0 76.6 73.3 200.2 198.8 268.5 333. 313. 400.
exchange factors, although in a linear, well-behaved manner [9]. Thus one should compare groupings and spacings rather than absolute numbers. Again, there are no formal selection rules for PES, and levels separated by less than 1 eV are unlikely to be resolved. Thus, comparing the calculated ionization energies with the ESCA results it can be seen that the general features agree qualitatively. The calculated splitting of the NH3 2aI and Cl 3s orbitals is larger in the cis complex than in the trans complex, and this is confirmed by the ESCA results. The NH3 le orbit& are hardly affected by the rest of the molecule and are nearly degenerate. The NH3 3al orbitals (the lone pairs) are markedly affected, however, and the molecular orbitals formed as their sum and difference are quite widely separated. The highest orbitals in both compounds are formed from chlorine 3p atomic orbitals and are quite close together. The Pt 5d orbit& are split into distinct groups but the splitting is different in the two compounds. A possible interpretation of the spectra is as follows. The peak at 8.7 eV in the cis-Pt(NH&C& spectrum is due to the NH%le orbit&, a NH3 2a, orbital and one composed mainly of Pt 5d,z but with a little NH3 3a,. The peak at 6.0 eV is made up of Pt 4d,,, 5d,,, 5d,, while the highest peak is mainly Cl 3p but with one orbital that is mainly NH3 3aI. The energy levels of the trans complex are split into four groups. The lowest consists of NH3 le and 3aI orbitals and corresponds to the peak at 7.8 eV. The next consists of Pt 5d,z and 5d,, and corresponds to the peak at 5.6 eV. The third group comprises NH, 3a1, Pt 5d,,, 5d,, and is not clearly resolved in the spectra. The highest group is again Cl 3p orbitals.
173 TABLE 4 Partitioning of electron density. OUT refers to the eIectron density outside the outer sphere
OUT H(average) N Cl Pt Intersphere NH,
Trans-PtCl,(NH,),
Cis-PtCl,(NH,),
0.7945 0.1979 5.9140 14.7881 7 6.9487 1.1665 6.5077
0.7088 0.2666 5.7115 14.7957 77.0025 1.1666 6.5113
CONCLUSIONS
Any discussion of differences in the electronic structure of the molecules based on the individual molecular orbit& is of necessity subjective: the total wavefunction is invariant to any unitary transformation, and any set of MOs could be used. It is probably safer to look at the electron density as a whole, this being the only invariant. In Table 4 we record the SCF-Xcz partitioning of total electron density into the intersphere and atomic sphere regions. There is little experience available as how to interpret the numbers rigorously, and there is almost certainly no relationship between them and the ordinary Mulliken gross atom populations produced by conventional SCF-MO theory. However, it is probable that comparisons of such numbers between rather similar molecules will be relevant. The electron density in the Pt and spheres is the same in either molecule, although the NH3 groups are a little different, but the total NH3 electron density is the same in either molecule. The implication therefore is that the different reactivities of the two isomers are due to steric rather than electronic effects. REFERENCES 1 B. Rosenberg, L. van Camp, J. E. Trusko and V. H. Mansour, Nature (London), 222 (1969) 385. 2 H. B. Gray, in R. L. Carlin (Ed.), Transition Metal Chemistry, Vol. 1, Arnold, London, 1965. 3 M. Boring, J. H. Wood and J. W. Moskowitz, J. Chem. Phys., 61 (1974) 3800. 4 J. T. Waber and F. W. Averill, J. Chem. Phys., 60 (1974) 4466. 5 J. C. Slater, Quantum Theory of Molecules and Solids, Vol. 4, McGraw-Hill, New York, 1974. 6 J. W. D. Connolly, in H. F. Schaefer III (Ed.), Modem Theoretical Chemistry, Vol. 7, Plenum Press, New York, 1977. 7 K. Schwarz, Phys. Rev. B,, 5 (1972) 2466. 8 M. R. Truter and C. W. Milburn, J. Chem. Sot. A, (1966) 1609. 9 Unpublished results,