Inorg. Chem. 1990,29, 2496-2501
2496
Contribution from the Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7, and Chemistry Division, National Research Council of Canada,+ Ottawa, Ontario, Canada K1 A OR9
Electronic Structure of Square-Planar cis-Bis(trifluoromethyl)platinum(II) Complexes from UV Photoelectron Spectra and SCF-MS-Xa Calculations Dong-Sheng Yang,* G. Michael Bancroft,**$R. J. Puddephatt,*,t and John S. Tses Received May 5, I989 UV photoelectron spectra of c i ~ - [ P t ( c F , ) ~ L(L2 ~ ] = 1,5-cyclooctadiene (COD), Me2NCH2CH2NMe2(TMED); L = PEt,, AsMe,) are reported, as well as MS-XCYcalculations on the model compounds [Pt(CF,)2(C2H4)2]and C~S-[P~(CF,)~(PH,)~]. The spectra are assigned by using M S - X a calculations and by comparison with the spectra of the corresponding dimethylplatinum(I1) complexes. The first two ionization bands (with lowest ionization energies) are assigned to platinum-ligand u orbitals, followed by four bands due to the essentially nonbonding platinum 5d orbitals. The nature of the Pt-CF, u bonding is discussed in terms of M S - X a results. In contrast to the R-CH, u bonds, the Pt-CF, u bonds have a significant C 2s and C-F antibonding character.
Introduction It has long been recognized that perfluoroalkyl transition-metal complexes in general have properties very different from those of the corresponding alkyl complexes. For example, the perfluoroalkyl compounds have enhanced thermal stabilitylS2 and shorter metal-carbon bond lengths compared t o their alkyl anal o g u e ~ .In~ ~platinum( ~ 11) complexes, perfluoroalkyl groups have a high NMR trans influence, comparable t o that of the methyl of methyl groups bound t o platinum(I1) g r o ~ p . ~Replacement ?~ by trifluoromethyl groups leads to deactivation of the metal center toward oxidative-addition reaction^.',^ A ligand L is more readily displaced from c i ~ - [ P t ( c F ~ ) ~than L ~ ]from cis-[PtMe2L2] Together these observations show that the platinum centers are more susceptible t o nucleophilic attack but less susceptible t o electrophilic attack when R = CF3 rather than CH3 in cis-[PtR2L2] complexes. This is expected because of t h e greater electronwithdrawing power of CF3 compared t o CH,. T h e unusual effects of fluorine as a substituent are normally attributed to three of its special properties: (a) its high electronegativity, (b) the three nonbonding electron pairs on F, and (c) the good energy match between the F 2s and 2p orbitals and the corresponding orbitals of the carbon atom. On the basis of these properties, it was suggested t h a t t h e unusual chemistry of the perfluoroalkyl transition-metal complexes was a t least in part due t o A back-bonding between filled d orbitals on the metal a t o m and u* orbitals of the perfluoroalkyl g r o ~ p . I , ~Later, it was suggested that the A back-bonding was not important,I0 and Fenske and Hall concluded from their approximate molecular orbital calculations on MeMn(CO), and (CF3)Mn(CO)511that the significant differences between the alkyl and perfluoroalkyl complexes could be understood in terms of u donation from an antibonding CF3 orbital and energy stabilization due t o t h e effects of charge on neighboring atoms. Since a detailed study of the electronic structure of square-planar cis-dimethylplatinum(I1) complexes has been undertaken in our group,I2 a further investigation of the electronic structure of the corresponding perfluoromethyl complexes would be valuable to help to understand their differences in chemical properties and reactions. In this paper, the UV photoelectron spectra of ~ i s - [ P t ( c F ~ ) ~ L ~ ] (L2 = 1,5-cyclooctadiene (COD), Me,NCH2CH,NMe2 (TMED); L = PEt3, AsMe3) and M S - X a calculations on the model compounds C ~ S - [ P ~ ( C F ~ ) ~ ( and C~H ~ i~s )- [~P]t ( c F , ) , ( P H ~ ) a~rle reported. T h e nature of the C-F bonding and the effect of the CF, groups on the MO energies and on the platinum bonding to t h e ligands L in these complexes a r e discussed.
Experimental Section [PtMe2(C0D)]'* was used as starting material for the preparation of [Pt(CF,),(COD)]. [PtMe,(COD)] (1.30 g, 3.9 mmol) was placed in a 20-mL thick-walled Carius tube and dissolved in 1.5 mL of dichloromethane. The solution was frozen and degassed twice by using conven'NRCC Contribution No. 30029. University of Western Ontario. #National Research Council of Canada 0020- 1669/90/ 1329-2496$02.50/0
tional freeze-thaw vacuum techniques. Trifluoromethyl iodide (1 1 mL) was condensed into the tube, and the Carius tube was sealed. The solution was warmed to room temperature and was allowed to stand for 5 days. The solution was filtered. The filtrate was passed through a Florisil column, eluting with dichloromethane, to give a orange-yellow solution first and then a very pale yellow solution. The volume of the pale yellow solution collected was reduced and n-pentane added to give white crystals. After the mixture was cooled at 0 "C for 1 h, the crystals of [Pt(CF,),(COD)] were filtered out and washed with a small amount of n-pentane: yield 0.83 g (48%); mp 179-181 OC. Other bis(trifluor0methyl)platinum(II) complexes were prepared by replacement of COD with the desired ligand L according to the literature methode2 The compounds were sublimed prior to running the spectra, and their purity and identity were checked by melting points and NMR spectra. Spectral data for the new compound with L = PEt, (white, mp 141-143 "C) are as follows. I9F NMR (CD2C12): 6 -21.65 (2J(Pt-F) = 617.3 Hz, PtCF,; 'J(P-F) = 65.8 Hz, CF,-Pt-PEt,). 31PNMR (CD2CIJ: 6 17.58 ('J(Pt-P) = 2840 Hz, Pt-PEt,). Anal. For C,,H3,,F6P2Pt. Calcd: C, 29.42; H, 5.67. Found: C, 29.51; H , 5.38. UV photoelectron spectra were recorded on a McPherson ESCA-36 spectrometer equipped with a hallow-cathode UV He lamp1) at temperatures within 30 OC of the melting point of each compound. The calibration and computer fitting of the spectra were performed as described before.I2 The calculations presented here were performed by using the relativistic version of the XCY scattered-wave method,I4 in which the relativistic radial wave functions for the Pt atom have been employed. The exchange (Y parameters used in each atomic region were taken from the Schwarz tabulationI5 except for hydrogen, for which 0.777 25 was used.I6 For the extramolecular and intersphere regions, a weighted average of the
King, R. B.; Bisnette, M. B. J . Organomet. Chem. 1964, 2, 15. Clark, H. C.; Manzer, L. E. J . Organomet. Chem. 1973, 59, 411. Churchill, M. R. Inorg. Chem. 1965, 4, 1734. Manojlovic-Muir, L.; Muir, K. W.; Solomun, T.; Meek, D. W.; Peterson, J. L. J . Organomer. Chem. 1978, 146, C26. Appleton, T. G.; Chisholm, M. H.; Clark, H. C.; Manzer, L. E. Inorg. Chem. 1972, 11, 1786.
Appleton, T.G.; Bennett, M. A. Inorg. Chem. 1978, 17, 738. Bennett, M. A,; Chee, H.-K.; Roberston, G . B. Inorg. Chem. 1979, 18, 1061. Appleton, T. G.; Clark, H. C.; Manzer, L. E. J . Organomer. Chem. 1974, 65, 275.
Appleton, T. G.;Hall, J . R.; Neale, D. W.; Williams, M. A. J . Organomer. Chem. 1984, 276, C73. Appleton, T. G . ; Berry, R.D.;Hall, J. R.;Neale, D. W. J . Organomet. Chem. 1988, 342, 399. Cotton, F. A,; McCleverty, J . A. J . Organomet. Chem. 1964, 2, 15. Clark. H. C.: Tsai. J . H. J . OrEanomet. Chem. 1967. 7. 515. Graham, W.'A. G.'Inorg. Chem. 1968, 7 , 315. Johnson; M. P. Inorg. Chim. Acta 1969, 3, 232.
Hall, M. B.; Fenske. R. F. Inorf. Chem. 1972, I/. 768. Yang, D. S.; Bancroft, G . M.; Dignard-Bailey, L.; Puddephatt, R. J.; Tse, J. S . Inorg. Chem., preceding paper in this issue. Bancroft, G. M.; Adams, J.; Coatsworth, L. L.; Bennewitx, C. D.; Brown, J . D.; Westwood, W. D.Anal. Chem. 1975, 47, 586. Johnson, K. H.; Adu. Quantum Chem. 1973, 7, 147. Herman, F.; William, A. R.; Johnson, K. H. J. Chem. Phys. 1974, 61, 3508. Case, D. A,; Yang, C. Y. Int. J. Quantum Chem. 1980,18, 1091. Cook, M.; Case, D. A. QCPE 1982, 14, 465. Schwartz, K. Phys. Rev. B. 1972,5,2466. Schwartz, K. Theor. Chim. Acta 1974, 34, 225.
Slater, J C. Int. J . Quantum Chem. 1973, 7s, 5 3 3 .
0 1990 American Chemical Society
Inorganic Chemistry, Vol. 29, No. 13, 1990 2491
cis-Bis(trifluoromethyl)platinum(II) Complexes uo
Ye2Pl(ADMe3)2
(CF3)2Pt(AsMe3)2
250
I
I 10.7 20,2
17.5
12.0
14.0
9.3
8.8
20.0
20.2 '
17.5
;
Ii.8
,
L2.0
b.3
14.4
11.7
0.0
7.9
8.6
7.2
8.1
Binding Energy (eV)
Binding Energy (eV)
'
17.2
9.3
10.0
Binding Energy (ev)
6.8'
Binding Energy (eV)
-
10.4
9.2
9.8
8.0
8.0
7.4
I
Binding Energy (eV)
Binding Energy (eV)
Figure 1. He 1 spectra of ~ i s - [ P t R ~ ( A s M e , and ) ~ ] [PtR2(COD)] (R = CHI and CF,). atomic a's was employed, the weight being the number of the valence electrons in the neutral atoms." Overlapping sphere radii were ~ s e d . ~ * J ~ L ; , ; : : . ; : : : :- I An I,, of 3 was used around outer-sphere region and Pt whereas I,, 11.5 10.0 10.1 9.4 8.7 8.0 values of 2, I , and 0 were used around P, C and F, and H atoms, reBinding Energy (eV) spectively. The geometries were based on the X-ray data of similar compounds, and the compounds were assumed to have C, symmetry.20 Figure 2. He I spectra of [Pt(CF,),(COD)] (a), C ~ S - [ P ~ ( C F ~ ) ~ ( P E ~ , ) ~ ] (b), and cis-[Pt(CF,),(AsMe,),] (c) (expanded first part only). Results and Discussion
General Features of the Spectra. Some representative spectra are shown in Figure 1, along with the corresponding spectra for the dimethylplatinum(I1) complexes for comparison. Further spectra of the complexes are given in Figures 2 and 3. By analogy, it is convenient to discuss the regions of the spectra in the ionization range > I 1.5 eV (bands labeled G-J(K)) and C 1 1 . 5 eV (bands labeled A-F) separately. It is interesting to note that the ionization bands in the range > I 1.5 eV are less shifted than those in the range < I 1.5 eV upon replacement of CH3 by CF3. This leads to a smaller energy separation between the two regions of the spectra when compared to the dimethylplatinum complexes, as indicated by the separation of the band G from the band F in Figure I . As for the dimethylplatinum compounds,'2 bands A-F are due to the contributions from mostly Pt and Pt-CF, orbitals. In the range > 1 1.5 eV, the spectra contain broad envelopes, which are fitted to the minimum number of bands, but which probably contain contributions from ionizations of many ligand-based M O s as well as the lowest energy (largest ionization energy) metal-ligand MO's. The highest IE band at 18 eV is absent in the spectra of the dimethylplatinum(l1) complexes and thus is assigned to ionizations of C-F u orbitals.21 The fluorine p lone pairs, which ionize at 14 eV, contribute to, for example, band H in the L = AsMe3 complex and band I in the Lz = COD complex, as
400
S 200
I
:
, 10.4
;
'
: 9.7
'
:
9.0
:
: 8.3
:
7.0
Binding Energy (eV)
-
-
, 1'1.1
1100-
(CF3),Pt(TMED) He1
880--
C 0 860--
U
440:;
S (17) Exchange factors for extramolecular and intersphere regions: Pt(CF3)2(PHJ*, 0.73550; Pt(CF3)2(C2H4)2, 0.74213; Pt(CF3)2, 0.73287. (18) Norman. J. G.. Jr. J . Chem. Phvs. . 1974.61.4630. Norman. J. G.. Jr. Mol. Phys. 1976, 31, 1191. (19) Atomic and outer-sphere radii (bohrs) are as follows. Pt(CF,)2(PH3)2: outer, 7.0803; Pt, 2.6577; C, 1.6794; P, 2.3080; F, 1.7048; H, 1.4040. Pt(CF3)2(C2H,)2: outer, 7.0200; Pt, 2.6577; C, 1.6794; F, 1.6802; H, 1.2100. Pt(CF,),: outer, 6.4698; Pt, 2.6577; C, 1.6794; F, 1.6802. (20) PtC = 2.058 A and LC-Pt-C = 88.0" (Manojlovic-Muir, L.;Muir, K. W.; Solomun, T.; Meek, D. W.; Pertson, J. L. J . Organomet. Chem. 1978,146 c26). C-F = 1.340 A and LC-P~-C= 88' 146, c26). C-F = 1.340 and LF-C-F = 109.5O (Hall, M. 9.; Fenske, R. F. Inorg. Chem. 1972, 1 1 , 768). Other geometric parameters are the same as
A
those in the corresponding dimethylplatinum(I1) compounds (see ref
12). (21) Drake, J. E.; Eujen, R.; Gorzelska, K. Inorg. Chem. 1982, 21, 1784.
2200.-
indicated by the relative area increases from the dimethylplatinum(I1) to the bis(trifluoromethyl)platinum(II) compounds (Figure l ) , as well as by the higher He II/He I intensity ratios G at least than those of the other bands in this r e g i ~ n . ~ ' "Band ~
2498 Inorganic Chemistry, Vol. 29, No. 13. 1990
Yang et al.
Table 1. Ionization Energies (ev), He II/He I Intensity Ratios, and Assignments of the Bands for cis-[Pt(CF,),L,]
(L2 = COD, TMED; L =
PEt,. AsMe,) band
B
IE 9.00 9.20 9.80
C D E F
10.12 10.43 10.90 11.26
A
;u'
-4.04
-ro.ot
COD He II/He I 1 .oo
0.87 0.86
0.82 0.93 0.96
TMED IE 8.26 8.45 8.80 9.00 9.49 9.96 9.96
10.41
He II/He I
1 .oo
AsMe, IE 8.42
assgnt 4a,
8.63
1.04
8.78
2b2
8.95 9.41 9.85 10.19
0.97 0.95 I .02 I .01
9.15 9.56 10.03 10.43
3a1 1 bl
1 .oo
assgnt 4a I
IE 8.33
0.87
2b2
0.85 0.96 0.96 I .04
3a1 1 a2 1 b, 2a,
PEt, He II/He I
'
2a I a2
I
a
Figure 4. Orbital correlation diagrams for cis-[PtMe2(C,H,), (a), ~ i s - [ P t ( c F , ) ~ ( c ~ (b), H ~ )and ~ ] ~ i s - [ P t ( c F ~ ) , ( P H , )(~cl) .
partially arises from the ionizations of As-C orbitals, since the [Pt(CF,),(TMED)] was also fitted to two peaks due to its ionization energy is approximately the same as that of As-C asymmetric shape with a separation of 0.20 f 0.04 eV. These orbitals in the free ligand AsMe, and remains almost unchanged splittings are similar to, but larger than, the splittings in the in both cis-[PtMe2(AsMe3),] and c~s-[P~(CF~)~(ASM~,)~].*~ The corresponding dimethylplatinum complexes and are probably He II/He I intensity ratios are much lower for the nonvibrational in origin.12 Second, the perfluoromethyl effect causes fluorine-containing bands in this high-IE region than for the bands a stabilization of all orbitals by 1 eV when compared with the at 1E's < I 1.5 eV, as expected for ligand-based ionizations. This corresponding methylplatinum complexes. Finally, the six bands region at > 1 1.5 eV will not be discussed further. often give He II/He I band area ratios very similar to those for In the region
