J. Phys. Chem. 1995, 99, 9933-9939
9933
A Theoretical Study on the Oxidative Addition of an Si-X Bond (X = H or Si) to M(PH3)2 (M = Pd or Pt). A Comparison of the Reactivity between Pt(PH3)2 and Pd(PH3)2 Shigeyoshi Sakaki," Masahiro Ogawa, and Mika Kinoshita
Department of Applied Chemistry, Faculty of Engineering, Kumamoto University, Kurokami, Kumamoto 860 Japan Received: December 12, 1994; In Final Form: March 21, 1995@
The oxidative addition of an Si-X a-bond (X = H or Si) to M(PH3):! (M = Pd or Pt) is investigated with ab initio MOMP4, SD-CI, and CCD methods. Geometries of reactants, transition states (TS), and products are optimized at the MP2 level. Although addition of an f-polarization function on Pt and Pd changes the activation energy (E,) and the reaction energy ( A E ) little, addition of a d-polarization function on P decreases the exothermicity considerably. Although the MPZoptimized geometries differ somewhat from the SCF-optimized ones, E, and A E calculated for the SCF geometries are almost the same as E, and A E for the MP2 geometries. E, and A E hardly change upon going from MP4SDQ to SD-CI and CCD, whereas they fluctuate somewhat at the MP3 and MP4DQ levels. The instability of Hartree-Fock wave function is not observed even at the TS. These results indicate that a single reference wave function can be used for these oxidative addition reactions. Unexpectedly, the Si-Si and Si-H oxidative additions to Pd(PH3)2 proceed with a lower E, than those to Pt(PH&, while the former are less exothermic than the latter. These results are explained in terms of bond energies and the electronic structure of the TS.
I. Introduction Oxidative additions of H-H, C-H, and Si-X a-bonds (X = H or Si) to low-valent transition metal complexes have received considerable attention in the last decade because such products of these reactions as transition metal hydride, alkyl, and silyl complexes serve as a key intermediate and/or an active species in many catalytic reactions.' In this regard, many theoretical works have been carried out on these reactions2 (see also refs 3- 10 for recent works). Considering that the saturated u-bond is broken during the oxidative addition, incorporation of electron correlation effects is expected to be indispensable for theoretical calculations of the oxidative addition. However, the geometry of the transition state (TS) was optimized at the SCF level and correlation effects on the TS geometry were not examined in previous works except for a recent pioneering work by Siegbahn and Svensson.Io In their work, the oxidative addition of water to Pd and Ni was investigated with the SCF and QCISD methods, in which correlation effects on the TS geometry were reported to be very small in the Pd reaction system but considerably large in the Ni reaction system. This result supports the idea that the geometries of the second-row transition metal complexes are successfully optimized at the SCF level. However, correlation effects on the TS geometry should be examined in various types of reactions because correlation effects depend upon the kinds of reaction. In the oxidative addition of C-H, C-C, and Si-X (X = H, C, or Si) bonds to Pt(PH3)2 and Pd(PH3)2, introduction of correlation effects significantly decreases the activation barrier (Ea)and considerably increases the exothermicity (Eexo).sThus, there is a need to investigate correlation effects on the TS geometries of these reactions, since their TS geometries are expected to be changed by introduction of correlation effects. In this work, the oxidative addition of an Si-X (X = H or Si) u bond to Pd(PH3)2 and Pt(PH3)2 is studied with ab initio MOMP2-MP4, SD-CI, and CCD (coupled cluster with double @
Abstract published in Advance ACS Abstracfs, June 1, 1995.
substitution) methods. This reaction is selected here because it is very important in organosilicon chemistry (see references cited in ref 8), and the Si-X a bond differs much from the 0 - H bond whose oxidative addition was investigated at the correlated level.I0 The main goals of this work are (1) to investigate correlation effects on the TS geometry and the basis set effects on Ea and AE, (2) to show whether E, and AE can be reliably estimated through SCF optimization followed by calculations including correlation effects, and (3) to clarify the factors determining the reactivities of Pt(PH3)2 and Pd(PH3)z. The third issue is worthy to investigate in detail because the oxidative addition to Pd(PH3)2 is calculated here to occur with a lower E, than that to Pt(PH3)2, against the general expectation that the Pt(0) complex is more favorable for the oxidative addition than the Pd(0) complex.' 11. Computations
Geometries of reactants, TS, and products were optimized at the MP2 level with the energy gradient technique, where the C, symmetry was adopted. The TS was determined by calculating the Hessian matrix. In all the calculations, the geometry of PH3 was taken to be the same as the experimental structure of the free PH3 molecule.I2 MP2-MP4, SD-CI, and CCDI3 calculations were performed with all core orbitals excluded from the active space. In CCD calculations, the contribution of single and triple substitutions was estimated through fourth-order perturbation using the double substitution functions (this method is abbreviated here as CCD(ST4)).I4 Gaussian 9215 and MELDI6 programs were used for these calculations. Two kinds of basis sets, BS I and BS 11, were used. In BS I, core electrons of Pt (up to 4f) and Pd (up to 3d) were replaced with the relativistic effective core potentials (ECPs)," and (5s 5p 3d)/[3s 3p 3d] and (5s 5p 4d)/[3s 3p 3d] basis sets were used for valence electrons of Pt and Pd, respectively. MIDI418 sets were employed for ligand atoms with the (4s)/[2s] setI9 used for H. In BS 11, a d-polarization function ( 5 = 0.34)18
0022-3654/95/2099-9933$09.00/0 0 1995 American Chemical Society
Sakaki et al.
9934 J. Phys. Chem., Vol. 99, No. 24, 1995 \
I1.m
HVSiqH
I H'
k>iy
I
;3 821 (4 448)
Complex Precursor
H
2 315 :p (23461
,
:4.419
,,
$1
I3.823
H
H
1
H
i
'\1.545
Product
ii
ti
Figure 1. Geometry changes in the oFidative additions of Si-H and Si-Si bonds to Pt(PH3)Z. Distances in A and angles in degrees (rounded to an integer). In parentheses are the SCF-optimizedvalues.
and a p-polarization function (5 = l.O)I9 were added to P and active H atoms, respectively, where the active H atom is the hydride ligand and the H of Si& that turns into the hydride ligand in the oxidation addition. Of course, the basis set of Si was augmented with a d-polarization functionIsb in both BS I and BS 11. Only in the investigation of the basis set effects was an f-polarization function20added to Pt and Pd atoms, and the Huzinaga-Dunning (12s 8p ld)/[6s 4p ld]19 basis set was used for Si, together with a (5s)/[3d] set for H.21 111. Results and Discussion
Geometry Changes in the Oxidative Addition. Geometry changes are shown in Figures 1 and 2, where in parentheses are the SCF-optimized values.2z The SCF-optimized values cannot be given for the oxidative addition of SiH4 to Pd(PH3)z, because the optimization of cis-PdH(SiH3)(PH3)~finally leads to the precursor complex, Pd(PH3)2(Si&), at the SCF leveLsb Since these geometrical changes have been discussed in our previous works,s we will focus the attention on a comparison of the MP2-optimized geometries with the SCF-optimized ones. For the Pt-Si bond of Pt(SiH&(PH&, the MP2-optimized distance is somewhat shorter than the SCF-optimized value and agrees well with the experimental Pt-Si distance of similar and complexes: ci~-Pt(SiHPr2)z(dppe)(R(Pt-Si) = 2.367 trun~-PtBr(SiMe3)(PEt3)~ (R(Pt-Si) = 2.330 For the PdSi bond of Pd(SiH&(PH&, the SCF-optimized Pd-Si distance is about 0.05 8, longer but the MP2-optimized value is about 0.03 8,longer than the experimentalvalue in Pd(SiHMe&(dcpe) Thus, MP2 optimization yields better (R(Pd-Si) = 2.358 geometries of the products.
Figure 2. Geometry changes in the oxidatiye additions of Si-H and Si-Si bonds to Pd(PH&. Distances in A and angles in degrees (rounded to an integer). In parentheses are the SCF-optimizedvalues.
We discuss the precursor complexes with the C3, like structure (Figure l), since the C3, like structure is more stable than the CzVlike structure by ca. 1 kcal/mol (MP4SDQBS II).z6 In the MP2 geometries, Si& and approach Pt more than in the SCF geometries. This is reasonable because the MP2 calculation involves the dispersion interaction which is in general important for the weak intermolecular interaction. Therefore, the precursor complex should be optimized at the MPZlevel. In the TS structure, considerable differences are observed between MP2 and SCF optimization. In the Si-H oxidative addition to Pt(PH3)2, the MP2 optimization gives longer Pt-H and Pt-Si distances, a shorter Si-H distance, and a larger PPtP angle than does the SCF optimization. Also in the Si-Si oxidative addition to Pt(PH3)2, the MP2 optimization gives a longer Pt-Si distance, a shorter Si-Si distance, and a considerably larger PPtP angle than does the SCF optimization. All these features indicate that introduction of correlation effects shifts the TS toward the reactant side. Basis Set Effects and Electron Correlation Effects. A binding energy (BE), an activation energy (E,), and a reaction energy (AE) were calculated with the MP4SDQ method, using various basis sets, where BE is a stabilizing energy of the precursor complex relative to reactants, E, is an energy difference between the TS and the precursor complex, and A E is an energy difference between the product and the reactants. A negative value represents a stabilization in energy for all these terms. As shown in Table 1, BE, E,, and AE change little upon addition of a p-polarization functionI9to the active H atom and an f-polarization functionz0 to Pt. Also, BE, E,, and AE are changed little by the use of the Huzinaga-Dunning [6s 4p Id] set19 for Si instead of MIDI-4*.I8 However, A E changes significantly upon addition of a d-polarization functionI8 on P, whereas BE and E, change little even in this case. A similar
Additions of
0 Bonds
J. Phys. Chem., Vol. 99, No. 24, 1995 9935
to Transition Metal Complexes
TABLE 1: Basis Set Effects" on the Binding Energy (BE): the Activation Energy (E,); and the Reaction Energy (my of the Si-H Oxidative Addition to Pt(PH3)z and Pd(PH3)* Pt Si
[3s 3p 3d] MIDI-4*e
SI
Hactiw
P
MIDI-4 -2.5 2.6 -27.2
BEb
E,' AEd
[3s 3p 3d] MIDI-4* [2s 1Pl MIDI-4 -2.5 2.3 -28.9
[3s 3p 3d] [6s 4p Id] [3s 1Pl MIDI-4 -2.6 2.5 -27.7
Pt(PH3)z [3s 3p 3d lfl MIDI-4* [2s 1Pl MIDI-4 -2.6 2.2 -28.7
[3s 3p 3d] MIDI-4* [2s 1Pl MIDI-4* -2.2 2.4 -21.9
[3s 3p 3d lfl MIDI-4* [2s 1Pl MIDI-4* -2.4 2.4 -21.9
Pd(PHd2 [3s 3p 3d] [3s 3p 3d] MIDI-4* MIDI-4* [2s 1Pl [2s 1Pl MIDI-4 MIDI-4* -2.6 -2.3 0.8 0.5 -6.6 -9.6
' MP4SDQ calculation (kcal/mol). The (4s)/[2s] set was used for H of SiH3 and PH3, when MIDI4 was used for P and Si. A (5s)/[3s] set was used for H of SiH,, when a [6s 4p ld] set was employed for Si. The energy stabilization of the precursor complex (the C3" type) relative to reactants. The energy difference between the TS and the precursor complex. The energy difference between the product and the reactants. e The superscript asterisk indicates the presence of a polarization function. TABLE 2: Electron Correlation Effectsa on the Activation Barrier (Ea)band the Reaction Energy (AE)e of the Oxidative Additions of Si-H and Si-Si Bonds to Pf(PH3)2 reactant H-SiH3 H3Si-SiH3 E, E, AE AE 8.3 -6.4 28.5 -15.5 HF MP2 2.o -24.2 16.9 -39.0 MP3 4.3 -21.7 20.1 -32.1 MP4DQ 0.6 -21.9 19.1 -34.0 MP4SDQ 2.4 -21.9 17.5 -34.8 20.1 -31.4 -2.2 -20.4 SDCI(D)d 18.0 -34.1 1.2 -22.7 SDCI(DS)d 19.0 -33.5 0.9 -22.1 SDCI(P)d 17.3 -34.4 2.8 -21.7 CCD(ST4)
TABLE 4: The Binding Energy (BE): the Activation in Oxidative Energy (E,): and the Reaction Energy Additions of Si& and Si2H6 to Pt(PH3)2 and Pd(PH3)zd
a BS I1 was used (kcal/mol). See footnote c of Table 1. See footnoted of Table 1. D, DS, and P in parentheses represent Davidson correction,26Davidson-Silver c~rrection,~' and Pople correction2*for higher order excitations.
values calculated for the SCF-optimized geometries.
TABLE 3: Lowest Eigenvalue of the Instability Matrix at the Transition State reaction internal RHF RRHF CRHF RRHF RUHF Pd(PH3)z+ SiH4" 0.174 0.161 0.112 0.150 0.084 Pt(PH& S i W 0.156 0.158 0.128 Pd(PH3)2 Si&' 0.162 0.087 0.160 0.156 Pt(PH3)2 + a ECPs and basis sets for Pt, Pd, and PH3 are taken to be the same as those in BS I. Huzinaga-Dunning (12s 8p ld)/[6s 4p Id], (5s lp)/ [3s lp], and (5s)/[3s] were used for Si, the active H, and the other H atoms of SiH4. BS I was used.
-
-
+ +
but smaller change of A E is observed in the Si-H oxidative addition to Pd(PH3)2, when the d-polarization function is added to P. Thus, BS 11, which is augmented with p- and dpolarization functions on the active H and P atoms respectively, seems to be reliable for investigating these oxidative addition reactions. BE, Ea, and A E are calculated with MPZMP4SDQ, SD-CI, and CCD(ST4) methods, using BS 11, as shown in Table 2. Introduction of correlation effects significantly decreases Ea and significantly increases the exothermicity (Eexo)in both Si-H and Si-Si oxidative additions. Although Ea and A E fluctuate somewhat at the MP2 and MP3 levels, they change little at the MP4SDQ, SD-CI(DS), SD-CI(P), and CCD(ST4) levels, except for the SD-CI(D) level, where D, DS, and P represent Davidson,*' Davidson-Silver,** and Pople correction^^^ for higher order excitations. Also, no instability of the Hartree-Fock wave function30is observed even at the TS, as shown in Table 3. All these results indicate that introduction of correlation effects is important for quantitative estimates of E, and AE, expectedly, and that a single-reference wave function can be used to represent the TS. The reason will be discussed below.
BE Ea AE BE E, AE
Oxidative Addition of SiH4 -2.2 (-0.1) -2.3 0.5 2.4 (1.6) 1.2 -6.6 -21.9 (-19.8) -22.7 Oxidative Addition of -2.5 -3.0 (-2.3) -3.1 (-0.7) 13.3 (14.0) 17.5 (16.9) -18.0 -17.1 (-17.2) -34.8 (-35.2) -34.1 -2.3
-2.5 2.8 -2 1.7 -3.4 17.3 -34.4
See footnotebof Table 1. See footnote c of Table 1. See footnote d of Table 1. BS I1 set was used (kcal/mol). In parentheses are the
Comparisons of E, and AE between Pd and Pt Reaction Systems. Ea and AE are summarized in Table 4, where in parentheses are values calculated for the SCF geometries. Several interesting results are found. The first is that Ea and A E for MP2 geometries are almost the same as those for SCF ones whereas introduction of correlation effects changes the TS geometry somewhat, unlike in the H20 oxidative addition to Pd.'O From this result, we can expect that E, and AE can be reliably estimated through SCF optimization followed by calculations at the correlated level, even when the SCF geometries differ somewhat from the geometries at the correlated level.31 This expectation is worthy of note from the practical point of view, since the SCF optimization of TS is much less time consuming than the optimization at the correlated level. The second is the difference in exothermicity between Pt and Pd reaction systems. As shown in Table 4, both oxidative additions of Si-H and Si-Si bonds to Pd(PH3)2 are much less exothermic than those to Pt(PH3)2, as has been interpreted in terms of the bond energies.8 Here we only report the bond energies in Table 5 because they were recalculated for the MP2 geometries. It is noted that a significant difference in the bond energy is not observed between the MP2 geometry and the SCF one and that the R"-R bond energy is greater than the Pdl'-R bond energy. The latter result is explained in terms of the sd hybridization as follows. In Figure 3, orbital energies calculated for the triplet state of M(PH3)2 are given, where the geometry of M(PH3)2 was taken to be the same as it in cis-M(SiH3)z(PH3)2 (M = Pd or Pt). Pd has the d orbital at a lower energy and the s orbital at a higher energy than does Pt. This is consistent with the fact that the ground state electron configuration is d9s' for Pt and d'O for Pd. Because the s-d energy difference is greater in Pd(PH& than in &(pH&, the sd hybridization occurs less easily in Pd(PH3)2 than in Pt(PH3)2: Accordingly, the Pd"-R bond is weaker than the Pt"-R bond,
Sakaki et al.
9936 J. Phys. Chem., Vol. 99, No. 24, 1995 TABLE 5: Bond EnerdesOb Related to the Pt-H (1)' (ad MP2 56.7 (59.9)b 56.5 MP3 59.3 (62.2) 59.1 MP4DQ 59.1 (62.1) 59.0 MP4SDQ 58.7 (61.7) 58.5 exptl other theor
Oxidative Addition Reactions Pt-SiH, 59.9 (64.4)b 56.4 (60.3) 57.1 (59.4) 57.6 (61.5)
Pd-Hd 49.4 (47.0)b 50.1 (49.1) 51.5 (49.5) 52.1 (48.9)
Pd-SiH3 50.6 (47.0)b 46.7 (43.3) 48.0 (44.6) 48.8 (45.4)
Si-H 92.1 (82.9)b 93.8 (84.2) 94.2 (84.5) 94.3 (84.6) 90.3e 91.38 9 1.8h
Si-Si 80.7 (72.9)b 80.6 (73.2) 80.2 (72.5) 80.5 (72.7) 74.w
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kcaymol. BS I1 was used. In parentheses are values calculated for the SCF-optimized geometries. These values were calculated by considering the following reaction: Pt(PH3)2 H2 ci~-Pt(H)2(PH3)2.These values were estimated by considering the Pt-SiH3 bond energy and the following reaction: Pt(PH3)2 S i b cis-PtH(SiH3)(PH,)z.e Reference 32. fReference 33. g Reference 34. Reference 35.
+
-
+
supermolecule relative to the distorted A and B molecules (eq
2)
++ -9.22
4-9.66
-10.20
+
-11.60 Pt(PH3)t Pd(PH3)z
SIHJ
*CHI
Figure 3. Orbital energy levels (UHFIBS I1 calculations were carried out for the triplet states of Pt(PH3)z and Pd(PH3)z and the doublet states of 'SiH3 and 'CH3) of Pt(PH3)2, and Pd(PH&, 'SiH3, and 'CH3. The geometry was of 'CH3 was taken to be the same as it in cis-M(SiH3)~(PH3)2.
since the sd hybridization should occur and form two equivalent
M-R bonds in MR2(PH3)2. The third is that the oxidative addition to Pd(PH3)2 proceeds with a lower Ea than that to Pt(PH3)2, which is of considerable interest as described in the Introduction. This result will be discussed below. The fourth is the difference in reactivity between the Si-H and Si-Si bonds; interestingly, the Si-H oxidative addition proceeds with nearly no barrier but the Si-Si oxidative addition requires considerable activation energy, whereas the former is much less exothermic than the latter. This result has been discussed in our previous work,* and the discussion is omitted here. Electronic Structure of the Transition State. We will discuss the electronic structure of the TS and the reactivities of Pt(PH3)2 and Pd(PH3)2, using such terms as the deformation energy (DEF), the interaction energy (IhT), the electrostatic (ES) interaction, the exchange (EX) repulsion, the chargetransfer (CT) interaction from A to B and that from B to A, where A and B represent fragment molecules in an A-B supermolecule. These terms have been used in the energy decomposition analysis.36 In the supermolecule, A and B cause geometry changes so as to stabilize the supermolecule. DEF is a destabilization energy to distort A and B from their equilibrium geometries to the distorted ones taken in the supermolecule (see eq 1). INT is a stabilization energy of the
where subscripts dist and eq mean the distorted and equilibrium geometries, respectively. The ES is the usual Coulombic interaction between AdlSt and Bdist. The EX repulsion is a destabilization interaction arising from the overlap of occupied and A positive value represents a orbitals between hist destabilization in energy for all these terms and vice versa. The TS of the Si-Si oxidative addition is examined here because this TS is easy to analyze due to its symmetrical structure. The CT interaction from the metal d, orbital to the Si-Si u* orbital plays an important role in breaking the Si-Si bond in the oxidative addition, as pointed out p r e v i o u ~ l yThis .~~ u* CT interaction is involved in the Bl representation. d, Also, the CT interaction from the Si-Si u orbital to the vacant sp, orbital of the metal would occur in the oxidative addition. This u sp, CT interaction is involved in the AI representation. The total difference density map given in the left panel Figure 4 is not useful very much for understanding the electron distribution at the TS, where the solid line represents an increase in density and the dashed line a decrease in density. There is only one feature to be noted as follows: the electron density decreases little in the region between the two Si atoms. The difference density maps of the AI and BI representations (middle and right panels of Figure 4), on the other hand, present meaningful information on the electron distribution. In the A, representation, the electron density decreases in region B but increases in region C (see the middle panel of Figure 4 for regions A, B, and C). These changes can be attributed not to sp, CT interaction but to the EX repulsion between the u Pt(PH3)z and as follows; since the u orbital of Si2H6 gives rise to the EX repulsion with occupied u orbitals of Pt(PH3)2 including thet'F dZ2,s, and pa orbitals, the electron density moves from these dZ2,s, and po orbitals to px and p4 orbitals so as to avoid the (5 electrons of Si2H6 (see Figure 4 for x and y axes). This means that the electron density decreases in region B but increases in region C. In spite of the EX repulsion, the electron density increases in region A. This increase would be interpreted in terms of the polarization of Si& caused by Pt(PH3)2. In the difference density map of the BI representation (Figure 4 right panel), the electron density slightly increases in region D but decreases in region E. These changes are caused by the u* CT interaction. However, this CT interaction does d, not explain all the electron redistribution in the Bl representation. For instance, the electron density increases in region F, whereas the d, u* CT interaction is expected to decrease the density in the d, orbital. This is explained again in terms of the EX
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Additions of u Bonds to Transition Metal Complexes
4
J. Phys. Chem., Vol. 99, No. 24, 1995 9931
x
Figure 4. Difference density maps (A@ = @(P~(PH~)~(S~ZH~))TS - Q ( P ~ ( P H ~TS) ~-)Q ) ~( S ~ ~TS. H at~ the ) ~SD-CI ~ level. Contour values are fO.01, *0.005, fO.OO1, f0.0005, and fO.OOO1. The MELD program was used for the calculations) at the transition state of the Si-Si oxidative addition to R(PH3)2: (left) total difference density, (middle) AI representation, and (right) B I representation. TABLE 6: Distortion Energy (DEF), Interaction Energy (INT) kcallmol unit), and Orbital Populations at the TS of the Si-Si Oxidative Addition to Pd(PH& and Pt(PH3)Z Pt Pd 6.9 9.1 1.6 14.4 -8.0 -0.014 -0.072 0.067 -0.069 2.171
8.6 8.9 7.2 10.3 -7.8 -0.005 -0.043 0.034 -0.053 2.095
-1.0
0.0
1.0
kcal/mol. MP4SDQBS I1 calculation. 'AB = @(P~(PH~)~(S~ZH~))TS R - @(Pt(PH3)2)," TS - g(Si2H& TS. A positive value represents an Figure 5. Electrostatic potentials of Pt(PH& and Pd(PH&. Geomincrease in the population and a negative value a decrease in the etries of Pt(PH3)2 and Pd(PH3J2were taken to be the same as those in population. In M(PH& which takes the same structure as in the TS. the TS. The HFBS I1 calculation was carried out. repulsion between the occupied u(b1) orbitals of Si& and the of the Pd reaction system. This means that the Pt reaction occupied Pt d, orbital, because this EX repulsion pushes away system receives either the weaker stabilizing interaction or the the electron density from region E to region F. greater EX repulsion than the Pd reaction system. From the above discussion, the character of the TS is The bonding nature of the TS can be deduced from the summarized as follows: Although CT from the Pt d, orbital to population change. As listed in Table 6, the d, orbital the cr* orbital takes place expectedly, this CT interaction population decreases at the TS only a little, but its decrease is is not strong but weak. On the other hand, the EX repulsion slightly greater in the Pt reaction system than in the Pd reaction contributes considerably to the electron distribution. Consistent system. The electron population of slightly decreases, with these features, the electron density decreases only a little and its decrease is greater in the Pt reaction system than in the in the region between the two Si atoms, the Si-Si bond slightly Pd reaction system: Accordingly, the po orbital population lengthens, and the Pt-Si distance is still long at the TS. All slightly increases, and its increase in the Pt reaction system is these features suggest that the TS is characterized to be reactantgreater than that in the Pd reaction system. All these results like. Since the CT interaction is weak and the Si-Si bond suggest that the Pt reaction system involves the slightly stronger weakens only a little, nondynamical correlation is not important Si& u* and (7 M po CT interactions than M d, and a single-reference wave function can be used to represent the Pd reaction system. Thus, these CT interactions are not the TS. A Comparison of the Reactivity between Pt(PH3)2 and Pdresponsible for the higher Ea value of the Pt reaction system. The strength of the ES interaction would be deduced from the (PH3)2. The reactivity of Pd(PH3)2 was briefly compared with electrostatic (ES) potential shown in Figure 5. Apparently, Ptthat of Pt(PH3)2 in our previous work,8b in which we pointed (PH3)2 exhibits much greater ES potential than Pd(PH3)2, that the d, orbital energy was similar in Pt(PH3)2 and Pd(PH& indicating that the ES interaction would lower the E, of the Pt when their structures were distorted like those in the TS. reaction system, too. One remaining factor of the higher Ea However, we could not find a clear reason for why the oxidative value of the Pt system is the EX repulsion between Pt(PH& addition to Pd(PH3)2 proceeded with a lower E, than that to and Si2H6. In the difference density map (Figure 4, middle Pt(PH3)2. panel), we have certainly seen that the u electrons of Si2H6 cause To find the reason, we compare the Si-Si oxidative addition the EX repulsion with u electrons of Pt. This EX repulsion between the Pt and Pd reaction systems because the E, of this would become greater, as the electron population of the Pt 6s reaction differs between Pt and Pd to a greater extent than that orbital increases. As shown in Table 6, the Pt 6s orbital of the Si-H oxidative addition. As shown in Table 6, DEF of population of Pt(PH3)2 is greater than the Pd 5s orbital Pt(PH3)2 is only 1.7 kcaymol smaller than that of Pd(PH3)2, population of Pd(PH3)2, and the Pt 6s population decreases more and DEF of is similar in Pt and Pd reaction systems. Thus, than the Pd 5s population at the TS. This means that Pt(PH3)2 DEF is not responsible for the E, difference. On the other hand, causes greater EX repulsion with Si2H6 than does Pd(PH3)2, INT of the Pt reaction system is considerably smaller than that a
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Sakaki et al.
9938 J. Phys. Chem., Vol. 99, No. 24, 1995 which leads to the higher E, of the Pt reaction system than that of the Pd reaction system. Of course, this conclusion is correct, when the EX repulsion is a main contributor to E,. If the TS is product-like and the activation energy is necessary to break the X-X IT bond, the CT interaction from the metal d, orbital to the X-X IT*orbital would become much more important. In such a case, the oxidative addition to Pt(PH3)2 would proceed with a lower E, value than that to Pd(PH3)z. Finally, we mention the reason that the Pt 6s population in Pt(PH3)2 is greater than the Pd 5s one. Since the ground state is d9s1for Pt but d10 for Pd, the Pd 4d orbital lies at a lower energy than the Pt 5d orbital, and the Pd 5s orbital lies at a higher energy than the Pt 6s orbital (see Figure 3). This suggests 6s promotion occurs more easily than the Pd that the Pt 5d 4d 5s one. Accordingly, the 6s electron population of Pt becomes greater than the 5s population of Pd.
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M(PH3)2 and Si2H6 (or SiH4) at the TS. Pt(PH3)2 is more favorable for the CT interaction but less favorable for the EX repulsion than Pd(PH3)z as follows: because the 6s-5d energy difference in Pt is smaller than the 5s-4d energy difference in Pd (Figure 3), the 5d electrons more easily promote to the 6s orbital in Pt than the 4d electrons to the 5s orbital in Pd. Since the 6s (and 5 s ) electrons yield the EX repulsion with Si2H6, Pt(PH3)2 is less favorable than Pd(PH3)2 for these oxidative additions. This conclusion is correct, when the EX repulsion is a main contributor to E,.
Acknowledgment. All these calculations were carried out with NEX SX-3 and Hitachi M-680 computers of Institute for Molecular Science (Okazaki, Japan) and IBM RS-6000/340 and 360 workstations of our laboratory. This work was supported in part by grants from Ministry of Education, Culture, and Science (Nos. 06227256 and 04243102).
IV. Concluding Remarks Electron correlation effects on geometry are investigated by comparing MP2-optimized geometries with SCF-optimized ones. Expectedly, the MP2 optimization yields better geometries than does the SCF optimization. Also, several interesting differences are observed in the TS geometry between the SCF and MP2 optimizations. Those differences indicate that introduction of correlation effects makes the TS more reactant-like. In spite of the considerable differences between the MP2 geometry and the SCF one, MP4SDQ values of E, and A E for SCF geometries are almost the same as those for MP2 geometries. This result is worthy of note from the practical point of view because the less expensive SCF optimization followed by correlated calculation seems to yield reliable E, and A E values. Basis sets effects on Ea and A E are examined in the Si-H oxidative addition to Pt(PH3)2. Although addition of an f-polarization function on Pd and Pt changes E, and AE unexpectedly little, addition of a d-polarization function on P decreases the exothermicity considerably. Thus, the basis set of P should be augmented with the d-polarization function to estimate reliably Ea and AE. Introduction of correlation effects significantly lowers Ea and considerably increases the exothermicity. Although Ea and A E fluctuate somewhat at MP2-MP4DQ, they change little upon going from MP4SDQ to SD-CI and CCD(ST4). The instability of the Hartree-Fock wave function was not observed even at the TS. These results suggest that a single-reference wave function can be used for these oxidative addition reactions. The electronic structure of the TS is investigated from the difference density map and Mulliken population analysis. Although CT from the metal d, orbital to the Si-Si IT*orbital is important to break the Si-Si bond, this CT interaction is weak at the TS. Also, the CT from the Si2H6 IT orbital to the metal sp, orbital is very weak at the TS. Consistent with these features, the Si-Si bond slightly lengthens and the Pt-Si distance is very long at the TS. Besides these CT interactions, the EX repulsion between Pt(PH3)2 and Si2H6 contributes considerably to the electron distribution. Because of the weak CT interactions and the slight lengthening of the Si-Si bond, nondynamical correlation is not important at the TS. This would be a main reason that a single-reference wave function can be used even for the TS. Unexpectedly, the Si-H and Si-Si oxidative additions to Pd(PH3)2 proceed with a lower E, than those to Pt(PH3)2. This result would be related to the electronic structure of TS. The CT interaction from the metal d, orbital to the Si-X IT*orbital is still very weak, but considerable EX repulsion arises between
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