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A theoretical study of the homogeneous catalysis of the D2-H2O exchange by ruthenium chloride complexes
Journal of Molecular GztaZysis. 3 (1977178) @ Ekevier Sequoia S-A., Lausanne -Printed
A THEORETICAL STUDY THE D&H,0 EXCHANGE
0.
NOVARO*
Instituto 20, D-F_
337
337 - 349 in the Netherlands
OF THE HOMOGENEOUS CATALYSIS OF BY RUTHENIUM CHLORIDE COMPLEXES
_
de Fkica, (Mexico)
Uniuersidad
National
Authoma
de M&cico.
Apdo_
20-364,
Mkico
J. PINEDA Comisick (Received
Federal
de EZectricidad
February
(Mexico)
16,1977)
The homogeneous catalysis of D2-Hz0 exchange by ruthenium chloride is investigated by an all-valence electron, self-consistent field study of the reaction coordinate for the process. The following reaction stages are described: the heterolytic scission of a D, moIecule by the catalyst complex (RuCl,)* with a subsequent substitution of a chIorine by a hydrogen; the substitution of an OH- in the coordination sphere of the Ru atom_ In the fiit stage, HCl is liberated_ Then the reaction proceeds via the c&transfer of the OH- towards the coordinated sphere of Ru, thus reforming the chloride complex. Each of these processes is analysed by considering the energies, charge distribution and bond orders through the whole reaction coordinate_
1. Introduction The study of ruthenium chloride complexes and their catalytic activation of molecular hydrogen was initiated some time ago by Halpem, Harrod and co-workers [l - 3]_ In particular, the homogeneous catalysis of the isotopic exchange between hydrogen and liquid water, which has been proposed as an alternative procedure for heavy water production 141, has been esfablished [5] as stemming &om a heterolytic splitting of hydrogen_ A study of the molecular foundations of this reaction seems to be interesting for the following: although the excellent work of the above cited authors [l - 53 seems to have fully established the reaction mechanism, a quantum mechanical analysis would help to understand more deeply the process at a molecular level_ Also, some of the intermediate species proposed in the
*Consultant of Institute
Mexicano
de1 Petrdleo.
mechanism which are hard to detect experimentahy can be studied only through theoretical methods The purpose of the present paper is to provide such a theoretical description of the mechanism of the hydrogen-water exchange. This is achieved through an ah-valence electron, self-consistent molecular orbital study of the reaction pathway, from the entrance of the reactants into the coordination sphere of Ru to the liberation of the products_ The program *used has been discussed and applied to several other catalytic reactions [6,7] _ Halpem and James [5] propose the entrance of Da into the coordination sphere of an Ru(II1) complex, followed by the heterolytic breaking of Da and liberation of D+Cl-, hence, a spontaneous exchange between D’ and Ha0 in solution has been assumed- However, these authors also observe an alternative pH-independent exchange process. We here propose this altemative process to proceed via a coordinated hydroxide_ We shah therefore describe the coordination of OH to the ruthenium site and show that the reaction between the OH and D moieties can indeed take place within the Ru coordination sphere, finahy liberating a deuterated water mo1ecule. In our model, the catalyst is shown to be responsible for the fact that no activation barrier exists for this reaction, showing that the possibility of pH-independent exchange [5] between D’ and Hz0 is, indeed, confirmed. For our purpose each individual molecukr interaction has to be studied theoretically, and although the order of entrance of the Da and OH- into the coordination sphere is not crucial, we shah analyze first the heterolytic breaking, i-e., the rate-determining step [5], then the substitution of Cl- by OH- (in ref_ 2 this is considered the first step, as we shah see, the ordering is of no consequence)_ Then the formation of the HDO as a coordinated movement of the Ru ligands will be proposed (and justified)_ Finally, the liberation of the products will be analyzed_ The active species hasbeen proposed to be hexa-coordinated Ru chloride complexes [2, 5] _ Among such complexes we here study (RuCI~)~-_ All the reaction steps discussed above are schematically represented in Fig. I-
2, Method
The calculations were performed within an SCF-MO CNDO approximation with a program adapted to include transition metals [7] and which has been tested on several catalytic processes [S, 71 as mentioned above. All the relevant parameters have been reported before [7 j except for ruthenium for which the following were estimated_ Slater orbital exponents; s-function 1.296 9, p-funct- 0,756, d-funct_ 3-203; the valence state ionization potentials were s- 5-6, p- 2.45, d- 7.37. Finally, the interatomic distances for the coordination complexes involved in the different stages of the process were taken from the literature [S] _
339
[*u
Cls]‘-
+
D+
b-
[Ru
Cl&j’-
(Ru
ClrD
+
DC
+
Cl-
H”
+
Cl-
(a) [Ru
C1sD-j
‘-
c
li20
______+
Fig. l_ Reaction mechanism [RuCI,] 3- compIex_
for the homogeneous
OH]
‘-
catalysis
-I-
of Dz-Hz0
exchange
by
3. Results As mentioned above, it is convenient to divide the process into four sequences that will be discussed in succession, from the entrance of the reactants into the coordination sphere of the ruthenium catalyst, to the liberation of the products_ Since this product is again a water moIecule we distinguish it by supposing that the reactants are really D2 and H,O. This is actually the case for this reaction [l - 51, but the molecular orbital method used in the present. calculations cannot differentiate between D and H atoms
(a) Heterolyfic breaking of D, A very important breakthrough in the understanding of the mechanism of the isotope exchange by ruthenium catalysts WZIS the evidence obtained by Halpem and James 153 showing that the complex can induce heterolytic breaking of hydrogen_ We shall here follow their general outline and try to provide a detailed molecular orbital description of such a spIitting of Hz. As mentioned above, we shall name our reactant D, for clarity, although the
340 TABLE
1 and the
ChargepolarizationoftheDa-s bond.AteachstepthedistancehetweenDa clo.sestCIatom isvaried.AlsotheDa-Db distance isvaried Step
1
Da-Db distance 0.746 Da-Cldistance 2.13 Da&age t-o-09 Db charge -0.09
2 0.746 1.85 to.07 -0.08
3 0.746 1.57 +0_20 -0.05
‘4 0.747 2.28 +0_38 -0.27
5 0.755 1.275 +0.26 -0.10
6
7
O-77 1.02 1.274 J-33 +0_27 +0.26 -0-17 -0.27
8 1.27 1.28 +0.26 -0.32
present studies of the electronic structure cannot differentiate between isotopes. In Table 1 we show the effect on the charge distribution of the D2 molecule as it approaches the ( RuC16)3- compIex. For clarity we have called D, the atom closest to the chlorine and I&, the one closer to Ru, considering that the molecule moves parallel to one of the Ru-Cl bonds. We are, in fact, proposing a four-center interaction: Cl-RU
Da-D, This interaction could polarize and weaken the bond in the Dz molecule while labilizing one Ru-Cl while forming a new Ru-D bond, thus giving a molecular orbital description of the proposed 153 heterolytic breaking. One objection could arise because of the possible role of the interactions of the other Cl ligand with DB_ We shal1 show that they are quite negligibleFrom Table 1, we immediately see a net polarization of charge between D, and D,_ This polarization is a manifestation of heterolytic breaking and starts from the outset, even before the diatomic distance is relaxed. The final steps in Table 1 assume a relaxation of this distance and this can be justified by the results depicted in Fig. 2. The bond orders between D, and Ru, D, and its closest Cl atom, as well as between Ru and Cl and the atoms Db and D,, are represented in Fig_ 2 as a function of the eight successive steps of TabIe 1, We observe the weakening of the D,-Db bond mentioned above and also the replacement of a chlorine by the Db atom as the Ru-Cl bond weakens and two new bonds (Da-C1 and RU-Db) form, On the other hand, the marginal bond orders that arise from the interactions of D, and Db with the rest of the chlorines in the ruthenium coordination sphere are too weak to affect- the above picture- They are, at most, of the order of one hundredth in evew one of the steps of Table 1 and Fig. 2, in spite of the fact that the entrance of D, into the Ru sphere necessarily brings it near to one or more of the other chlorines_ The reason for this, however, is quite simple_ In approaching Ru, Db (the deuterium atom that approaches the other Cl’s the
341
Fig. 2. Changes in the most relevant bond orders during the heterolytic breaking of I& by the catalyst complex.. The eight steps of Table 1 are depicted by the corresponding numbers of the abscissa.
most) is gaining negative charge, thus making it highly unlikely that it iriteracts with a coordinated chloride_ The results of Table 1 and Fig. 2 give just the molecular orbital verification of the heterolytic splitting proposed by Halpem and James as the rate determining step in their mechanism. The following three steps refer to our proposal for the pEGindependent exchange observed by Halpem and James 151 which here is interpreted to occur uiu coordinated hydroxide_ (6) Substitulion of a chlorine by OH The substitution of an OH- into the coordination sphere and a Cl dissociation can be easily explained through a hydrolytic equilibrium situation [21_ Consequently, we shall describe only the most relevant points. We calculated the interaction of the whole ruthenium complex with an Ha0 molecule showing that the splitting of the water molecule as OH- and HY is quite favorable, even without considering solvent effects, because of the polarizing influence of the Ru complex. Furthermore, the entrance of the
OH- into the coordination sphere greatly enhances the possibility of a Cl dissociating_ AR this is described succinctly in step (b) of Fig_ 1. The principal points concerning this step are, as the OH- enters, the Ru-CI bond immediately weakens, its original population going from its value of 0.60 when the OH- is effectively outside the coordination sphere, to O-16 when OH- is at a distance of 2.25 A from the Ru, and to O-09 (essentially liberating the chlorine) when it reaches its equilibrium distance of 2-03 A from Ru- During this process the Ru-OH bond population grows from a value of 0.04 to 0.35 and 0.52 for the same configurations_ fc) Reaction
between
the coordinated
D and OH
moieties
In order to understand why the hydride and the OH radical react readily while coordinated to the Ru atom we shall begin to analyze all the possible movements of the ligands on the Ru coordination sphere_ Starting from the octahedraI structure we first allow every coordinated atom to oscillate around its equilibrium position_ These oscillatory motions naturally bring the particular atom nearer to, or farther ikom the others. All oscillations of up to 5 degrees bring the total energy of the complex up a few kcal/ mole, with one exception, i-e_, the movement of the OH moiety towards the hydride_ This means that aZZdisplacements of the chlorines, as well as the
2
t
Fig_ 3. Geometrical configurations of the [RuCI~OHD] tion of the OH moiety (steps 0 - 3) and the detachment OH (steps 4 - 65
3- complex of D from
Y
during the cis-migraRu and its fastening to
343
L 51
,
Fig_ 4_ Total energy during the process of Fig. 3.
hydride itseff about the “normal” octahedral structure imply a notable activation barrier, The highest barrier of all is found when we try to move the OH away from the hydride. On the other hand, no barrier at all ismet by displacing it towards the positive y-axis, as the total energy gradually lessens as OH- advances (see Fig. 3); this is clearly a &-migration towards the hydride, D_ In fact, as the OH group moves, the energy is always lowered, as shown in Fig. 4, So, from its original octahedral position (which in Fig. 3 was taken as lying on the z-axis) up to a 45 degree displacement on the YZ plane (towards the D lying on the y-axis in Fig. 3) the energy simply decreases, ie., no activation barrier is present. We now explain, on the basis of the molecular orbitals involved, why this is so. First, the reason why an OH (as opposed to a Cl) can migrate towards the hydride can be understood from an analysis of the molecular orbitals of the whole ruthenium complex, There are twenty-four molecular orbitals occupied (or semi-occupied) by the valence electrons of the complex. Aithough none of them can be associated exclusively to a single bond or interaction there are some dominant atomic orbital contributions that essentially classify them in different groups, playing different roles in the complex_ Starting from those molecular orbitals higher in energy, the first
314 four are essentially formed from the ruthenium d-orbitals, some of them interacting effectively with the OH group. We shall have more to say about these molecular orbitals in the latter part of this subsection, showing how they change during the OH movement (see Fig. 3) and their role in the fact that such a movement presents no energy barrier, as shown in Fig. 4. In any case, the fact that the OH-Ru interactZon is concentrated in the highest energy molecular orbitals is a measure of the lability of this bond_ Far lower in energy lies a group formed from the Ru p-orbitals interacting with the chlorine and hydride orbit&s_ These are separated from the highest lying orbitals mentioned above by 4 eV or more, which is the cause for the little mobility of these ligands (H and Cl). The lowest lying group of molecular orbitals is almost exclusively associated with individual atoms (they are mostly s-orbit%& from Ru and the Cl atoms) and contribute little to bond formation_ We shall study the OH movement in greater detail, but its greater mobility as compared with the Cl atoms can be assigned to this molecular orbital ordering_ In other words, the lability of the Ru-OH bond is far greater than any other bond in the complex. In several studies on other transition metal complexes [S, 71 a similar relation between the lability of a certain bond (i-e_, its association with the highest occupied molecular orbitals) and its capacity to migrate has been encountered. We shall now proceed to justify the other movement depicted in Fig. 3, that of D towards OH_ As the OH reaches its furthermost position, which is at 45 o from the z-axis (from then on an important barrier stops the cismigration), the D atom, originally fixed on the y-axis gradually becomes free to move_ The explanation for this is implicit in Fig_ 5, where the bond orders of the Ru-D, Ru-OH and O-D bonds are depicted as a function of the steps described in Fig. 3_ It is seen that as the OH leaves the z-axis (the octahedralposition), notonlydoesanO-Dbondbeginto formbutalsothe Ru-OH and, more notably the Ru-D bond, weaken. Indeed, this is so marked that D is essentially freed to move as represented in the last steps of Fig. 3 and these lower the energy of the system still further as Fig. 4’ shows. The effect of this D-movement is also overt in Fig_ 5 where, in the last stages, the Ru-D t-ond is effectively severed and an HDO molecule is formed (the O-D bond becomes as populated as the original OH bond). A notable characteristic of Fig. 5 is the well-defined transition state which the system traverses at step 3 of the &-migration. All bonds change abruptly, producing a multiple crossing of the curves. We shall now try to present a deeper explanation of the cause of this energy lowering during the reaction coordinate, in terms of the molecular orbit&s of the system_ The understanding of the OH movement is a little more involved and we shall postpone it for a moment and consider the causes of the D movement at the latter stages. Table 2 shows the net charges on the OH moiety and in the hydride, D, during the &-migration. It is seen that the OH movement not only weakens the Ru-D bond, as was shown in Fig_ 5, but, indeed, affects the charge distribution so as to make D positive
345
Fig. 5. Changes TABLE
in the bond
orders during
the process
of Fig. 3.
2
Charge in the coordinated hydride D as the OH advances towards it during the cis-migration. Also the charge on the OH moiety is reported_ Steps 0 to 6 correspond to Fig. 3 Step
0
Charge on the coordinated hydride, D Charge on the OH moiety
Oxygen atom Hydrogen atom
1
2
-6.41
-632
-0.05
-0.81
-6.76
-667
+0.16
+0_14
+0_17
3
so-31 -6-53 +0.23
4
+0.34 -6-50 1-o-25
5
+0.28 -0.42 +0.23
6
+0.20 -0.31 +-o-23
at the “transition state” step 3. These two effects are not independent, of course, as the Ru-D bond, of necessity, is highIy polar, as, indeed, are all Ru bonds in this complex, and its breaking inverts the charge in D. Thus, the D atom acquires a positive charge and is then attracted by the highIy negative oxygen site. As both the oxygen and the hydride have alI but severed their
346
STEP
0
r
z
1
Fig_ 6. Changes in the energies of the four highest $3 and $4) during the process of Fig_ 3.
(
occupied
5
molecular
6
orbitals
(@I,
@2,
Ru bond (especially D) one can readily under&and that the D movement towards OH is so energetically favourable. To understand why the OH &-migration is also favoured we must look to the highest (energy) molecular orbit&. In Fig. 6 the m.o. energies for the four highest occupied m.o.‘s are given as a func_fZonof the coordinate of Fig. 3. It is seen that $J1 and 9 4 are quite sensitive t6 the movements described, while I& and 1,0~change rather monotonically_ These last two molecular orbitaIs are formed essentially from the ruthenium dxz and d,, atomic orbil;als and, from symmetry considerations, should not intervene heaviIy in this reaction (see Fig. 3). The other two m_o_‘s, xJ1and tid, are more complicated: in Tables 3 and 4 the dominant contributions from the different atoms are given (for simpIicity non-negligible contributions from the chlorine p-orbital& which have no bearing on the process and are insensitive to the OH and D movements, are not reported)_ It is clear that originalIy the d, ruthenium orbital essentially degenerates in energy with d,, and d_ (see JI=, gbz, and $J~in Fig. 6), while e4 is the very important m-o_ responsible for most of the D and 0 bonding to Ru (through its dr2_,,z and dg orbitals)_ As soon as the OH group starts to move, however, d, starts to contribute more end more to 15~
347 TABLE
3
Dominant atomic orbital contributions to the highest Fig. 6) as they change during the k-migration Steps
0
Atomic
0.00 -O_Ol 0.99
0.02 -0.23
0.95
Steps
s
0
Pr
($1
of
2
3
4
5
6
-0-16 -0.52
-0.29 -0-75
-0-36 -0.74
-0.41 -0.73
-0-42 -0-73
0.74
O-22
O-06
0.00
0.00
to the +4
0
1
2
0.25
O-30
0.32
0.11
O-22
O-26
molecular
3
orbital
of Fig. 6 as they
4
5
6
0.19
0.13
O-10
0.09
-0.07
-0.02
O-06
--0.05
orbit&
d,z Ru dXz-y= d ,= D
orbital
4
Dominant atomic orbital contributions change during the &r-migration
Atomic
molecular
orbital
dxl_y2 Ru d,a d YZ
TABLE
1
occupied
0.01 -0-59 O-14
0.10 -0.57 o-11
O-48 -0-47 0.10
O-90 -9.19
0.10
0.95 -609
O-11
O-96 -0-03
0.10
0.97 -0.02 O-07
in compensation, Gr gradually becomes more and more dominated by di _ Through these changes, $1 and Qq retain the same character, I$~ always remains an inert, uninteracting d-function of Ru while *a belongs to the RuOH bond, first as d,z then as d,. Thus, we see that the movement of the OH in the yz plane does not really imply an unfavourable situation, it maintains its interaction with Ru, d, gradually replacing d;_ Thus, no bond breaking is necessary until the new D-O bond has started to form, and thus no energy barrier appears in this part of the process_ and,
(d)
Liberation
of the product
The deuterated species, DHO, is easily liberated, as the low final Ru-0 bond order of Fig. 5 suggests, This can be helped by the entrance of two Cl atoms (or an H and a Cl) to the Ru coordination sphere. We calculated the movement of two Cl’s towards the ruthenium, obtaining a lowering of the energy while the two new Ru-Cl bonds are formed, hence weakening the Ru-HDO interaction. At an Ru-Cl distance of 3.35 A (Le., 1 A further than its equilibrium distance [S] ) the RuCl bond is stronger than the Ru-0 bond, which afterwards reduces down to negligible values (O-04 when the Cl reach their f&ml positions)_
A THEORETICAL STUDY THE D&H,0 EXCHANGE
0.
NOVARO*
Instituto 20, D-F_
337
337 - 349 in the Netherlands
OF THE HOMOGENEOUS CATALYSIS OF BY RUTHENIUM CHLORIDE COMPLEXES
_
de Fkica, (Mexico)
Uniuersidad
National
Authoma
de M&cico.
Apdo_
20-364,
Mkico
J. PINEDA Comisick (Received
Federal
de EZectricidad
February
(Mexico)
16,1977)
The homogeneous catalysis of D2-Hz0 exchange by ruthenium chloride is investigated by an all-valence electron, self-consistent field study of the reaction coordinate for the process. The following reaction stages are described: the heterolytic scission of a D, moIecule by the catalyst complex (RuCl,)* with a subsequent substitution of a chIorine by a hydrogen; the substitution of an OH- in the coordination sphere of the Ru atom_ In the fiit stage, HCl is liberated_ Then the reaction proceeds via the c&transfer of the OH- towards the coordinated sphere of Ru, thus reforming the chloride complex. Each of these processes is analysed by considering the energies, charge distribution and bond orders through the whole reaction coordinate_
1. Introduction The study of ruthenium chloride complexes and their catalytic activation of molecular hydrogen was initiated some time ago by Halpem, Harrod and co-workers [l - 3]_ In particular, the homogeneous catalysis of the isotopic exchange between hydrogen and liquid water, which has been proposed as an alternative procedure for heavy water production 141, has been esfablished [5] as stemming &om a heterolytic splitting of hydrogen_ A study of the molecular foundations of this reaction seems to be interesting for the following: although the excellent work of the above cited authors [l - 53 seems to have fully established the reaction mechanism, a quantum mechanical analysis would help to understand more deeply the process at a molecular level_ Also, some of the intermediate species proposed in the
*Consultant of Institute
Mexicano
de1 Petrdleo.
mechanism which are hard to detect experimentahy can be studied only through theoretical methods The purpose of the present paper is to provide such a theoretical description of the mechanism of the hydrogen-water exchange. This is achieved through an ah-valence electron, self-consistent molecular orbital study of the reaction pathway, from the entrance of the reactants into the coordination sphere of Ru to the liberation of the products_ The program *used has been discussed and applied to several other catalytic reactions [6,7] _ Halpem and James [5] propose the entrance of Da into the coordination sphere of an Ru(II1) complex, followed by the heterolytic breaking of Da and liberation of D+Cl-, hence, a spontaneous exchange between D’ and Ha0 in solution has been assumed- However, these authors also observe an alternative pH-independent exchange process. We here propose this altemative process to proceed via a coordinated hydroxide_ We shah therefore describe the coordination of OH to the ruthenium site and show that the reaction between the OH and D moieties can indeed take place within the Ru coordination sphere, finahy liberating a deuterated water mo1ecule. In our model, the catalyst is shown to be responsible for the fact that no activation barrier exists for this reaction, showing that the possibility of pH-independent exchange [5] between D’ and Hz0 is, indeed, confirmed. For our purpose each individual molecukr interaction has to be studied theoretically, and although the order of entrance of the Da and OH- into the coordination sphere is not crucial, we shah analyze first the heterolytic breaking, i-e., the rate-determining step [5], then the substitution of Cl- by OH- (in ref_ 2 this is considered the first step, as we shah see, the ordering is of no consequence)_ Then the formation of the HDO as a coordinated movement of the Ru ligands will be proposed (and justified)_ Finally, the liberation of the products will be analyzed_ The active species hasbeen proposed to be hexa-coordinated Ru chloride complexes [2, 5] _ Among such complexes we here study (RuCI~)~-_ All the reaction steps discussed above are schematically represented in Fig. I-
2, Method
The calculations were performed within an SCF-MO CNDO approximation with a program adapted to include transition metals [7] and which has been tested on several catalytic processes [S, 71 as mentioned above. All the relevant parameters have been reported before [7 j except for ruthenium for which the following were estimated_ Slater orbital exponents; s-function 1.296 9, p-funct- 0,756, d-funct_ 3-203; the valence state ionization potentials were s- 5-6, p- 2.45, d- 7.37. Finally, the interatomic distances for the coordination complexes involved in the different stages of the process were taken from the literature [S] _
339
[*u
Cls]‘-
+
D+
b-
[Ru
Cl&j’-
(Ru
ClrD
+
DC
+
Cl-
H”
+
Cl-
(a) [Ru
C1sD-j
‘-
c
li20
______+
Fig. l_ Reaction mechanism [RuCI,] 3- compIex_
for the homogeneous
OH]
‘-
catalysis
-I-
of Dz-Hz0
exchange
by
3. Results As mentioned above, it is convenient to divide the process into four sequences that will be discussed in succession, from the entrance of the reactants into the coordination sphere of the ruthenium catalyst, to the liberation of the products_ Since this product is again a water moIecule we distinguish it by supposing that the reactants are really D2 and H,O. This is actually the case for this reaction [l - 51, but the molecular orbital method used in the present. calculations cannot differentiate between D and H atoms
(a) Heterolyfic breaking of D, A very important breakthrough in the understanding of the mechanism of the isotope exchange by ruthenium catalysts WZIS the evidence obtained by Halpem and James 153 showing that the complex can induce heterolytic breaking of hydrogen_ We shall here follow their general outline and try to provide a detailed molecular orbital description of such a spIitting of Hz. As mentioned above, we shall name our reactant D, for clarity, although the
340 TABLE
1 and the
ChargepolarizationoftheDa-s bond.AteachstepthedistancehetweenDa clo.sestCIatom isvaried.AlsotheDa-Db distance isvaried Step
1
Da-Db distance 0.746 Da-Cldistance 2.13 Da&age t-o-09 Db charge -0.09
2 0.746 1.85 to.07 -0.08
3 0.746 1.57 +0_20 -0.05
‘4 0.747 2.28 +0_38 -0.27
5 0.755 1.275 +0.26 -0.10
6
7
O-77 1.02 1.274 J-33 +0_27 +0.26 -0-17 -0.27
8 1.27 1.28 +0.26 -0.32
present studies of the electronic structure cannot differentiate between isotopes. In Table 1 we show the effect on the charge distribution of the D2 molecule as it approaches the ( RuC16)3- compIex. For clarity we have called D, the atom closest to the chlorine and I&, the one closer to Ru, considering that the molecule moves parallel to one of the Ru-Cl bonds. We are, in fact, proposing a four-center interaction: Cl-RU
Da-D, This interaction could polarize and weaken the bond in the Dz molecule while labilizing one Ru-Cl while forming a new Ru-D bond, thus giving a molecular orbital description of the proposed 153 heterolytic breaking. One objection could arise because of the possible role of the interactions of the other Cl ligand with DB_ We shal1 show that they are quite negligibleFrom Table 1, we immediately see a net polarization of charge between D, and D,_ This polarization is a manifestation of heterolytic breaking and starts from the outset, even before the diatomic distance is relaxed. The final steps in Table 1 assume a relaxation of this distance and this can be justified by the results depicted in Fig. 2. The bond orders between D, and Ru, D, and its closest Cl atom, as well as between Ru and Cl and the atoms Db and D,, are represented in Fig_ 2 as a function of the eight successive steps of TabIe 1, We observe the weakening of the D,-Db bond mentioned above and also the replacement of a chlorine by the Db atom as the Ru-Cl bond weakens and two new bonds (Da-C1 and RU-Db) form, On the other hand, the marginal bond orders that arise from the interactions of D, and Db with the rest of the chlorines in the ruthenium coordination sphere are too weak to affect- the above picture- They are, at most, of the order of one hundredth in evew one of the steps of Table 1 and Fig. 2, in spite of the fact that the entrance of D, into the Ru sphere necessarily brings it near to one or more of the other chlorines_ The reason for this, however, is quite simple_ In approaching Ru, Db (the deuterium atom that approaches the other Cl’s the
341
Fig. 2. Changes in the most relevant bond orders during the heterolytic breaking of I& by the catalyst complex.. The eight steps of Table 1 are depicted by the corresponding numbers of the abscissa.
most) is gaining negative charge, thus making it highly unlikely that it iriteracts with a coordinated chloride_ The results of Table 1 and Fig. 2 give just the molecular orbital verification of the heterolytic splitting proposed by Halpem and James as the rate determining step in their mechanism. The following three steps refer to our proposal for the pEGindependent exchange observed by Halpem and James 151 which here is interpreted to occur uiu coordinated hydroxide_ (6) Substitulion of a chlorine by OH The substitution of an OH- into the coordination sphere and a Cl dissociation can be easily explained through a hydrolytic equilibrium situation [21_ Consequently, we shall describe only the most relevant points. We calculated the interaction of the whole ruthenium complex with an Ha0 molecule showing that the splitting of the water molecule as OH- and HY is quite favorable, even without considering solvent effects, because of the polarizing influence of the Ru complex. Furthermore, the entrance of the
OH- into the coordination sphere greatly enhances the possibility of a Cl dissociating_ AR this is described succinctly in step (b) of Fig_ 1. The principal points concerning this step are, as the OH- enters, the Ru-CI bond immediately weakens, its original population going from its value of 0.60 when the OH- is effectively outside the coordination sphere, to O-16 when OH- is at a distance of 2.25 A from the Ru, and to O-09 (essentially liberating the chlorine) when it reaches its equilibrium distance of 2-03 A from Ru- During this process the Ru-OH bond population grows from a value of 0.04 to 0.35 and 0.52 for the same configurations_ fc) Reaction
between
the coordinated
D and OH
moieties
In order to understand why the hydride and the OH radical react readily while coordinated to the Ru atom we shall begin to analyze all the possible movements of the ligands on the Ru coordination sphere_ Starting from the octahedraI structure we first allow every coordinated atom to oscillate around its equilibrium position_ These oscillatory motions naturally bring the particular atom nearer to, or farther ikom the others. All oscillations of up to 5 degrees bring the total energy of the complex up a few kcal/ mole, with one exception, i-e_, the movement of the OH moiety towards the hydride_ This means that aZZdisplacements of the chlorines, as well as the
2
t
Fig_ 3. Geometrical configurations of the [RuCI~OHD] tion of the OH moiety (steps 0 - 3) and the detachment OH (steps 4 - 65
3- complex of D from
Y
during the cis-migraRu and its fastening to
343
L 51
,
Fig_ 4_ Total energy during the process of Fig. 3.
hydride itseff about the “normal” octahedral structure imply a notable activation barrier, The highest barrier of all is found when we try to move the OH away from the hydride. On the other hand, no barrier at all ismet by displacing it towards the positive y-axis, as the total energy gradually lessens as OH- advances (see Fig. 3); this is clearly a &-migration towards the hydride, D_ In fact, as the OH group moves, the energy is always lowered, as shown in Fig. 4, So, from its original octahedral position (which in Fig. 3 was taken as lying on the z-axis) up to a 45 degree displacement on the YZ plane (towards the D lying on the y-axis in Fig. 3) the energy simply decreases, ie., no activation barrier is present. We now explain, on the basis of the molecular orbitals involved, why this is so. First, the reason why an OH (as opposed to a Cl) can migrate towards the hydride can be understood from an analysis of the molecular orbitals of the whole ruthenium complex, There are twenty-four molecular orbitals occupied (or semi-occupied) by the valence electrons of the complex. Aithough none of them can be associated exclusively to a single bond or interaction there are some dominant atomic orbital contributions that essentially classify them in different groups, playing different roles in the complex_ Starting from those molecular orbitals higher in energy, the first
314 four are essentially formed from the ruthenium d-orbitals, some of them interacting effectively with the OH group. We shall have more to say about these molecular orbitals in the latter part of this subsection, showing how they change during the OH movement (see Fig. 3) and their role in the fact that such a movement presents no energy barrier, as shown in Fig. 4. In any case, the fact that the OH-Ru interactZon is concentrated in the highest energy molecular orbitals is a measure of the lability of this bond_ Far lower in energy lies a group formed from the Ru p-orbitals interacting with the chlorine and hydride orbit&s_ These are separated from the highest lying orbitals mentioned above by 4 eV or more, which is the cause for the little mobility of these ligands (H and Cl). The lowest lying group of molecular orbitals is almost exclusively associated with individual atoms (they are mostly s-orbit%& from Ru and the Cl atoms) and contribute little to bond formation_ We shall study the OH movement in greater detail, but its greater mobility as compared with the Cl atoms can be assigned to this molecular orbital ordering_ In other words, the lability of the Ru-OH bond is far greater than any other bond in the complex. In several studies on other transition metal complexes [S, 71 a similar relation between the lability of a certain bond (i-e_, its association with the highest occupied molecular orbitals) and its capacity to migrate has been encountered. We shall now proceed to justify the other movement depicted in Fig. 3, that of D towards OH_ As the OH reaches its furthermost position, which is at 45 o from the z-axis (from then on an important barrier stops the cismigration), the D atom, originally fixed on the y-axis gradually becomes free to move_ The explanation for this is implicit in Fig_ 5, where the bond orders of the Ru-D, Ru-OH and O-D bonds are depicted as a function of the steps described in Fig. 3_ It is seen that as the OH leaves the z-axis (the octahedralposition), notonlydoesanO-Dbondbeginto formbutalsothe Ru-OH and, more notably the Ru-D bond, weaken. Indeed, this is so marked that D is essentially freed to move as represented in the last steps of Fig. 3 and these lower the energy of the system still further as Fig. 4’ shows. The effect of this D-movement is also overt in Fig_ 5 where, in the last stages, the Ru-D t-ond is effectively severed and an HDO molecule is formed (the O-D bond becomes as populated as the original OH bond). A notable characteristic of Fig. 5 is the well-defined transition state which the system traverses at step 3 of the &-migration. All bonds change abruptly, producing a multiple crossing of the curves. We shall now try to present a deeper explanation of the cause of this energy lowering during the reaction coordinate, in terms of the molecular orbit&s of the system_ The understanding of the OH movement is a little more involved and we shall postpone it for a moment and consider the causes of the D movement at the latter stages. Table 2 shows the net charges on the OH moiety and in the hydride, D, during the &-migration. It is seen that the OH movement not only weakens the Ru-D bond, as was shown in Fig_ 5, but, indeed, affects the charge distribution so as to make D positive
345
Fig. 5. Changes TABLE
in the bond
orders during
the process
of Fig. 3.
2
Charge in the coordinated hydride D as the OH advances towards it during the cis-migration. Also the charge on the OH moiety is reported_ Steps 0 to 6 correspond to Fig. 3 Step
0
Charge on the coordinated hydride, D Charge on the OH moiety
Oxygen atom Hydrogen atom
1
2
-6.41
-632
-0.05
-0.81
-6.76
-667
+0.16
+0_14
+0_17
3
so-31 -6-53 +0.23
4
+0.34 -6-50 1-o-25
5
+0.28 -0.42 +0.23
6
+0.20 -0.31 +-o-23
at the “transition state” step 3. These two effects are not independent, of course, as the Ru-D bond, of necessity, is highIy polar, as, indeed, are all Ru bonds in this complex, and its breaking inverts the charge in D. Thus, the D atom acquires a positive charge and is then attracted by the highIy negative oxygen site. As both the oxygen and the hydride have alI but severed their
346
STEP
0
r
z
1
Fig_ 6. Changes in the energies of the four highest $3 and $4) during the process of Fig_ 3.
(
occupied
5
molecular
6
orbitals
(@I,
@2,
Ru bond (especially D) one can readily under&and that the D movement towards OH is so energetically favourable. To understand why the OH &-migration is also favoured we must look to the highest (energy) molecular orbit&. In Fig. 6 the m.o. energies for the four highest occupied m.o.‘s are given as a func_fZonof the coordinate of Fig. 3. It is seen that $J1 and 9 4 are quite sensitive t6 the movements described, while I& and 1,0~change rather monotonically_ These last two molecular orbitaIs are formed essentially from the ruthenium dxz and d,, atomic orbil;als and, from symmetry considerations, should not intervene heaviIy in this reaction (see Fig. 3). The other two m_o_‘s, xJ1and tid, are more complicated: in Tables 3 and 4 the dominant contributions from the different atoms are given (for simpIicity non-negligible contributions from the chlorine p-orbital& which have no bearing on the process and are insensitive to the OH and D movements, are not reported)_ It is clear that originalIy the d, ruthenium orbital essentially degenerates in energy with d,, and d_ (see JI=, gbz, and $J~in Fig. 6), while e4 is the very important m-o_ responsible for most of the D and 0 bonding to Ru (through its dr2_,,z and dg orbitals)_ As soon as the OH group starts to move, however, d, starts to contribute more end more to 15~
347 TABLE
3
Dominant atomic orbital contributions to the highest Fig. 6) as they change during the k-migration Steps
0
Atomic
0.00 -O_Ol 0.99
0.02 -0.23
0.95
Steps
s
0
Pr
($1
of
2
3
4
5
6
-0-16 -0.52
-0.29 -0-75
-0-36 -0.74
-0.41 -0.73
-0-42 -0-73
0.74
O-22
O-06
0.00
0.00
to the +4
0
1
2
0.25
O-30
0.32
0.11
O-22
O-26
molecular
3
orbital
of Fig. 6 as they
4
5
6
0.19
0.13
O-10
0.09
-0.07
-0.02
O-06
--0.05
orbit&
d,z Ru dXz-y= d ,= D
orbital
4
Dominant atomic orbital contributions change during the &r-migration
Atomic
molecular
orbital
dxl_y2 Ru d,a d YZ
TABLE
1
occupied
0.01 -0-59 O-14
0.10 -0.57 o-11
O-48 -0-47 0.10
O-90 -9.19
0.10
0.95 -609
O-11
O-96 -0-03
0.10
0.97 -0.02 O-07
in compensation, Gr gradually becomes more and more dominated by di _ Through these changes, $1 and Qq retain the same character, I$~ always remains an inert, uninteracting d-function of Ru while *a belongs to the RuOH bond, first as d,z then as d,. Thus, we see that the movement of the OH in the yz plane does not really imply an unfavourable situation, it maintains its interaction with Ru, d, gradually replacing d;_ Thus, no bond breaking is necessary until the new D-O bond has started to form, and thus no energy barrier appears in this part of the process_ and,
(d)
Liberation
of the product
The deuterated species, DHO, is easily liberated, as the low final Ru-0 bond order of Fig. 5 suggests, This can be helped by the entrance of two Cl atoms (or an H and a Cl) to the Ru coordination sphere. We calculated the movement of two Cl’s towards the ruthenium, obtaining a lowering of the energy while the two new Ru-Cl bonds are formed, hence weakening the Ru-HDO interaction. At an Ru-Cl distance of 3.35 A (Le., 1 A further than its equilibrium distance [S] ) the RuCl bond is stronger than the Ru-0 bond, which afterwards reduces down to negligible values (O-04 when the Cl reach their f&ml positions)_