Density Functional Theory Study of Substitution at the Square-Planar

Jun 10, 2009 - N-9037 Tromsø, Norway, and ‡University of the Free State, 9300 Bloemfontein, Republic of South Africa. Received February 18, 2009...
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Organometallics 2009, 28, 3710–3715 DOI: 10.1021/om900133s

Density Functional Theory Study of Substitution at the Square-Planar Acetylacetonato-dicarbonyl-rhodium(I) Complex Kathrin Helen Hopmann† and Jeanet Conradie*,†,‡ †

Center for Theoretical and Computational Chemistry and Department of Chemistry, University of Tromsø, N-9037 Tromsø, Norway, and ‡University of the Free State, 9300 Bloemfontein, Republic of South Africa Received February 18, 2009

Density functional theory results show that the ligand substitution of PPh3 for CO in the squareplanar β-diketonato complex [Rh(acac)(CO)2] to give the monosubstituted [Rh(acac)(CO)(PPh3)] occurs in two reaction steps. Nucleophilic attack by PPh3 results in temporary dissociation of an acetylacetonato oxygen and formation of a square-planar intermediate, while carbonyl dissociation first occurs in the second step. The unexpected course of the reaction is rationalized by the order of the trans influence of the involved ligands: PPh3>CO>Oacac. Introduction Rhodium(I) complexes with bidentate β-diketonato ligands are well-known catalysts for hydroformylation of olefins.1-3 The first complexes of the type [Rh(β-diketonato)(CO)2] were reported by Bonati and Wilkinson in 1964.4 They showed that substitution reactions of [Rh(β-diketonato)(CO)2] with olefins as nucleophiles lead to replacement of both carbonyl groups, while reactions involving triphenylphosphine (PPh3) or triphenylarsine (AsPh3) lead to substitution of only one of the carbonyl ligands.4 Reaction of the symmetric β-diketonato complex [Rh(acac)(CO)2] (acac = acetylacetonato) with stoichiometric amounts of PPh3 results in formation of the monosubstituted [Rh(acac)(CO) (PPh3)] complex (Scheme 1).5,6 Understanding the detailed mechanism for CO substitution in [Rh(β-diketonato)(CO)2] complexes can provide valuable information about the reactivity of rhodium(I) β-diketonato systems. Substitution reactions of squareplanar low-spin d8 compounds such as rhodium(I) and platinum(II) complexes have been investigated both experimentally6-9 and theoretically.10-12 The limiting cases of ligand substitution reactions may be defined as *To whom correspondence should be addressed. E-mail: conradj.sci@ ufs.ac.za. (1) Mieczynska, E.; Trzeciak, A. M.; Zi olkowski, J. J. J. Mol. Catal. 1993, 80, 189–200. (2) Trzeciak, A. M.; Mieczynska, M.; Zi olkowski, J. J. Top. Catal. 2000, 11/12, 461–468. (3) Jin, H.; Subramaniam, B.; Ghosh, A.; Tunge, J. AIChE J. 2006, 52, 2575–2581. (4) Bonati, F.; Wilkinson, G. J. Chem. Soc. 1964, 3156–3160. (5) Trzeciak, A. M.; Zi olkowski, J. J. Inorg. Chim. Acta 1985, 96, 15– 20. (6) Serron, S.; Huang, J.; Nolan, S. P. Organometallics 1998, 17, 534– 539. (7) van Eldik, E.; Aygen, S.; Kelm, H.; Trzeciak, A. M.; Zi olkowski, J. J. Transition Met. Chem. 1985, 10, 167–171. (8) Simanko, W.; Mereiter, K.; Schmid, R.; Kirchner, K.; Trzeciak, A. M.; Ziolkowski, J. J. Organomet. Chem. 2000, 602, 59–64. (9) Mureinik, R. J. Coord. Chem. Rev. 1978, 25, l–30. (10) Cooper, J.; Ziegler, T. Inorg. Chem. 2002, 41, 6614–6622. (11) Lin, Z.; Hall, M. B. Inorg. Chem. 1991, 30, 646–651. (12) Burdett, J. K. Inorg. Chem. 1977, 16, 3013–3025. pubs.acs.org/Organometallics

Scheme 1. Substitution of PPh3 for CO in [Rh(acac)(CO)2] Leads to the Formation of Monosubstituted [Rh(acac)(CO)(PPh3)]

the dissociative mechanism (involving loss of a ligand prior to nucleophilic attack by the entering group) and the associative mechanism (involving formation of a fivecoordinated intermediate prior to loss of a ligand).13 If bond formation to the entering group and dissociation of a ligand occur at the same transition state, the reaction is defined as an interchange mechanism.13 Depending on the degree of bond formation and bond dissociation at the transition state, an interchange mechanism can be further classified as associative or dissociative interchange.13 A few examples of dissociative substitution have been reported for rhodium(I) and platinum(II) complexes,14,15 but most substitution reactions at square-planar low-spin d8 systems seem to occur through an interchange mechanism (involving formation of a single trigonal-bipyramidal transition state) or an associative mechanism (involving two transition states separated by a trigonal-bipyramidal intermediate).10-12 In this study we report the density functional theory (DFT) results of the ligand substitution mechanism for the formation of [Rh(acac)(CO)(PPh3)] from the precursors [Rh(acac)(CO)2] and PPh3 (Scheme 1). The replacement of a carbonyl ligand by triphenylphosphine is shown to occur through two steps, involving formation of a square-planar intermediate. The course of the reaction is rationalized by the (13) Housecroft, C. E.; Sharpe. A. G. Inorganic Chemistry, 2nd ed.; Pearson Education Limited, 2005. (14) Bossio, R. E.; Hoffman, T. N. W.; Cundari, T. R.; Marshall, A. G. Organometallics 2004, 23, 144–148. (15) Romeo, R.; Mons u Scolaro, L.; Plutino, M. R.; Fabrizi De Biani, F.; Bottari, G.; Romeo, A. Inorg. Chim. Acta 2003, 350, 143–151.

Published on Web 06/10/2009

r 2009 American Chemical Society

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order of the trans influence of the involved ligands: PPh3 > CO>Oacac.16

Computational Details Calculations were performed with the Gaussian 03 progam package.17 Geometries were optimized with the hybrid DFT functional B3LYP, which is composed of the Becke 88 exchange functional18 in combination with the LYP correlation functional.19 Optimizations were done in gas phase with the tripleζ basis set 6-311G(d,p) on all atoms except rhodium. The LANL2DZ basis set,20 corresponding to the Los Alamos ECP plus DZ, was used for rhodium. Additional calculations employing an added f-polarization function on rhodium (exponent = 0.4,21 referred to as LANL2DZ(f)) or the double-ζ basis set SDD on rhodium yielded similar results (Supporting Information, Tables S2 and S3). A qualitatively similar mechanism to that reported here for B3LYP was also obtained with the classical pure functional BLYP and the newer “pure” functional OPBE (Supporting Information, Tables S4 and S5). Approximate transition states were located using linear-transit techniques, and the transition states were then geometryoptimized fully without any restraints. Frequency calculations at the same level of theory as geometry optimizations were performed on all optimized structures to ensure that minima exhibit only positive frequencies, while transition states exhibit only one imaginary frequency. Thermochemical data and temperature corrections (298.15 K) were obtained from frequency calculations. Additional thermochemistry calculations at 333.15 K were performed with the freqchk functionality implemented in Gaussian 03 (Table 1 and Supporting Information, Table S1). Solvent effects on the potential energy surface were not computed here, as the reaction of Rh(acac)(CO)2 with PPh3 generally is performed in very low-polarity media such as benzene5 or hexane.22 Polarizing effects of the surroundings should thus be small.

Results and Discussion The reaction of [Rh(acac)(CO)2] with PPh3 leads to the substitution of one carbonyl ligand by the triphenylphosphine group, resulting in formation of the [Rh(acac)(CO)(PPh3)] product (Scheme 1). A pure dissociative mechanism in which (16) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973, 10, 335–422. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (18) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (19) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (20) (a) Dunning, T. H. Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, pp 1-28. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284–298. (d) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (21) Nsouli, N. H.; Mouawad, I.; Hasanayn, F. Organometallics 2008, 27, 2004–2012. (22) Conradie, J.; Lamprecht, G. J.; Otto, S.; Swarts, J. C. Inorg. Chim. Acta 2002, 328, 191–203.

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the Rh-CO bond is cleaved prior to PPh3 attack is possible if the cost of dissociating a CO ligand is lower than the activation barrier for an associative mechanism.11 To test this scenario, we computed the free energy (ΔG) associated with transformation of the four-coordinated [Rh(acac)(CO)2] complex into separated fragments, that is, the three-coordinated [Rh(acac) CO] complex and free CO. This analysis provides a ΔG value of 134 kJ mol-1. The computed energy is much higher than the expected barrier for the substitution reaction, and a dissociative mechanism is thus dismissed. A priori, the most likely reaction scenarios involve either an associative mechanism (involving two transition states and the formation of a five-coordinated trigonal-bipyramidal intermediate with subsequent CO release)12 or a one-step interchange mechanism (involving simultaneous PPh3 attack and CO release).10 Interestingly, neither such a transition state nor a five-coordinated intermediate could be located in our study. Instead, our results show that the reaction proceeds through two steps, involving formation of a fourcoordinated square-planar intermediate and temporary loss of one Rh-Oacac bond. The details of our findings are discussed below. Reactants. The reactant complexes for the studied reaction are shown in Figure 1. The [Rh(acac)(CO)2] molecule adopts a square-planar geometry, as expected for rhodium(I) complexes. The optimized bond lengths can be compared to the X-ray crystal structure of [Rh(acac)(CO)2].23 While both rhodium-oxygen bonds have a length of 2.07 A˚ in the optimized structure, they are both 2.04 A˚ in the crystal structure. The rhodium-carbonyl bonds are both 1.88 A˚ in the optimized structure, compared to 1.82 and 1.83 A˚ in the crystal structure. These small deviations of 0.03 to 0.05 A˚ in the metal-ligand bond lengths are not considered critical here.24 The carbon-carbon and carbon-oxygen bonds are very close to the experimental structure (bond, calculated, experimental): (C-Oacac, 1.27 A˚, 1.28 A˚) and (C d O, 1.14/ 1.14 A˚, 1.14/1.16 A˚). Transition State 1. As the PPh3 group approaches the reactant [Rh(acac)(CO)2], the reaction proceeds through transition state 1 (TS1, Figure 2a). The three main features of TS1 can be summarized as (i) a nucleophilic attack of the PPh3 phosphorus on the rhodium atom, (ii) cleavage of one rhodium-acetylacetonato bond, and (iii) rotation of one carbonyl ligand. The overall structure of TS1 appears to be pseudotrigonal bipyramidal (TBP), with the attacking phosphor, one carbonyl, and one acetylacetonato bond in the equatorial plane (Figure 2A). The optimized distance between the attacking phosphor, atom and rhodium at the transition state is 2.68 A˚. Formation of the new rhodium-phosphorus bond is accompanied by cleavage of one of the rhodium ligand bonds. However, the dissociating bond is not between rhodium and carbonyl but is instead one of the rhodiumacetylacetonato bonds. The displaced oxygen atom of acetylacetonato is positioned 2.28 A˚ from the Rh ion, which is a displacement of 0.21 A˚ compared to the reactant structure. In comparison, the second oxygen-rhodium bond length has only changed 0.02 A˚ from the reactant to the transition state. The two carbonyl ligands exhibit basically the same (23) Huq, F.; Skapski, A. C. J. Cryst. Mol. Struct. 1974, 4, 411–413, Cambridge ID ACABRH02. (24) Hehre, W. J. A Guide to Molecular Mechanisms and Quantum Chemical Calculations; Wavefunction Inc.: Irvine, CA, 2003; pp 153, 181.

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Table 1. Computed Energies for the Reactant, Transition States, Intermediate, And Product during the Formation Reaction of [Rh(acac)(CO)(PPh3)] from PPh3 and [Rh(acac)(CO)2]a electronic energy0K/kJ mol-1

ΔH298K/kJ mol-1

ΔS298K/J K-1 mol-1

reactant 0.0 0.0 0.0 TS1 23.9 20.9 -101.7 intermediate 17.0 16.7 -82.2 TS2 49.8 45.6 -88.4 product 15.3 14.5 -32.4 product, separated 19.3 14.7 45.0 a All energy values are given compared to the reactant. ΔG values are given at both 298.15 and 333.15 K.

Figure 1. DFT-optimized reactant complexes, viz., triphenyl phosphine (PPh3) and [Rh(acac)(CO)2]. Distances are indicated in angstroms. Inset shows angles and the square-planar geometry of the rhodium complex.

bond distances as in the reactant structure. However, the displacement vectors of the imaginary frequency of the transition state show that one of the carbonyl ligands exhibits a rotation-like movement around the axis of the pseudotrigonal bipyramid (Figure 2a). The observed movement is not arbitrary, but comprises an essential feature of TS1, because it ensures that the TBP geometry of TS1 immediately converts back to a four-coordinated squareplanar intermediate structure (Figure 2b). In this way the rhodium complex minimizes the geometric distortion associated with ligand replacement. Analysis of the transition state eigenvector shows major contributions from two bonds and two angles. As could be expected, the two bonds are the forming phosphorus-rhodium bond and the scissile oxygen-rhodium bond. The two angles are the equatorial Oacac-rhodium-carbonyl angle (142.5° at the transition state) and the equatorial phosphorus-rhodium-carbonyl angle (133.2° at the transition state). The third equatorial angle between phosphor, rhodium, and the acac-oxygen is only 83.8° at the transition state. This relatively small angle is in agreement with previous DFT studies on exchange reactions of rhodium(I) complexes that found that the small angle minimizes the repulsion between the electron concentrations on the central atom and the electron concentrations on the entering and leaving ligands.11 The imaginary frequency obtained for TS1 is -52.95 cm-1, which appears rather low. However, this is not a surprising result, considering the contributions of several heavy atoms to the reaction coordinate (as the associated frequency will be inversely proportional to the square root of the reduced mass of the contributing atoms). The computed energies for TS1 are given in Table 1. The electronic energy alone indicates a barrier of only 23.9 kJ mol-1 for this step. Thermochemical quantities were computed at 298.15 K (Table 1) and for comparison also at 333.15 K, which is closer to the experimental temperature (above 60 °C,22 Table 1 and Supporting Information Table S1). The calculated

ΔG298K/kJ mol-1

ΔG333K/kJ mol-1

0.0 51.3 41.2 71.9 24.2 1.3

0.0 54.8 44.1 75.0 25.3 -0.3

enthalpy of activation is small, 20.9 kJ mol-1 (298.15 K), indicating that one rhodium-Oacac bond has relatively low bond strength. The large negative entropy of activation of -101.7 J K-1 mol-1 is consistent with an associative interchange mechanism. However, as entropy calculations are very sensitive to low-frequency modes, this value should be considered as an estimation only. Including the entropy effects, the overall activation free energy, ΔGq, is computed as 51.3 kJ mol-1 (298.15 K). The geometric consequences of TS1, involving the loss of a bond to the acac ligand instead of a carbonyl ligand, might appear surprising at first. However, the course of the reaction is easily understood by considering that the choice of leaving group will be heavily influenced by the ligand trans to it. The thermodynamic trans influence is a ground state phenomenon leading to a weakening of the metal-ligand bond in the trans position.25 The trans influence of a carbonyl group is stronger than that of a Oacac group.16 It is thus more likely that nucleophilic attack of PPh3 on [Rh(acac)(CO)2] will result in loss of a rhodium-Oacac bond than in loss of a CO ligand. Intermediate. The optimized intermediate formed following TS1 is visualized in Figure 2b. It is noteworthy how the complex has regained the preferred square-planar structure. Rhodium(I) compounds generally have a strong preference for planar, four-coordinated geometries, and the fact that the reaction proceeds through a four-coordinated intermediate instead of a five-coordinated trigonal-bipyramidal complex is thus no surprise. The rhodium-phosphorus bond is reduced to 2.45 A˚ in the intermediate structure, while the rhodium-Oacac bond is fully dissociated with a distance of 2.54 A˚. The rotated CO ligand shows a slightly elongated bond of 1.92 A˚, which is likely to be due to the trans influence of the PPh3 ligand. Thus, by allowing binding of triphenylphosphine to rhodium prior to carbonyl dissociation, the stronger trans influence of the triphenylphosphine unit over that of the CO group16 can be utilized to weaken the rhodium-carbonyl bond, thereby promoting the subsequent dissociation of CO. The charge distribution and bond length of the acac ligand at the intermediate have changed significantly compared to the reactant structure. At the reactant, the single negative charge of the acac ligand is symmetrically delocalized between the two oxygen atoms, with a Mulliken atomic charge distribution of -0.484 on each oxygen atom. At the intermediate, the conjugated system instead shows an asymmetric charge distribution, -0.410 on the free Oacac and -0.499 on the rhodium-bound Oacac. The change in conjugation is also seen from the C-Oacac and C-C (acac backbone) bond lengths, which are symmetric in the reactant (1.27 A˚ for both (25) Jordan, R. B. Reaction Mechanisms of Inorganic and Organometallic Systems, 3rd ed.; Oxford University Press: New York, 2007.

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Figure 2. (a) DFT-optimized pseudo-TBP transition state 1 (Scheme 2), involving the attack of PPh3 on [Rh(acac)(CO)2], cleavage of one Rh-Oacac bond, and rotation of one CO ligand (indicated by the arrow). Breaking and forming bonds are shown with dashed lines. (b) DFT-optimized square-planar intermediate during the substitution reaction of PPh3 for CO in [Rh(acac)(CO)2]; see Scheme 2.

Figure 3. (a) DFT-optimized pseudo-TBP transition state 2 (Scheme 2), involving attack of acetylacetonato on rhodium and release of CO to form the DFT-optimized square-planar product displayed in (b).

C-O, and 1.42 A˚ for both C-C) but have changed to 1.25 A˚ (free C-O) and 1.28 A˚ (coordinated C-O) and 1.42 A˚ (C-C adjacent to free C-O) and 1.39 A˚ (C-C adjacent to coordinated C-O). The conjugated nature of the acac ligand thus allows asymmetric delocalization of the negative charge to the axial Oacac oxygen, thereby facilitating cleavage of the equatorial rhodium-Oacac bond. The relative electronic energy of the intermediate is computed as 17.0 kJ mol-1 (Table 1). The calculated Gibbs free energy is 41.2 kJ mol-1 (298 K), which is only slightly below the TS1 energy (Table 1). Thus, despite the square-planar geometry of the intermediate, the complex is energetically unstable. Transition State 2. From the intermediate, the reaction proceeds through a second transition state, TS2, which results in formation of the [Rh(acac)(CO)(PPh3)] product. The overall structure of the transition state is again pseudotrigonal bipyramidal (Figure 3a). The main elements of this transition state include (i) nucleophilic attack of the free oxygen atom of the acac ligand on rhodium, (ii) cleavage of the rhodium-carbonyl bond, and (iii) movement of rhodium and the acac ligand to result in formation of a new squareplanar geometry. At TS2, the attacking oxygen atom is located 2.31 A˚ from the rhodium atom, while the distance to the leaving carbonyl group is elongated to 2.11 A˚ (Figure 3a). Not surprisingly, the main contribution to the transition state eigenvector comes from the scissile rhodium-carbonyl bond. The second

main contribution involves the angle between rhodium and the C-O atoms of the leaving carbonyl group, which has changed from near linear (176.4°) at the intermediate to 149.9° at the transition state. The angle between leaving and incoming group is again relatively small (100.4°), while the other two equatorial angles of the pseudotrigonal bipyramid are 134.9° (P-Rh-Oacac) and 124.4° (P-Rh-CCO leaving). The two axial ligands exhibit similar bond lengths to that at the intermediate, but the equatorial phosphorus ligand has moved closer to the rhodium, with a bond length of 2.34 A˚ compared to 2.45 A˚ at the intermediate. The imaginary frequency of TS2 is slightly larger than for TS1 and is computed as -86.1 cm-1. The displacement vectors of the imaginary frequency of the TS2 relates to the movement of CO away from rhodium and the loose Oacac into the SQP plane. The thermochemical analysis shows that the enthalpy of activation for TS2 is 45.6 kJ mol-1, which is about twice the magnitude computed for TS1 (20.9 kJ mol-1, Table 1) or 28.9 kJ mol-1 above the intermediate state. The overall computed Gibbs energy of activation is 71.9 kJ mol-1, about 20 kJ mol-1 higher than TS1. Products. Following TS2 is the [Rh(acac)(CO)(PPh3)] product formation, shown in Figure 3b. The product is square planar, as expected. The CO molecule is fully dissociated, with a distance of 4.34 A˚ to the rhodium atom. The RhOacac bonds are slightly different in length, 2.09 and 2.11 A˚, respectively. The difference is in agreement with the larger

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Scheme 2. DFT-Calculated Reaction Mechanism for the Substitution of PPh3 for CO in [Rh(acac)(CO)2]a

a Dashed lines indicate bonds that are broken or formed. The reactant, intermediate, and product adopt a square-planar geometry, while the transition states are trigonal bipyramidal.

trans influence of PPh3 relative to CO, resulting in a longer Rh-Oacac bond trans to PPh3. The C-O bonds of the acac ligand are both 1.27 A˚, as in the reactant. The phosphorusrhodium bond is reduced to 2.31 A˚. Compared to the X-ray crystal structure of [Rh(acac)(CO)(PPh3)],26 the bond lengths are slightly overestimated by 0.05 to 0.07 A˚. In the experimental structure, the rhodium-phosphorus bond is 2.24 A˚, while the Rh-CCO bond is only 1.80 A˚ (1.85 A˚ calculated). The Rh-Oacac bond trans to PPh3 is 2.09 A˚, while the other Rh-Oacac bond is 2.03 A˚. It is interesting to note that both the experimental and theoretical results indicate that the trans influence of the PPh3 ligand is stronger than the trans influence of the CO ligands in β-diketone rhodium(I) complexes.16,26 The computed Gibbs free energy is 24.2 kJ/mol, indicating that the formed product complex is somewhat higher in energy than the reactant complex (Table 1). The reaction is driven to completion by the evaporation of CO. Computing the energies of [Rh(acac)(CO)(PPh3)] and CO separately and adding them up results in a Gibbs free energy value of only 1.3 kJ mol-1 (Table 1) for the full reaction at 25 °C. At the experimental conditions above 60 °C,22 the reaction proceeds spontaneously (ΔGCO> Oacac).16,26 The stronger trans influence of CO versus Oacac promotes cleavage of the rhodium-oxygen bond at the first transition state, while the simultaneous rotation of one carbonyl ligand allows immediate re-formation of the square-planar geometry, thus minimizing the geometric distortion associated with ligand displacement. Cleavage of an acetylacetonate bond is facilitated by the conjugated nature of the bidentate ligand, which allows asymmetric delocalization of

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the negative charge, thereby reducing the anionic character of the displaced oxygen. It is noteworthy that the stepwise substitution reaction observed here might not be accessible without a bidentate ligand such as β-diketonato. If the ligand displaced at the first transition state was of monodentate character, it would most likely be lost at the intermediate stage, and the desired CO substitution would not occur. However, the bidentate nature ensures that the displaced Oacac remains in the vicinity of the rhodium atom at the square-planar intermediate. The second reaction step is facilitated by the trans influence of the PPh3 ligand (PPh3 > CO), which promotes cleavage of the rhodium-carbonyl bond and simultaneous re-formation of the Rh-Oacac bond

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to yield the product complex [Rh(acac)(CO)(PPh3)]. The overall results indicate that the lability of the β-diketonato ligand might be crucial for the reactivity of rhodium(I) β-diketonato complexes.

Acknowledgment. This work was supported by the Research Council of Norway and the National Research Fund of the Republic of South Africa. Supporting Information Available: Tables S1-S5 containing additional calculations at different levels of theory and optimized Cartesian coordinates for selected systems. This material is available free of charge via the Internet at http://pubs.acs.org.