DFT Investigation of the Mechanism of Phosphine-Thioether

Sep 5, 2012 - ABSTRACT: The rearrangement of the phosphine-thioether ligand in. 1,2-(Peq,Seq)-Os3(CO)10(Ph2PCH2CH2SMe) to 1,1-(Peq ...
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DFT Investigation of the Mechanism of Phosphine-Thioether Isomerization in the Triosmium Cluster Os3(CO)10(Ph2PCH2CH2SMe): Migratory Preference for the Formation of an Edge-Bridged Thioether versus a Phosphine Moiety David A. Hrovat,†,‡ Ebbe Nordlander,*,§ and Michael G. Richmond*,‡ †

Center for Advanced Scientific Computing and Modeling and ‡Department of Chemistry, University of North Texas, Denton, Texas 76203, United States § Inorganic Chemistry Research Group, Chemical Physics, Center for Chemistry and Chemical Engineering, Lund University, SE-221 Lund, Sweden S Supporting Information *

ABSTRACT: The rearrangement of the phosphine-thioether ligand in 1,2-(Peq,Seq)-Os3(CO)10(Ph2PCH2CH2SMe) to 1,1-(Peq,Sax)-Os3(CO)10(Ph2PCH2CH2SMe) was investigated by electronic structure calculations. The chelated isomer lies 2.5 kcal/mol lower in energy than its bridged counterpart, and the barrier computed for the mechanism is in agreement with the results from our earlier experimental study. Phosphine-thioether isomerization occurs via three distinct steps that involve the migration of the CO and SMe groups in a plane that is perpendicular to the trimetallic core. One of the intermediates on the reaction surface corresponds to the 50e cluster Os3(CO)9(μ-CO)(μ-Ph2PCH2CH2SMe), whose edge-bridging thioether moiety functions as a 4e donor ligand. Alternative mechanisms involving ligand dissociation/association and merry-go-round sequences are energetically prohibitive.

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INTRODUCTION The ability of bidentate hemilabile ligands to modulate the coordination number at a mononuclear metal complex during autogeneous catalysis is a phenomenon that has been successfully exploited in hydrogenation and hydroformylation reactions.1 Such compounds also serve as model systems that provide insight into the properties of allosteric enzymes.2 The generation of a latent coordination site is readily achieved through the dissociation of the weakly bound arm of the chelated hemilabile ligand, which in turn enhances substrate binding, turnover rate, and chemical selectivity during catalysis. As a class of ligands, hemilabile ligands are diverse, and the vast majority of systems contain a strong anchor moiety in the form of a phosphine, along with a weaker donor moiety based on oxygen (P,O), nitrogen (P,N), phosphine oxide (P,PO), or sulfur (P,S). Our interest in such ligands involves their coordination chemistry and fluxionality at polynuclear clusters.3 In particular, we have studied the reaction of the phosphine thioethers Ph2PCH2CH2SR (where R = Me, p-tolyl), Ph2P(CH2)2S(CH2)2S(CH2)2PPh2, and S(CH2CH2PPh2)2 with Os3(CO)12−n(MeCN)n (where n = 0−2), and these studies have provided irrefutable evidence for the intramolecular exchange of the hemilabile PS ligand system about the Os3 polyhedron.4−6 An integral aspect of the exchange process is the isomerization of the ancillary PS ligand from the bridged (1,2-P,S) to the chelated (1,1-P,S) form of the cluster. We have studied these isomerizations kinetically, and they are well-behaved and exhibit first-order kinetics;4 however, the mechanism responsible for © 2012 American Chemical Society

the permutation of the ancillary PS ligand about the cluster frame remains equivocal. Related to the fluxional behavior of the PS ligand in Os3(CO)10(PS) clusters is the lability of the ancillary ligands in the related clusters Os3(CO)10(PP), whose diphosphine isomerization behavior has been thoroughly investigated by us experimentally and computationally.7,8 DFT calculations indicate that the concerted bridge-to-chelate isomerization of the diphosphine ligand in Os3(CO)10(PP) clusters occurs through a merry-go-round sequence that involves the simultaneous rearrangement of a phosphine moiety and a pair of CO groups in a plane coincident with the metallic frame. This phenomenon is illustrated in eq 1 in the case of the cluster containing the ligand 1,2-bis(diphenylphosphino)benzene (dppz).8

Received: July 9, 2012 Published: September 5, 2012 6608

dx.doi.org/10.1021/om300640t | Organometallics 2012, 31, 6608−6613

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Scheme 1

of imaginary frequencies through calculation of the vibrational frequencies, using the analytical Hessian. Zero imaginary frequencies (positive eigenvalues) correspond to an intermediate or minimum, whereas an imaginary frequency (negative eigenvalue) designates a transition state. All transition states on the potential energy surface were evaluated by IRC calculations, and these calculations have unequivocally established the reactant and product species associated with each transition-state structure. The computed frequencies were used to make zero-point and thermal corrections to the electronic energies, and the reported potential energies and enthalpies are quoted in kcal/mol relative to the specified standard.12 The natural charges and Wiberg bond indices were computed using Weinhold’s natural bond orbital (NBO) program, as executed by Gaussian 09.13,14 The geometry-optimized structures have been drawn with the JIMP2 molecular visualization and manipulation program.15

In order to establish the mechanism associated with the rearrangement of the phosphine-thioether ligand in 1,2-(Peq,Seq)Os3(CO)10(Ph2PCH2CH2SMe) to 1,1-(Peq,Sax)-Os3(CO)10(Ph2PCH2CH2SMe), we have performed DFT calculations, and these results are presented herein. The lowest energy manifold for the conversion of 1,2-(Peq,Seq)-Os3(CO)10(Ph2PCH2CH2SMe) to 1,1-(Peq,Sax)-Os3(CO)10(Ph2PCH2CH2SMe) does not involve a merry-go-round sequence but rather a series of in-plane migrations of CO and SMe moieties about selected Os−Os vectors. The ability of the thioether moiety to function as a 4e donor and to promote the expansion of the cluster core is discussed.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION I. Pairwise Exchange Mechanism. The mechanism responsible for the isomerization of the thiophosphine ligand in Os3(CO)10(Ph2PCH2CH2SMe) was addressed through the use of electronic structure calculations. Starting from 1,2-(Peq,Seq)Os3(CO)10(Ph2PCH2CH2SMe), the kinetic product of ligand substitution from the reaction between Os3(CO)10(MeCN)2 and thiophosphine,4 the first step in the isomerization involves an in-plane migration of the MeS moiety and CO groups across the non-phosphine-substituted Os−Os vector. Scheme 1 outlines this first exchange step, where the conrotatory motion of

Computational Methodology and Modeling Details. All calculations were performed with the hybrid DFT functional B3LYP, as implemented by the Gaussian 09 program package.9 This functional utilizes the Becke three-parameter exchange functional (B3),10 combined with the correlation functional of Lee, Yang, and Parr (LYP).11 The Os atoms were described by Stuttgart−Dresden effective core potentials (ecp) and SDD basis set, while a 6-31G(d′) basis set was employed for the remaining atoms. The initial input structures for clusters 1,2-(Peq,Seq)-Os3(CO)10(Ph2PCH2CH2SMe) and 1,1-(Peq,Sax)-Os3(CO)10(Ph2PCH2CH2SMe) were taken from X-ray data already reported by us.4 All reported geometries were fully optimized and evaluated for the correct number 6609

dx.doi.org/10.1021/om300640t | Organometallics 2012, 31, 6608−6613

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Figure 1. Optimized structures for the ground-state minima A1−A4 and the corresponding transition states.

agreement with our earlier kinetic data on this reaction, where a value of ΔH⧧ = 18.8 kcal/mol was found.4,17 Species A3 was computed to be higher in enthalpy than A1 by 18.4 kcal/mol. Accompanying the formation of A3 is the expansion of the P,S-ligand-bridged Os−Os bond from a computed distance of 2.986 Å in A1 to 3.434 Å in A3. This polyhedral expansion may be traced to the fact that the MeS moiety has undergone a donicity change from a 2e donor in clusters A1 and A2 to a 4e donor in A3. The ability of thioethers to serve as 4e donor ligands is not without precedent,18 and the addition of two extra electrons to A3 from the MeS moiety gives rise to a 7 skeletal electron pair (SEP) count, which in turn promotes the cleavage of one of the Os−Os vectors in keeping with the tenets of polyhedral skeletal electron pair (PSEP) theory.19 Additional support for the increased electron count in intermediate A3 is also seen in the selected natural charges and Wiberg bond indices that are displayed in Table 1. The charge on the phosphorus atom in each species on the potential energy surface in the transformation of A1 to A4 is invariant at ca. 1.35. This charge uniformity is consistent with the η1 coordination mode and 2e donor nature of the ancillary phosphine ligand in each of the different species. In contrast, the charge on the sulfur atom shows the largest deviation in species A3, where the chalcogen functions as a 4e donor. Here the charge on sulfur goes from a value of 0.57 in A2 to a high of 0.75 in A3, followed by a decrease to 0.55 in the final product A4. Further corroboration for a breaking of the P,S-ligand-bridged Os−Os vector during the isomerization is revealed in the Wiberg bond index for the Os1−Os2 vector in the various species. The anticipated polyhedral expansion in the formation of A3 is accompanied by a significant decrease in the Wiberg index of the Os1−Os2 vector relative to the other Os−Os bonds in A3 and the other reaction species. Completion of the thioether transit across the Os1−Os2 vector in A3 leads to the net isomerization of the phosphinethioether ligand and formation of the thermodynamically more stable cluster A4. The barrier for this last step (TSA3A4) is computed to lie 0.8 kcal/mol lower in enthalpy than TSA2A3, the highest transition structure in the isomerization. Relative to A1, the ligand-chelated isomer A4 is enthalpically favored by 2.5 kcal/mol, in concert with the thermolysis experiments, where A1 transforms cleanly to give A4 without material loss. Finally, we compute a Keq value of 49 for the transformation involving A1 → A4 at room temperature on the basis of the Gibbs free energy change for this reaction.20

the six migrating groups furnishes the bridging isomer A2; here the thioether moiety is transformed from an initial equatorial position in A1 to an axial site at the cluster. This in-plane migration represents a prevalent manifold for the exchange of CO and phosphine ligands between adjacent metal centers in polynuclear metal clusters.4−8,16 Figures 1 and 2 show the

Figure 2. Potential energy surface for the isomerization of A1 to A4. Energy values are ΔE (ΔH) in kcal/mol with respect to A1.

geometry-optimized structures and the potential energy surface for the conversion of 1,2-(Peq,Seq)-Os3(CO)10(Ph2PCH2CH2SMe) to 1,1-(Peq,Sax)-Os3(CO)10(Ph2PCH2CH2SMe). The transition state TSA1A2 lies 10.9 kcal/mol higher in energy than the starting species A1, and the product species, A2, is only marginally less stable than the starting cluster, lying 0.8 kcal/mol above A1. At first glance, it would appear that the MeS moiety in A2 is positioned for a concerted transfer across the thiophosphine-bridged Os−Os vector to furnish, in one step, the anticipated chelating isomer 1,1-(Peq,Sax)-Os3(CO)10(Ph2PCH2CH2SMe). However, our calculations subsequently confirmed that the direct transformation of A2 to give the thermodynamically more stable isomer 1,1-(P eq ,S ax )Os3(CO)10(Ph2PCH2CH2SMe) (A4) through a concerted manifold was prohibitive. Instead, a second in-plane migration of the MeS moiety and CO groups occurs across the thiophosphine-bridged Os−Os vector through TSA2A3, followed by the formation of the 50e cluster A3. The latter transition state serves as the rate-limiting step for this reaction, and the computed enthalpic barrier of 20.6 kcal/mol is in excellent 6610

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Table 1. Selected Natural Charges and Wiberg Bond Indices for Clusters A1−A4 and the Corresponding Transition Statesa

A1

TSA1A2

A2

Os1 Os2 Os3 P S

−1.45 −1.28 −1.32 1.35 0.60

−1.41 −1.35 −1.29 1.35 0.59

−1.45 −1.23 −1.37 1.36 0.57

Os1−Os2 Os1−Os3 Os2−Os3 Os1−P Os2−P Os2−S Os1−S Os1−C(μ)b Os2−C(μ)b

0.44 0.45 0.49 0.80

0.44 0.47 0.26 0.79

0.47 0.45 0.44 0.80

0.69

0.63

0.65

TSA2A3 Atomic Charge −1.34 −1.09 −1.40 1.35 0.59 Wiberg Index 0.18 0.40 0.47 0.78 0.23 0.61 0.68 0.81

A3

TSA3A4

A4

TSA2A4_alt

−1.33 −1.24 −1.41 1.36 0.75

−1.35 −1.11 −1.41 1.36 0.62

−1.25 −1.37 −1.37 1.33 0.55

−1.23 −1.12 −1.29 1.16 0.55

0.12 0.39 0.39 0.79

0.16 0.40 0.44 0.79

0.42 0.49 0.43 0.79

0.56 0.58 0.75 0.70

0.31 0.61 0.69 0.77

0.28 0.34 0.32 0.33 0.38 0.63

0.62

a Atom numbers based on the numbering scheme for the Os3 cluster depicted above. bTerminal Os−CO Wiberg indices are on the order of ca. 1.00−1.25.

II. Alternative Isomerization Mechanisms Involving Merry-Go-Round and Ligand Dissociation Manifolds. The isomerization of diphosphine ligands about triosmium clusters has been shown to occur through a concerted scrambling of equatorial CO and phosphine ligands via a merry-goround process.6,7,21 In these examples, where the diphosphine ligands possess an unsaturated backbone, the single-step exchange process is kinetically favored over alternative, multistep sequences involving pairwise ligand exchanges across a single Os−Os vector. In order to rule out a possible merry-go-round manifold in the conversion of 1,2-(Peq,Seq)-Os3(CO)10(Ph2PCH2CH2SMe) to 1,1-(Peq,Sax)-Os3(CO)10(Ph2PCH2CH2SMe), the energetics associated with the migration of the phosphine ligand to the MeS-substituted osmium center in species A2 were computationally modeled. The conversion of A2 directly to A4 was found to take place through an in-plane exchange of two CO ligands and the phosphine group in a manner similar to that reported by us for diphosphine ligands. The transition structure for this merry-go-round process is represented by TSA2A4_alt (Figures 1 and 2), and it is calculated to be 5.5 kcal/mol higher in enthalpy than TSA2A3, allowing us to rule out this particular merry-go-round contribution in our isomerization reaction. Scheme 2 illustrates two other potential merry-go-round sequences originating from A1. Here the conversion of the bridged ligand in A1 to the chelated isomer 1,1-(Peq,Seq)Os3(CO)10(Ph2PCH2CH2SMe) may be envisioned through merry-go-round sequences involving a pair of CO groups in tandem with the migration of a phosphine or thioether moiety. These reactions would then be followed by an equatorial-toaxial rearrangement of the thioether moiety in 1,1-(Peq,Seq)Os3(CO)10(Ph2PCH2CH2SMe) via an in-plane migration

Scheme 2

analogous to that in A2 → A3 to ultimately furnish A4. This sequence has the advantage of accomplishing in two steps what the mechanism depicted in Figure 2 required to complete in three separate elementary reactions. The migration of CO and phosphine ligands in the merry-go-round process depicted in path a occurs without problems, in keeping with our earlier work, and the resulting transition structure lies 35.3 kcal/mol 6611

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Figure 3. Optimized structures for the ground-state minima A5 and A6 and the corresponding transition states.

16.4 kcal/mol above TSA2A3. In contrast, the dissociation of the thioether moiety is 12.9 kcal/mol lower in energy than TSA1A5, and these data clearly support the enhanced lability of the thioether versus phosphine moiety in these ligand dissociation scenarios. Each dissociation product displays a weak van der Waals interaction with a neighboring hydrogen on an aryl ring (A5) and the tethering −CH2CH2− moiety (A6). Such intermediates may serve as entry points into ligand degradation manifolds involving C−H bond activation, and the reactivity of these and related cluster-based species are under active investigation by our groups.

above A1 (not shown). In the case of a migrating thioether moiety (path b) all attempts to locate a viable transition structure failed. These optimizations either collapsed back to 1,2-(Peq,Seq)Os3(CO)10(Ph2PCH2CH2SMe) or rose too high in energy to be useful. The merry-go-round processes examined here are energetically unfavorable relative to the potential energy surface for the A1 → A4 evolution illustrated in Figure 2. Given the hemilabile nature of the phosphine-thioether ligand in A1, an isomerization manifold that is initiated by a dissociation of one arm of the bidentate ligand to give the reactive cluster Os3(CO)10(η1-P-S) or Os3(CO)10(η1-S-P) could not, a priori, be eliminated from consideration. Ligand redistribution at the unsaturated intermediate, followed by recoordination of the pendant arm of the dissociated ligand, could then lead to the net isomerization of the phosphine-thioether ligand. The dissociation of the phosphine and thioether moieties in A1 was investigated and found to proceed through the transition structures TSA1A5 and TSA1A6, respectively, to afford the corresponding 46e unsaturated cluster, as shown in Figures 3 and 4.

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CONCLUSIONS

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ASSOCIATED CONTENT

The mechanism for the isomerization of 1,2-(Peq,Seq)-Os3(CO)10(Ph2PCH2CH2SMe) to 1,1-(Peq,Sax)-Os3(CO)10(Ph2PCH2CH2SMe) has been established using DFT calculations. The isomerization takes place by a nondissociative process, and the modeled mechanism yields activation parameters in accord with our published kinetic data. The phosphine moiety is present only as a spectator donor entity and does not participate in the isomerization reaction. A three-step mechanism that proceeds through in-plane migrations of CO groups and the thioether moiety in a plane coincident with an Os−Os bond has been computationally demonstrated. Our data have also provided irrefutable evidence for the preferential dissociation of the thioether over the phosphine moiety in 1,2-(Peq,Seq)-Os3(CO)10(Ph2PCH2CH2SMe). S Supporting Information *

Text giving the complete ref 9 and tables giving the atomic coordinates of all optimized stationary points and transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 4. Potential energy surface for the dissociation of the Ph2PCH2CH2SMe ligand from A1. Energy values are ΔE (ΔH) in kcal/mol with respect to A1.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.N.); cobalt@ unt.edu (M.G.R.).

Both ligand dissociations exhibit higher activation barriers relative to TSA2A3, with the greater of the two barriers found for the liberation of the phosphine ligand, whose transition structure lies

Notes

The authors declare no competing financial interest. 6612

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Organometallics 1995, 14, 698. (d) Huang, S.-H; Watson, W. H.; Carrano, C. J.; Wang, X.; Richmond, M. G. Organometallics 2010, 29, 61. (17) The kinetics for the isomerization of 1,2-(P eq ,S eq )Os3(CO)10(Ph2PCH2CH2SMe) to 1,1-(Peq,Sax)Os3(CO)10(Ph2PCH2CH2SMe) were investigated by NMR spectroscopy and spectrophotometric methods,4 and the Eyring activation parameters were determined as ΔG⧧ = 23.1 kcal/mol, ΔH⧧ = 18.8 kcal/mol, and ΔS⧧ = −14 eu. (18) For several reports where thioethers function as 4e donor ligands, see: (a) Churchill, M. R.; Ziller, J. W.; Dalton, D. M.; Keister, J. B. Organometallics 1987, 6, 806. (b) Adams, R. D.; Pompeo, M. P. Organometallics 1990, 9, 2651. (c) Hauptman, E.; Shapiro, R.; Marshall, W. Organometallics 1998, 17, 4976. (d) Adams, R. D.; Kwon, O.-S.; Smith, M. D. Inorg. Chem. 2002, 41, 5525. (e) Reynolds, M. A.; Guzei, I. A.; Angelici, R. J. Inorg. Chem. 2003, 42, 2191. (19) Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1990. (20) The Gibbs free energy value for the equilibrium of A1 → A4 is computed as ΔG⧧ = −2.3 kcal/mol. (21) See also: (a) Watson, W. H.; Poola, B.; Richmond, M. G. J. Organomet. Chem. 2006, 691, 4676. (b) Watson, W. H.; Poola, B.; Richmond, M. G. Polyhedron 2007, 26, 3585.

ACKNOWLEDGMENTS Financial support from the Robert A. Welch Foundation (No. B-1093-MGR), the National Science Foundation (No. CHE0741936), and the European Union (TMR Network Metal Catalysts in Catalysis and Organic Synthesis, MECATSYN) is much appreciated. M.G.R. also acknowledges support in the form of a research fellowship from the Grace and Philip Sandblom Foundation. We thank Prof. Michael B. Hall (TAMU) for providing us a copy of his JIMP2 program, which was used to prepare the geometry-optimized structures reported here.

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REFERENCES

(1) (a) Roch-Neirey, C.; Le Bris, N.; Laurent, P.; Clément, J.-C.; des Abbayes, H. Tetrahedron Lett. 2001, 42, 643. (b) Grützmacher, H. Angew. Chem., Int. Ed. 2008, 47, 1814. (c) Nandi, M.; Jin, J.; RajanBabu, T. V. J. Am. Chem. Soc. 1999, 121, 9899. (d) Hu, X.-P.; Huang, J.-D.; Zeng, Q.-H.; Zheng, Z. Chem. Commun. 2006, 293. (e) Charette, A. B.; Côté, A.; Desrosiers, J.-N.; Bonnaventure, I.; Linsday, V. N. G.; Lauzon, C.; Tannous, J.; Boezio, A. A. Pure Appl. Chem. 2008, 80, 881. (2) (a) Yoon, H. J.; Kuwabara, J.; Kim, J.-H.; Mirkin, C. A. Science 2010, 310, 66. (b) Wiester, M. J.; Ulmann, P. A.; Mirkin, C. A. Angew. Chem., Int. Ed. 2011, 50, 114. (3) (a) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg. Chem. 1999, 48, 233. (b) Braunstein, P; Naud, F Angew. Chem., Int. Ed. 2001, 40, 680. (4) Persson, R.; Monari, M.; Gobetto, R.; Russo, A.; Aime, S.; Calhorda, M. J.; Nordlander, E. Organometallics 2001, 20, 4150. (5) Persson, R.; Stchedroff, M. J.; Uebersezig, B.; Gobetto, R.; Steed, J. W.; Prince, P. D.; Monari, M.; Nordlander, E. Organometallics 2010, 29, 2223. (6) Persson, R.; Stchedroff, M. J.; Gobetto, R.; Carrano, C. J.; Richmond, M. G.; Monari, M.; Nordlander, E. Eur. J. Inorg. Chem. 2012, submitted. (7) (a) Watson, W. H.; Wu, G.; Richmond, M. G. Organometallics 2005, 24, 5431. (b) Watson, W. H.; Wu, G.; Richmond, M. G. Organometallics 2006, 25, 903. (c) Hunt, S. W.; Yang, L.; Wang, X.; Richmond, M. G. J. Organomet. Chem. 2011, 696, 1432. (8) Zhang, X.; Kandala, S.; Yang, L.; Watson, W. H.; Wang, X.; Hrovat, D. A.; Borden, W. T.; Richmond, M. G. Organometallics 2011, 30, 1253. (9) Frisch, M. J. et al. Gaussian 09, Revision E.01; Gaussian, Inc., Wallingford, CT, 2009. (10) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (11) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (12) (a) In our calculations on the different osmium clusters reported here, numerous low-frequency vibrations (10−100 cm−1) were encountered in the minima and transition states. Within the harmonic oscillator approximation, these frequencies will serve as a source of error for the temperature-corrected free energy values. Accordingly, we have avoided using free energies in our discussions in the absence of corroborating experimental data. (b) Cramer, C. J. Essentials of Computational Chemistry, 2nd ed.; Wiley: New York, 2004. (c) Watson, L. A.; Eisenstein, O. J. Chem. Educ. 2002, 79, 1269. (13) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (14) Wiberg, K. B. Tetrahedron 1968, 24, 1083. (15) JIMP2, version 0.091, a free program for the visualization and manipulation of molecules: (a) Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972, 11, 768. (b) Manson, J.; Webster, C. E.; Hall, M. B. Texas A&M University, College Station, TX, 2006; http://www.chem.tamu.edu/ jimp2/index.html. (16) For related exchange sequences that promote the migration of carbonyl groups about a cluster polyhedron, see: (a) Alex, R. F.; Pomeroy, R. K. Organometallics 1987, 6, 2437. (b) Washington, J.; Takats, J. Organometallics 1990, 9, 925. (c) Cooke, J.; Takats, J. 6613

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