Comparison of the One-Electron Oxidations of CO-Bridged vs

Article pubs.acs.org/Organometallics

Comparison of the One-Electron Oxidations of CO-Bridged vs Unbridged Bimetallic Complexes: Electron-Transfer Chemistry of Os2Cp2(CO)4 and Os2Cp*2(μ-CO)2(CO)2 (Cp = η5‑C5H5, Cp* = η5‑C5Me5) Derek R. Laws,† R. Morris Bullock,*,‡ Richmond Lee,§ Kuo-Wei Huang,*,§ and William E. Geiger*,† †

Department of Chemistry, University of Vermont, Burlington, Vermont 05405 United States Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352 United States § KAUST Catalysis Center and Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: The one-electron oxidations of two dimers of half-sandwich osmium carbonyl complexes have been examined by electrochemistry, spectro-electrochemistry, and computational methods. The all-terminal carbonyl complex Os2Cp2(CO)4 (1, Cp = η5-C5H5) undergoes a reversible oneelectron anodic reaction at E1/2 = 0.41 V vs ferrocene in CH2Cl2/0.05 M [NBu4][B(C6F5)4], giving a rare example of a metal−metal bonded radical cation unsupported by bridging ligands. The IR spectrum of 1+ is consistent with an approximately 1:1 mixture of anti and gauche structures for the 33 e− radical cation in which it has retained all-terminal bonding of the CO ligands. Density functional theory (DFT) calculations, including orbital-occupancy-perturbed Mayer bond-order analyses, show that the highest-occupied molecular orbitals (HOMOs) of anti-1 and gauche-1 are metal−ligand delocalized. Removal of an electron from 1 has very little effect on the Os−Os bond order, accounting for the resistance of 1+ to heterolytic cleavage. The Os−Os bond distance is calculated to decrease by 0.10 Å and 0.06 Å as a consequence of one-electron oxidation of anti-1 and gauche-1, respectively. The CO-bridged complex Os2Cp*2(μ-CO)2(CO)2 (Cp* = η5-C5Me5), trans-2, undergoes a more facile oxidation, E1/2 = −0.11 V, giving a persistent radical cation shown by solution IR analysis to preserve its bridged-carbonyl structure. However, ESR analysis of frozen solutions of 2+ is interpreted in terms of the presence of two isomers, most likely anti-2+ and trans-2+, at low temperature. Calculations show that the HOMO of trans-2 is highly delocalized over the metal−ligand framework, with the bridging carbonyls accounting for about half of the orbital makeup. The Os−Os bond order again changes very little with removal of an electron, and the Os−Os bond length actually undergoes minor shortening. Calculations suggest that the second isomer of 2+ has the anti all-terminal CO structure.

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INTRODUCTION A small number of complexes are known in which simple piano-stool moieties, consisting only of carbonyl and cyclopentadienyl (Cp) or substituted Cp ligands, are linked by metal−metal bonds unsupported by bridging ligands (Scheme 1).1−5 Major themes in characterization of these complexes have included their stabilities relative to carbonyl-bridged isomers and their tendencies to undergo homolytic cleavage into the monomeric radicals from which they are intellectually derived.6−8 Of the Group 6 dimers, only the chromium system has a sufficiently weak metal−metal bond to allow controlled production of 17-electron radicals by thermally-driven shifts of the dimer/monomer equilibrium (Scheme 2).7 Analogous thermally induced access to Group 7 piano-stool radical chemistry was recently described for the dicationic rhenium system [Re2Cp2(CO)6]2+, which in spite of being isolectronic with the essentially undissociated9 W2Cp2(CO)6 dimer, readily © XXXX American Chemical Society

Scheme 1. Known Group 6 to Group 9 Dileptic Unbridged Piano-Stool Complexes

Special Issue: Organometallic Electrochemistry Received: December 18, 2013

A

dx.doi.org/10.1021/om401213y | Organometallics XXXX, XXX, XXX−XXX

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1, while undergoing an increase in contribution of the more polar gauche isomer in higher polarity solvents.14 The Cp* complex 2 has been found only in its trans-bridged form.14−16 However, the (unbridged) anti isomer of 2 was actually computed to have a slightly lower free energy compared to trans-2.14 The close-lying energies of these two bridged isomers become relevant in our discussion of the electron spin resonance (ESR) spectra obtained for 2+. The electrochemical literature of the Group 6 family, some of which goes back over 50 years, is concentrated on the bridged iron complex Fe2Cp2(μ-CO)2(CO)2. As summarized by Mann et al.,17 early work established that the reduction of this dimer cleaves the iron−iron bond, giving the 18-electron anion [FeCp(CO)2]− (eq 1).9a,18,19 In separate low-temperature studies, both Parker et al.20a and Murray et al.20b were able to detect, by cyclic voltammetry (CV), the radical anion [Fe2Cp2(μ-CO)2(CO)2]− prior to its cleavage.

Scheme 2. Dimer/Monomer Equilibrium Resulting From Thermal Homolysis of Cr2Cp2(CO)6

equilibrates with the monometallic radical cation [ReCp(CO)3]+.2 In addition to thermally or photochemically induced homolysis reactions,6−8 dimers may be subject to heterolytic electron-transfer-induced cleavage as well. The present paper explores this possibility for two complexes with osmium− osmium bonds, namely, the unbridged complex Os2Cp2(CO)4 (1) and the bridged complex Os2Cp*2(μ-CO)2(CO)2 (Cp* = η5-C5Me5, 2), and evaluates the results in the context of previous literature on the electrochemistry of Group 6 to Group 9 metal−metal bonded dimers.

Fe2Cp2 (μ‐CO)2 (CO)2 + 2e− → 2[FeCp(CO)2 ]−

(1)

Oxidation of the bridged iron dimer gives a reactive radical cation for which cleavage is observed in media containing either donor solvents (eq 2)21a,b or nucleophilic anions.21c Fe2Cp2 (μ‐CO)2 (CO)2 − 2e− + 2NCMe → 2[FeCp(NCMe)(CO)2 ]+

(2)

The anodically produced radical cations [Fe2(η5-C5R5)2(μCO)2(CO)2]+ (R = H, Me) and [Fe2(η5-C5Me4CF3)2(μCO)2(CO)2]+ were sufficiently stable in dichloromethane to be spectroscopically characterized by both IR and ESR measurements.17,21c,22 Electrochemical information on the ruthenium dimers is more limited. Metal−metal cleavage occurs in the reduction of Ru2Cp2(CO)4.20 One-electron oxidations have been reported for Ru2Cp2(CO)4 and some of its Cp-derivatized analogues.23 The cathodic reduction of 1 has been reported to be irreversible,15 most likely forming the monoanion [OsCp(CO)2]−, in concert with its iron and ruthenium counterparts as shown in eq 1. Whereas we will make some comments on the reductive cleavage of 1, the present paper concentrates on the anodic oxidations of the osmium complexes 1and 2 and on the structural, bonding, and chemical properties of their previously unreported radical cations.

Drawings of the four possible systematic isomers of this series are shown specifically for the Cp complex in Scheme 3. Scheme 3. Idealized Structures of the Four Possible Isomers of Os2Cp2(CO)4

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EXPERIMENTAL SECTION

Materials and Instrumentation. Except where noted, Schlenk or drybox procedures under nitrogen were used throughout this study. Electrochemical measurements were carried out under nitrogen in a Vacuum Atmospheres drybox using a Princeton Applied Research model 273A potentiostat. Os2Cp2(CO)4 (1) was prepared as previously described,14 and Os2Cp*2(μ-CO)2(CO)2 (Cp* = η5C5Me5, 2) was prepared by a modification of the method given in that paper. Thus, a solution of OsCp*(CO)2H (142 mg, 0.371 mmol) in CH2Cl2 (5 mL), added to a solution of [Ph3C][PF6] (72 mg, 0.21 mmol) at room temperature, gave a yellow solution. An IR spectrum verified the formation of {(μ-H)[OsCp*(CO)2]2}+ (νCO = 2033, 2010, 1975 cm−1). Addition of DBU (1,8-diazabicycloundec-7-ene; 50 mL, 0.3 mmol) gave a lighter yellow solution. Hexane (20 mL) was added, and the solution was evaporated to about 5 mL volume, giving a yellow precipitate that was washed with hexane (5 mL), isopropanol (3 mL), and CH3CN (2 mL) to give 2 (70 mg, 50% yield). IR: (CH2Cl2) 1909, 1708 cm−1, in agreement with spectral properties previously reported.14 [NBu4][B(C6F5)4] was prepared by metathesis of [NBu4]Br with K[B(C6F5)4] (Boulder Scientific Co.) in MeOH,

The rich literature relating to the relative contributions and interconversions of the iron and ruthenium congeners has been nicely summarized by Bitterwolf.10 In general, the carbonyl bridged structures are thermodynamically favored for both the Fe and Ru systems, but significant contributions from the unbridged (all-terminal) isomers may be present, depending on the medium as well as any substituents on the cyclopentadienyl rings. Whereas all-terminal isomers contribute less than 1% to the solution makeup of Fe2Cp2(CO)4,10,11 the ruthenium analogue Ru2Cp2(CO)4 displays more balanced proportions of the bridged and unbridged structures, with as many as three of the four possible isomers being observed by temperaturedependent infrared (IR) spectroscopy.12,13 In contrast, equilibria between bridged and unbridged isomers have not been observed for the osmium analogues. Thus, the Cp complex 1 retains its all-terminal structures, anti-1 and gaucheB


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Table 1. Carbonyl Region IR Bands for Os and Fe Dimers in Two Different Redox States and under Different Medium Conditions cmpd

νCO (cm−1)

Os2Cp2(CO)4, 1 Os2Cp2(CO)4, 1 Os2Cp2(CO)4, 1 [Os2Cp2(CO)4]+, 1+ Os2Cp*2(μ-CO)2(CO)2, 2 [Os2Cp*2(μ-CO)2(CO)2]+, 2+

1969, 2002, 2002, 2075, 1909, 1969,

1932 1959, 1921 1959, 1921 2015, 1994 1709 1820

trans-Fe2Cp2(μ-CO)2(CO)2 trans-[Fe2Cp2(μ-CO)2(CO)2]+

1955, 1773 2023, 1934

trans-Fe2Cp*2(μ-CO)2(CO)2 trans-[Fe2Cp*2(μ-CO)2(CO)2]+

1922, 1747 1987, 1884

νCO shifts from neutral (cm−1)

medium

average +70 (see text for assignments) + 60 (terminal) + 111 (bridging) + 78 (terminal) + 161 (bridging) + 65 (terminal) + 137 (bridging)

recrystallized three times from CH2Cl2/OEt2, and dried at 80 °C under vacuum for 24 h.24 CH2Cl2 was dried over calcium hydride and collected by vacuum transfer prior to use. IR spectroelectrochemistry was carried out either under argon using a fiber-optic “dip” probe25 (Remspec, Inc.) connected to a Mattson FT-IR or by removing samples from electrolysis solutions. ESR spectra were recorded on a Bruker X-band instrument. Electrochemistry. All potentials in this paper are referred to the ferrocene/ferrocenium couple26 employing the in situ method of Gagne.26c The experimental reference was a Ag/AgCl electrode in the electrolyte solution, separated from the working compartment by a fine porosity frit. Polished glassy carbon disks of 1 mm or 2 mm diameter (Bioanalytical Systems) were used as the working electrodes. Other experimental details are largely identical to those previously described.27 Standard voltammetric diagnostics28,29 were used to assign diffusion-control, chemical reversibility, and electrochemical reversibility to the redox reactions. Computational Details. Density functional theory calculations were performed on gas-phase osmium complexes employing the Gaussian 09 suite of programs30 at ωB97X long-range corrected hybrid density functional.31 LANL2DZ ECP32 was applied on Os and all electron Pople basis set 6-31G(d)33,34 for the rest of the atoms. Frequency calculations were used to find minima and to determine zero-point energies (ZPE).

ref

hexane THF CH2Cl2/[NBu4][B(C6F5)4] CH2Cl2/[NBu4][B(C6F5)4] CH2Cl2/[NBu4][B(C6F5)4] CH2Cl2/[NBu4][B(C6F5)4]

14 14 this this this this

CH2Cl2/[NBu4][PF6] CH2Cl2/[NBu4][PF6]

17 17

CH2Cl2/[NBu4][PF6] CH2Cl2/[NBu4][PF6]

17 17

work work work work

observation of separate waves for two species interconverting at the rate expected for the isomers of 1. The fact that, as shown below, only a single anodic peak attributable to one-electron oxidation is seen in CV scans of 1 mostly likely arises from interconversion between the anti and gauche isomers that is fast on the CV time scale.36 Although the unbridged anti structure was calculated to be the most stable form of complex 2 in the gas phase,14 only the trans-bridged isomer has been observed, either in solution or in the solid state.14−16 IR spectra in dichloromethane/[NBu4][B(C6F5)4] electrolyte also do not detect the presence of an allterminal isomer (Table 1). Thus, the anodic electrochemistry of 2 is assigned to oxidation of the trans bridged isomer. As will be shown, although room-temperature IR spectra of the radical cation 2+ also identify only a bridging-CO structure, ESR spectra of 2+ suggest the presence of both the trans-bridged and anti-unbridged isomers in frozen solutions. Electrochemistry of Os2Cp2(CO)4 (1). Reduction of 1. Scanning solutions of Os2Cp2(CO)4 (1) in CH2Cl2/0.05 M [NBu4][B(C6F5)4] negatively from 0 V produced an irreversible cathodic wave of apparent two-electron height at Epc = −2.91 V (scan rate 0.1 V s−1), just prior to solvent breakdown (Figure S1 in Supporting Information). Potential reversal after this reduction showed a single anodic product wave at Epa = −1.19 V. On the basis of a number of precedents for metal− metal bonded dimers,9a,17−21 the cathodic wave is attributed to a two-electron ECE process (two electron-transfer reactions sandwiched around a chemical reaction), and the anodic product wave is assigned to oxidation of the reductively generated 18-electron anion [OsCp(CO)2]−. This process is delineated in Scheme 4, where uptake of one electron by 1 gives the unstable radical anion [Os2Cp2(CO)4]−, which undergoes heterolytic cleavage to give the 18-electron anion [OsCp(CO)2]− and the 17-electron radical OsCp(CO)2. The latter is reduced to its monoanion at this potential, completing the two-electron reduction of 1. The reductively induced cleavage of 1 is consistent with the fact that its lowest

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RESULTS AND DISCUSSION Favored Isomers in Electrolyte Media. Calculations have shown that the anti and gauche isomers of neutral 1 lie sufficiently close in energy in the gas phase to account for their IR-detected coexistence in solution.14 The fact that only two bands are predicted for the anti isomer, compared to three for the gauche isomer,35 simplifies the isomeric analysis, a key observable being the highest energy band near 2000 cm−1, which is compatible only with the gauche isomer.14 The relative intensity of this band compared to the other νCO bands was used to show that the gauche to anti ratio increased from about 1:50 in hexane to 1:1 in THF.14 In the present work, a similar IR analysis of 1 in dichloromethane/[NBu4][B(C6F5)4] indicates a slight excess of the anti over the gauche isomer under our conditions prior to application of electrochemical potentials. It is important to keep in mind the much different time scales on which the IR and electrochemical measurements respond. Interconversion between anti and gauche isomers is slow on the IR time scale (∼10−13 s), allowing spectroscopic identification of the separate isomers. In contrast, the comparatively long time scale of cyclic voltammetry, CV (∼10−1 s for a 1 V scan at 10 V s−1), dictates against the

Scheme 4. Overall Electron-Transfer Mechanism for Oxidation and Reduction of 1

C


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Figure 1. Positive-going cyclic voltammetry scans of anodic oxidations of 1 mM Os2Cp2(CO)4, 1, in CH2Cl2/0.05 M [NBu4][B(C6F5)4], scan rate 0.1 V s−1, 2 mm glassy carbon, room temperature. The dashed line shows a scan to more positive potentials.

Figure 2. Cyclic voltammogram following bulk anodic electrolysis at Eappl = 0.7 V of 2.7 mM Os2Cp2(CO)4, 1, in CH2Cl2/0.05 M [NBu4][B(C6F5)4], scan rate 0.1 V s−1, 2 mm glassy carbon, 243 K.

of 243 K. As shown in Figure 2, two follow-up product waves were seen at Epc values of −1.05 V and −1.64 V. Although these products are unassigned, IR spectra taken after electrolysis established that they (or it) still contain(s) only terminal CO ligands. Figure 3 shows a comparison of the carbonyl-region IR spectra before and after anodic electrolysis of 1.38 Three bands of 1 at 2002, 1959, and 1921 cm−1 (dashed-line) were replaced by six significant bands in the electrolysis solution. With the goal of assigning the spectral features arising from 1+, we remeasured the spectrum after exposing the solution to ambient air, a condition that often results in decomposition of reactive organometallic radical cations. On the basis that only the bands at 2075, 2015, and 1994 cm−1 disappeared (Figure S2 in Supporting Information), we assign those bands to the radical cation 1+. The highest energy band at 2075 cm−1 clearly arises from the gauche isomer, since assigning it to the anti isomer would require a shift of + 116 cm−1 for νsym from the value of 1959 cm−1 in the neutral anti isomer, an increase we consider to be too large. One-electron oxidations of both bridged and unbridged dinuclear complexes display much smaller increases in terminal CO frequencies, shifts of + 45 to + 65 cm−1 being

unoccupied molecular orbital (LUMO) is predominantly Os− Os antibonding in character. Oxidation of 1. Cyclic voltammograms of 1 obtained by scanning positive from 0 V showed two anodic waves (Figure 1), the first of which (E1/2 = 0.41 V) is a diffusion-controlled reversible one-electron process. Fitting of the voltammetric diagnostics to Nernstian behavior29 established that any chemically coupled equilibria, such as the gauche/anti isomerizations mentioned above, have to be fast and reversible on the CV time scale. The second, more positive, wave (Epa = 1.15 V, scan rate 0.1 V s−1) has the general appearance of a fully irreversible one-electron process.37 The overall voltammetry of Os2Cp2(CO)4 is therefore consistent with the electron-transfer series of Scheme 4. Radical Cation [Os2Cp2(CO)4]+, (1+). Bulk anodic electrolysis of 1 at an Eappl of 0.7 V changed the solution from yellow to purple and passed exactly 1.0 F, confirming a one-electron oxidation. CVs and linear scan voltammograms after electrolysis showed, however, that the radical cation 1+ had undergone significant decomposition, with only about 25% of the expected concentration being observed even at the reduced temperature D


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gauche/anti‐Os 2Cp2 (CO)4 ⇌ [gauche/anti‐Os2Cp2 (CO)4 ]+ + e−

E1/2 = 0.41 V (3) +

A comment on the thermal stability of 1 is in order. Although this radical has a lifetime of only between 10 s and a few minutes at 243 K,42 its mere detection is noteworthy. Radical cations of unsupported metal−metal bonded organometallic systems are rare,43 as will be reviewed in the Discussion. The relative longevity of 1+ compared to 1− may be attributed to the fact that the Os−Os bond order is essentially unchanged when an electron is removed. The structure and bonding of 1+ and 1− are discussed in the calculation section of this paper. Improved Preparation of Os2Cp*2(μ-CO)2(CO)2 (2). The synthesis of Os2Cp*2(μ-CO)2(CO)2 was previously reported based on hydrogen atom abstraction from the osmium hydride OsCp*(CO)2H using 2,2′-azo-bis-isobutyrylnitrile (AIBN).14 Subsequent experiments have shown that the dimer 2 is reproducibly generated by this route but that impurities formed in the reaction can lead to difficulties in obtaining pure product. We developed a new preparation shown in Scheme 5 that leads

Figure 3. IR spectra at room temperature of solutions of 2.7 mM Os2Cp2(CO)4, 1, in CH2Cl2/0.05 M [NBu4][B(C6F5)4] before electrolysis (dashed line) and after exhaustive electrolysis at Eappl = 0.7 V. The arrows designate the three bands of the neutral complex. The bands are 2075 cm−1, 2015 cm−1, and 1994 cm−1 are assigned to 1+. See the text for details.

typical.22,39−42 Assigning the 2075 cm−1 band to a gauche isomer of 1+, a shift of + 73 cm−1 from the highest energy CO band of gauche-1, is more consistent with literature precedents. Assignment of the two remaining strong absorptions at 2015 cm−1 and 1994 cm−1 is aided by the DFT-calculated IR bands of gauche-1+ and anti-1+, which are shown schematically in Figure 4. It is predicted that, within limits of resolution, only

Scheme 5. Improved Method for the Preparation of 2

to higher yields of 2. Hydride transfer from OsCp*(CO)2H to Ph3C+44 leads to an Os cation that then reacts with the second equivalent of OsCp*(CO)2H, producing the cationic hydridebridged complex {(μ-H)[OsCp*(CO)2]2}+. Deprotonation of this complex by the hindered organic base DBU (1,8diazabicycloundec-7-ene) produces 2. Anodic Electrochemistry of Os2Cp*2(μ-CO)2(CO)2 (2). A cathodic reduction wave was not observed for Os2Cp*2(μCO)2(CO)2, 2, out to the background limit of −2.5 V in CH2Cl2/0.05 M [NBu4][B(C5F5)4]. A facile oxidation process was observed, however, at E1/2 = −0.11 V having the general characteristics of a quasi-Nernstian one-electron process. An irreversible second oxidation was observed at Epa = 0.61 V (scan rate 0.1 V s−1) (Figure 5). Bulk anodic electrolysis at an Eappl of 0.3 V passed between 0.90 and 0.95 F as the originally yellow solution became dark green, confirming the one-electron oxidation of 2 to 2+ (eq 4). Reverse electrolysis at Eappl = −0.5 V regenerated neutral 2, apparently without decomposition.

Figure 4. Calculated terminal carbonyl-region IR spectra (in cm−1, unscaled) of anti- and gauche-1+.

two carbonyl bands will be observed for either isomer (there is a slight shoulder on the lower energy band of gauche-1+ due to a less intense asymmetric CO vibration), implying that the electrolysis solution must contain significant amounts of both. In fact, the calculated IR spectrum for a 1:1 mixture of gauche1+ and anti-1+ (Figure 4) is a reasonable reproduction of the three radical cation peaks in the experimental spectrum (calculated unscaled wavenumbers are in all cases higher than the experimental values. See comparative numbers in Table S1 of the Supporting Information). On this basis, we conclude that one-electron oxidation of 1 does not result in a dramatic change of its isomeric makeup (see eq 3). Attempts to record ESR spectra of 1+ were unsuccessful.

Os2Cp*2 (μ‐CO)2 (CO)2 ⇌ [Os 2Cp*2 (μ‐CO)2 (CO)2 ]+ + e−

E1/2 = −0.11 V

(4)

The simplicity of the redox process facilitated comparison of IR spectra going from 2 to 2+ (Figure 6), wherein shifts of + 60 E


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Figure 5. Positive-going cyclic voltammetry scans of anodic oxidations of 1 mM Os2Cp*2(μ-CO)2(CO)2, 2, in CH2Cl2/0.05 M [NBu4][B(C6F5)4], scan rate 0.1 V s−1, 2 mm glassy carbon, room temperature. The dash line shows a second scan to more positive potentials.

Figure 6. IR spectra at room temperature of solutions of 3 mM Os2Cp*2(μ-CO)2(CO)2, 2, in CH2Cl2/0.05 M [NBu4][B(C6F5)4] before electrolysis (dashed line) and after exhaustive electrolysis at Eappl = 0.3 V.

cm−1 for the terminal CO bands and + 111 cm−1 for the bridging carbonyl bands were observed (Table 1). The IR spectrum of 2+ shows definitively that the radical cation retains its bridging CO ligands. The fact that the IR shift in the bridging CO frequency is significantly greater than that of the terminal CO is consistent with earlier observations on iron analogues (Table 1).17,22 The absence of a pair of bands in the terminal-CO region is evidence that no detectable amount of an all-terminal isomer of 2+ is present under these conditions. Solutions of 2+ were ESR-active from room temperature to 153 K (Figure S3 in the Supporting Information). A single line was observed in fluid solutions (giso = 2.162) and a rhombic pattern was observed for frozen solutions (g1= 2.406, g2 = 2.199, g3 = 1.879; ⟨g⟩ = 2.161). The resolution of osmium hyperfine satellites was indicated only near the high field (g3) component of the sharpest line, recorded at 77 K. An expanded view of this spectral region is shown in Figure 7. At first glance, the four resolved satellite lines appear to be consistent with hyperfine coupling to a single Os atom (189Os, I = 3/2, 16.1% abundant). However, both the irregular spacings of the lines and their intensities relative to the major (unsplit) line argue against this interpretation.45 Rather, the satellite pattern

suggests the presence of hyperfine splittings from two different Os nuclei, here termed Os(A) and Os(B), with A3(A) = 81 G and A3(B) = 49 G, with four of the expected eight satellites being too highly overlapped to be resolved (see Figure 7). This scenario can be accounted for either in terms of (i) a single isomer of 2+ with two significantly different metal centers or (ii) two isomers of 2+ in roughly equal populations. As will be shown below, only the mixed-isomer model (ii), almost certainly involving the trans- and anti-isomers, is consistent with calculations on the radical cation 2+. Computational Analysis. To evaluate the effect of oneelectron oxidation and reduction on the stability of the Os complexes, the molecular orbitals (MOs) and the bond orders in the optimized structures of four species were analyzed: the gauche and anti-isomers of Os2Cp2(CO)4 (1) and the trans- and anti-isomers of Os2Cp*2(μ-CO)2(CO)2 (2). Because the Cp and Cp* groups are significantly larger than the CO ligands, all structures deviate from idealized anti/gauche and cis/trans geometries. For example, in anti-1, the (O)C−Os−Os angle is 76.6° rather than 109.5°, in gauche-1 the Cp−Os−Os−Cp torsional angle is 78.1°, rather than 60°, and in trans-1, the terminal (O)C−Os−Os angle is 92.5° rather than 120°. All the 189

F


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Figure 7. Enlarged figure of g3 region of 77 K ESR spectrum of 2+, showing the positions of the two quartets of hyperfine splittings for Os(A) and Os(B). Inserted is the overall rhombic spectrum at this temperature. The sample was obtained from the anodically oxidized solution of 3 mM Os2Cp*2(μ-CO)2(CO)2, 2, in CH2Cl2/0.05 M [NBu4][B(C6F5)4].

Table 2. Orbital-Occupancy-Perturbed Mayer Bond Orders of Selected Bonds in anti- and gauche-1a MO anti-1 Os−Os Os−COb C−Oc gauche-1 Os−Os Os−COd Os−COe C−Od C−Oe

reference

LUMO

HOMO

HOMO-1

HOMO-2

HOMO-3

HOMO-4

HOMO-5

0.68 0.95 2.10

0.57 (A) 0.95 2.09

0.90 (A) 0.94 2.09

0.72 0.88 2.03

0.73 0.94 2.01

0.82 (A) 0.91 2.08

0.80 (A) 0.92 2.05

0.86 (A) 0.93 2.04

0.69 0.95 0.94 2.10 2.13

0.57 (A) 0.95 0.94 2.09 2.12

0.68 0.88 0.91 2.05 2.06

0.95 (A) 0.96 0.86 2.00 2.07

0.81 (A) 0.92 0.93 2.01 2.04

0.85 (A) 0.89 0.94 2.04 2.04

0.76 0.93 0.90 2.02 2.05

0.92 (A) 0.91 0.92 2.03 2.05

a

Mayer bond orders after addition of an electron to alpha-spin unoccupied MO or after removal of an electron from alpha-spin occupied MO. Orbitals which are Os−Os antibonding are marked by the symbol A bAverage of four Os−CO bonds. cAverage of four C−O bonds. dAverage of two gauche to Cp groups. eAverage of two anti to Cp groups.

calculated structures can be visualized from the xyz file information provided in the Supporting Information. Owing to the importance of possible redox-induced changes in the Os−Os interaction, the contributions of individual MOs to the Os−Os bond were studied by orbital-occupancy-perturbed (OOP) Mayer bond-order calculations using the AOMix program.46 In this method, an electron is either added to an unoccupied MO (or removed from an occupied MO) of the original MO set, giving OOP states of the charged species for comparison with the reference state, which in this case is that of the neutral complex. The comparative OOP values give information about the bonding or antibonding character of an individual MO for a given bond. For example, a LUMO with an OOP value lower than that of the reference indicates that this orbital exhibits a degree of antibonding character (A) for the bond, favoring a decrease in the given bond order in the anion. The interpretation of changes in OOP bond-order values for occupied MOs is inverted, with a lower OOP value in that case indicating that the MO from which the electron was removed has bonding character for the given bond. In the

occupied MO case, a higher OOP value suggests antibonding character for a bond in that orbital. The OOP Mayer bond order values for the two calculated isomers of 1 are collected in Table 2. Os2Cp2(CO)4 (gauche and anti-1). The LUMOs of gauche-1 (Figure 8) and anti-1 are very similar, and the makeup of these orbitals is essentially as expected, the most important aspect being the significant Os−Os σ* character. The calculated lowering of the Os−Os OOP Mayer bond-value from 0.69 (gauche) or 0.68 (anti) to 0.57 in the LUMO of 1 (Table 2) suggests that the metal−metal bond will be weakened upon addition of an electron to 1. The Os−Os bond length is calculated to increase from 2.86 Å to 3.32 Å (Table 3), thus accounting for the fragility of the 35 e− radical anion 1−. The covalent radius of Os is 1.44 Å.47 More complex results are found when considering the filled orbitals of the two isomers of 1. Surprisingly, no Os−Os bonding character was identified in the OMOs from highest occupied molecular orbital (HOMO) to HOMO-19 (selected results of HOMO to HOMO-5 shown in Table 2), suggesting G


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that the Os−Os bonding interaction for 1 is highly diversified, in the sense of arising from partial contributions of several OMOs. Thus, no significant decrease in the Os−Os bond order is expected in the depopulation of an individual OMO. The HOMOs (Figure 9 for the gauche isomer) are not dominated

Figure 8. LUMOs of anti- and gauche-1.

Table 3. Bond Distances and Mayer Bond Orders (In Parentheses) Of Selected Bonds in anti- and gauche-(1−, 1, and 1+) Os−Os −

anti-1 anti-1 anti-1+ gauche-1−

3.315 2.855 2.755 3.320

gauche-1

2.856 (0.69)

gauche-1+

2.797 (0.57)

(0.38) (0.68) (0.68) (0.39)

Os−CO 1.854 1.869 1.899 1.851 1.860 1.868 1.874 1.903 1.904

Figure 9. HOMOs of anti- and gauche-1.

by simple Os−Os dz2−σ interactions. In fact, in the case of anti1, elimination of an electron from the HOMO removes some Os−Os antibonding character, and in the case of gauche-1, the OOP bond order value (Table 3) barely changes between that calculated for the HOMO (0.68) and the neutral reference state (0.69). The calculations show that the oxidation of 1 actually shortens the Os−Os bond length to 2.755 Å and 2.797 Å, for anti-1+ and gauche-1+, respectively, influenced in part by the smaller atomic radii of the more positive metal atoms. Figure 10 gives a picture of important bond distances in the redox family of gauche- and anti-1. Relevant to the electrochemical results, the calculations suggest that the slow decomposition of 1+ on the bulk electrolysis time scale does not arise from weakening of the Os−Os bond in the radical cation. However, as expected for metal carbonyl radical cations,2b,48,49 removal of an electron

C−O a

(1.12) (0.95)a (0.82)a (1.12)c (1.09)d (0.95)c (0.94)d (0.80)c (0.82)d

1.172 1.160 1.149 1.173 1.168 1.160 1.156 1.145 1.150

b

(1.99) (2.10)b (2.22)b (1.98)c (2.03)d (2.10)c (2.13)d (2.25)c (2.20)d

a

Average of four Os−CO bonds. bAverage of four C−O bonds. Average of two gauche to Cp groups. dAverage of two anti to Cp groups. c

H


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Figure 10. Stationary point structures of gauche- and anti-1, 1−, and 1+. Selected bond lengths are in angstroms and Mayer bond orders are in italic.

Figure 11. HOMOs of trans-2 (left) and anti-2 (right). The former contains a total of 22% osmium character, mixing Os dxz, dyz, px, and py. The latter totals 42% osmium character, split almost equally between Os p orbitals (18% px, 4% pz) and Os d orbitals (14%, dz2, dxz, dx2−y2), along with 6% Os s.

Table 4. Orbital-Occupancy-Perturbed Mayer Bond Orders of the Os−Os Bond in trans-2a MO

reference

LUMO

HOMO

HOMO-1

HOMO-2

HOMO-3

HOMO-4

HOMO-5

Os−Os Os−μCO μC−O Os−CO C−O

0.39 0.74 1.86 0.97 2.04

0.38 0.75 1.81 0.97 2.04

0.37 0.69 1.96 0.95 2.06

0.55 (A) 0.74 1.86 0.94 2.06

0.48 0.76 1.86 0.89 2.09

0.57 (A) 0.69 1.92 0.97 2.04

0.47 0.73 1.86 0.94 2.06

0.56 (A) 0.72 1.88 0.92 2.06

a

Mayer bond orders after addition of an electron to an alpha-spin unoccupied MO or after removal of an electron from an alpha-spin occupied MO. Orbitals which are Os−Os antibonding are marked by the symbol A.

removal of an electron. This value is consistent with the experimental value of + 67 cm−1 (Figure 3) observed for the oxidation, strengthening the IR assignments for the 33-electron system 1+ as being evidence for the presence of both the gauche and anti structures. Os2Cp*2(μ-CO)2(CO)2 (2). In the case of neutral 2, the experimentally observed isomer has the trans structure. However, since the energies of the trans (bridged) and anti (unbridged) structures are close, 14 both isomers were calculated, including an OOP Mayer bond order analysis. Owing to the fact that no reduction of 2 was observed

from 1 does result in weakening of the Os−C(O) bonds, the average Os−C(O) bond length increasing by 0.033 Å. The average C−O bond length decreases by 0.01 Å, consistent with a decrease in metal−CO back bonding. These changes make the metal centers more susceptible to nucleophilic attack with loss of carbon monoxide. However, since the radical decomposition products are unknown, further discussion of possible follow-up reactions would be speculative. We also note an important finding in the calculations of carbonyl IR stretching frequencies, where an average IR shift of + 61 cm−1 is obtained in going from gauche-1 to gauche-1+, reflecting the decreased back-bonding that accompanies I


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Table 5. Bond Distances and Mayer Bond Orders (In Parentheses) of Selected Bonds in trans-(2−, 2, and 2+) ‑

trans-2 trans-2 trans-2+ a

Os−Os

Os−μCOa

μC−Ob

Os−COc

C−Od

2.832 (0.37) 2.778 (0.39) 2.761 (0.37)

1.988 (0.79) 2.055 (0.74) 2.094 (0.64)

1.205 (1.78) 1.194 (1.86) 1.173 (2.02)

1.857 (1.10) 1.864 (0.97) 1.890 (0.85)

1.175 (1.95) 1.164 (2.04) 1.153 (2.14)

Average of four Os-μCO bonds. bAverage of two μC−O bonds. cAverage of two Os−CO bonds. dAverage of two terminal C−O bonds.

Table 6. Orbital-Occupancy-Perturbed Mayer Bond Orders of Selected Bonds in anti-2a MO

reference

LUMO

HOMO

HOMO-1

HOMO-2

HOMO-3

HOMO-4

HOMO-5

Os−Os Os−COb C−Oc

0.68 0.91 2.04

0.60 (A) 0.90 2.02

0.49 (B) 0.89 2.06

0.84 (A) 0.89 2.05

0.97 (A) 0.87 2.05

0.85 (A) 0.87 2.06

0.74 0.91 2.05

0.76 0.87 2.06

a

Mayer bond orders after addition of an electron to alpha-spin unoccupied MO or after removal of an electron from alpha-spin occupied MO. Orbitals which are Os−Os antibonding are marked by the symbol A, those which are bonding by B. bAverage of four Os−CO bonds. cAverage of four C−O bonds.

experimentally, our discussion focuses on the filled orbitals of 2.50 Considering first the experimentally verified trans-2, its HOMO (Figure 11, left side) is highly diversified, having a major contribution from the bridging carbonyl ligands, with only an 11% contribution from each of the osmium atoms. The singly-occupied molecular orbital (SOMO) computed for 33 e− trans-2+ has even less metal character, with the bridging carbonyls accounting for about half the spin density in the radical cation. The calculated spin densities for trans-2+ are 53% in (μ-CO)2, 24% in Cp*2, 18% in Os2, and 5% in the two terminal CO ligands. See Table S1 in the Supporting Information for an atom-by-atom listing. Bond-Order Analysis of trans-2. The bond order analysis in Table 4 indicates that removal of an electron from the HOMO results in only a slight lowering of the Os−Os bonding interaction, with a slight decrease actually predicted for the Os−Os distance in trans-2+ (Table 5). The expected consequences of reduced back-bonding on the bond lengths and IR frequencies of the bridging and terminal carbonyl groups are also manifest. Thus, there are overall shortenings of both the μ-C−O and terminal C−O bond lengths from 1.194 Å to 1.173 Å and 1.164 Å to 1.153 Å, respectively. Furthermore, the experimentally measured shifts in νCO are matched well by the unscaled calculated values: terminal CO, calculated + 59 cm−1, experimental + 60 cm−1; bridging CO, calculated + 121 cm−1, experimental + 111 cm−1. It is also worth noting that there is a weakening of the bond order between the metal and the bridging CO ligand in the radical cation (0.74 to 0.64, Table 5). Owing to the ambiguity of the isomeric identification at very low temperatures (see above), an orbital description of the energetically close-lying all-terminal CO structure, anti-2, must be considered.51 For this isomer, the HOMO (Figure 11, right side) has significant Os−Os bonding character (Table 6), although over half of the orbital is still found away from the metals. Interestingly, even though removal of an electron from the HOMO of anti-2 decreases the Os−Os bond order, the Os−Os bond distance actually decreases in going from 34 e− anti-2 to 33 e− anti-2+ (Table 7). Certainly, the smaller atomic radii of the more positively charged osmium atoms in 2+ become a factor, but similar nonintuitive relationships between bond orders and bond distances have been noted in other transition metal systems.52,54 It is important to note that the present complex retains a substantial Os−Os bond order (0.52)

Table 7. Bond Distances and Mayer Bond Orders (In Parentheses) of Selected Bonds in anti-(2−, 2, and 2+) ‑

anti-2 anti-2 anti-2+ a

Os−Os

Os−COa

C−Ob

3.549 (0.33) 2.928 (0.68) 2.857 (0.52)

1.859 (1.04) 1.866 (0.91) 1.894 (0.80)

1.175 (1.94) 1.164 (2.04) 1.155 (2.13)

Average of four Os−CO bonds. bAverage of four C−O bonds.

in the radical cation, suggesting reasonable kinetic stability for anti-2+. Two particularly important aspects of the calculations on anti-2+ are that (a) it is predicted to have about the same free energy as trans-2+ (Table 8) and (b) the two osmium atoms are electronically equivalent, each having a spin density of about 0.4 e− (Supporting Information, Table S2). Taken together, these data favor interpretation (ii) for the low-temperature ESR spectra of 2+ (see above), specifically the presence of two radical isomers at 77 K. The relative free energy ordering in Table 8 suggest that trans-2+ and anti-2+ are the favored lowtemperature isomers.51

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DISCUSSION The chemically irreversible reduction of 1 is simply another example of a dinuclear system that does not give a persistent radical anion, short-lived 35 e− [Fe2Cp2(CO)4]− being a rare example.21b The generally enhanced tendency toward metal− metal cleavage reactions of 35 e− and 33 e− radicals is traditionally rationalized by a simple molecular orbital model55 in which the HOMO and LUMO orbitals of the 34 e− precursors have bonding and antibonding metal−metal character, respectively (Figure 12). The more unusual species found in the present study is 33 e− 1+, which is an uncommon example of a verified radical cation derived from a metal−metal bonded system unsupported by bridging ligands. The few predecessors of this type of which we are aware are [Co2Cp2(CO)4]+, 3,3 and [M2Cp2(CO)4L2]+, 4, M = Mo; L = pyridine4 and M = Mo, W; L = PMe3.5

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Table 8. Relative Stability of Different Conformers of 1 and 2a

a

charge

anti-1

gauche-1

trans-1

cis-1

anti-2

gauche-2

trans-2

cis-2

0 1+ 1−

0 0 0

3.1 4.8 1.8

14.6 28.3 12.7

14.7 8.2 15.8

0 0 0

5.4 3.0 5.0

3.8 0.0 7.5

8.4 6.8 14.3

Free energy in kcal/mol, relative to the anti conformers, in the gas phase at wB97X/LANL2DZ/6-31G*.

In terms of the broader literature of iron-group complexes of the type M2(CO)4(η5-C5R5)2, the present osmium results add to the rich literature involving the various isomers possible for these complexes. It appears, however, to be the first example in which the redox chemistry of all-terminal vs CO-bridged isomers, differing only in R, have been compared. Calculation and experiment agree that at least three of the four possible idealized structures shown in Scheme 3 are close in energy in the 34 e− and 33 e− versions of 1 and 2, suggesting Scheme 6 as a model for the interplay between structure and redox state for the diosmium system.

Figure 12. Simple MO model of metal−metal bonded dimer based on metal-based SOMOs of radical monomers.

Scheme 6. Proposed Structures for 1 and 2 in Neutral and Oxidized Formsa

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ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

Three figures giving voltammograms, IR, and ESR spectra; Cartesian coordinates of calculated structures; and summary of computational results (31 pages total). This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Authors

a

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

In 2 there is evidence for significant redox-induced isomerization.

Notes

The authors declare no competing financial interest.

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Detailed computations such as those reported52,53 for [Co2Cp2(CO)2]0 and [(μ-H)M2Cp2(CO)2]+ can reveal a more nuanced picture of the relationship between metal− metal bonding, electron count, and charge in unsupported dimetallic complexes. Such is the case for 1 and 1+. The LUMOs of gauche-1 and anti-1 (Figure 8), while only 54% diosmium in character, are mainly metal−metal antibonding, resulting in significant weakening of the Os−Os bond when it is half-occupied in the radical anion, in concert with the simple metal-localized MO model. However, the HOMOs (Figure 9) are more delocalized over the metals (33% on each Os atom) and ligands. Surprisingly, removal of an electron from the HOMO is calculated to actually decrease the metal−metal bond length in 1+ and have very little effect on the Os−Os bond order. This result accounts conceptually for the fact that 1+ does not undergo cleavage on the CV time scale and points to the likelihood that the follow-up reaction of 1+ involves its electrophilicity rather than a reduced Os−Os bond strength. The CO-bridged complex trans-2 presents two contrasts to the all-terminal CO complex 1: the large negative shift of −520 mV in the oxidative E1/2 of 2, and the significantly longer lifetime of the 33 e− radical 2+. Some negative shift in E1/2 is due to the increased donor abilty of Cp* compared to Cp, but this could account for no more than half of that measured.56 In fact, the redox orbitals of 1 and 2 are significantly different, the latter being much more ligand-based with only 11% of the orbital located on each Os atom. It appears that trans-2+ is longer-lived than 1+ owing to its positive charge being more highly distributed over its molecular framework.

ACKNOWLEDGMENTS D.R.L. and W.E.G. acknowledge the support of the National Science Foundation under Grant CHE-0808909. K.-W.H. acknowledges financial support from KAUST. R.M.B. thanks the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences for support. Pacific Northwest National Laboratory is a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy. We thank Dr. S. I. Gorelsky for the discussion on the OOP analysis.

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N.; Millam, N. J.; Klene, M.; Knox, J. E.; 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.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (31) Chai, J.-D.; Head-Gordon, M. J. Chem. Phys. 2008, 128, 084106. (32) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (c) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (33) (a) Ditchfie, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724−728. (b) Hehre, W. J.; Ditchfie, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (c) Harihara, P.; Pople, J. Theor. Chim. Acta 1973, 28, 213. (34) (a) Miertuš, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117−29. (b) Miertuš, S.; Tomasi, J. Chem. Phys. 1982, 65, 239−245. (35) Although four carbonyl absorptions energies are calculated for gauche-Os2Cp2(CO)4; the two lowest energy bands are predicted to be unresolved in liquid media. (36) Owing to the preoxidation equilibrium between the two isomers, the oxidation of 1 formally follows a CE mechanism (see ref 28, pp 473−474). However, since the equilibrium is fast on the voltammetric time scale, the anodic process follows the diagnostics for a simple E mechanism which, in the interest of simplicity, we employ in our discussions of the process. (37) The width and peak height of the second wave are consistent with an irreversible one-electron process having a transmission coefficient (β) of about 0.3. No change in reversibility of the two anodic waves was observed with scan rate changes between 0.1 V s−1 and 2 V s−1. (38) No product bands were seen in the bridging carbonyl region of the spectrum. (39) Baker, R. T.; Calabrese, J. C.; Krusic, P. J.; Therien, M. J.; Trogler, W. C. J. Am. Chem. Soc. 1988, 110, 8392. (40) Treichel, P. M.; Rublein, E. K. J. Organomet. Chem. 1989, 359, 195. (41) Anderson, L. B.; Cotton, F. A.; Dunbar, K. R.; Falvello, L. R.; Price, A. C.; Reid, A. H.; Walton, R. A. Inorg. Chem. 1987, 26, 2717. (42) This estimate is based on the relative time scales of slow CV scans vs bulk anodic electrolysis. (43) Odd-electron species derived from metal-metal multiply bonded complexes are well-known. See (a) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms; John Wiley & Sons: New York, 1982. (b) Cotton, F. A.; Walton, R. A. Struct. Bonding (Berlin) 1985, 62, 1. (c) Schore, N. E.; Ilenda, C.; Bergman, R. G. J. Am. Chem. Soc. 1976, 98, 256. (d) Lopez, L. P. H.; Schrock, R. R.; Müller, P. Organometallics 2006, 25, 1978. (44) Cheng, T.-Y.; Bullock, R. M. Organometallics 2002, 21, 2325. (45) The irregularities in line spacing are too large to be attributed to second-order hyperfine effects (see Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic Resonance, John Wiley & Sons: New York, 1994; pp 72−73). Furthermore, the measured intensities of about 4% compared to the major line are inconsistent with coupling to a single 189 Os, which should give satellites of about 9.5% relative intensity. (46) (a) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis; University of Ottawa: Ottawa, Ontario, Canada, 2012; http://www.sg-chem.net/ (b) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 635, 187. (c) Gorelsky, S. I. J. Chem. Theory. Comput. 2012, 8, 908. (47) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Dalton Trans. 2008, 2832. (48) Elschenbroich, C. Organometallics, 3rd ed.; Wiley-VCH Verlag: Weinheim, Germany, 2005; pp363−374. (49) Geiger, W. E. Organometallics 2007, 26, 5738. For leading references, see pp 5757−5759. L


Organometallics

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(50) The calculations suggest that an anion of 2 would have an anti structure and retain a reasonably strong Os−Os bond, in contrast to the case of 1−. (51) An unresolved aspect of the “two isomer” interpretation of the low-temperature ESR spectra is it requires that the two radicals have approximately the same g-values. (52) Schugart, K. A.; Fenske, R. F. J. Am. Chem. Soc. 1986, 108, 5100. (53) Phillips, A. D.; Ienco, A.; Reinhold, J.; Böttcher; Mealli, C. Chem.Eur. J. 2006, 12, 4691. (54) Ernst, R. D.; Freeman, J. W.; Stahl, L.; Wilson, D. R.; Arif, A. M.; Nuber, B.; Ziegler, M. L. J. Am. Chem. Soc. 1995, 117, 5075. (55) Albrecht, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; John Wiley & Sons: New York, 1985; pp 326−327. (56) Lu, S.; Strelets, V. V.; Ryan, M. F.; Pietro, W. J.; Lever, A. B. P. Inorg. Chem. 1996, 35, 1013.

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Comparison of the One-Electron Oxidations of CO-Bridged vs

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