Trends in Homolytic Bond Dissociation Energies of Five- and Six

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Trends in Homolytic Bond Dissociation Energies of Five- and Six-Coordinate Hydrides of Group 9 Transition Metals: Co, Rh, Ir Vassiliki-Alexandra Glezakou, Roger Rousseau, Stephen T. Elbert, and James Alan Franz J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11655 • Publication Date (Web): 31 Jan 2017 Downloaded from http://pubs.acs.org on February 18, 2017

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The Journal of Physical Chemistry

Trends in Homolytic Bond Dissociation Energies of Five-and SixCoordinate Hydrides of Group 9 Transition Metals: Co, Rh, Ir Vassiliki-Alexandra Glezakou* 1 , Roger Rousseau 1 , Stephen T. Elbert 2 , and James A. Franz # 3 1

Basic and Applied Molecular Foundations, 2 Advanced Controls, 3Institute for Integrated

Catalysis Pacific Northwest National Laboratory, MS K1-83, P.O. Box 999, Richland WA 99352 Email: [email protected] #

Deceased.

1 ACS Paragon Plus Environment


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ABSTRACT The homolytic bond dissociation energies of a series of five- and six-coordinate mono- and dihydride complexes of the type HM(diphosphine)2 and [H2M(diphosphine)2]+ (where M = Co, Rh and Ir) are calculated and compared with experimental values. This work probes the relationship between the homolytic bond dissociation energies (HMBDEs) of these complexes in these two different coordination environments and formal oxidation states. The results of these calculations and previous experimental observations suggest that for M = Rh the HMBDE of the fivecoordinate HM(diphosphine)2 species are 0-2 kcal/mol larger than the HMBDE of the corresponding six-coordinate [H2M(diphosphine)2]+ species. For M = Ir the bond energies of the five- and six-coordinate complexes are nearly the same and for M = Co the six-coordinate species are 1-5 kcal/mol less than the corresponding five-coordinate species. Simplified models of large and complicated ligands seem to capture the essential trends and give very good estimates of these thermodynamic properties compared with experimentally available data that are difficult to obtain.

Keywords: Homolytic bond dissociation, metal hydrides, diphosphine ligand bite-angle



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Introduction Transition metal hydrides are very important reagents encountered in a wide range of important catalytic processes such as hydrogenation1, hydroformylation2, electrocatalytic production and oxidation of hydrogen3-10 and many others11. As a result, a fundamental understanding of those features controlling the strength of the M-H bond is important. The metalhydrogen bond can be cleaved in three different ways as shown in Figure 1, and the metalhydride complex can be the source of a hydrogen atom (black line), a proton (green line), or a hydride (red line). The reactivity of metal hydrides varies widely with the nature of the metal and the ligands, and the free energies associated with these reactions in solution can be determined using a variety of experimental methods12-14. Solution bond dissociation free energies for all three types of M-H bond cleavage have been determined experimentally for a number of five-coordinate monohydride complexes, HM(diphosphine)2 and [HM'(diphosphine)2]+ (where M=Co, Rh and M'=Ni, Pd, Pt)13,

15-24

.

Theoretical studies of the acidity and hydride donor abilities of these complexes have also been reported25-30. Owing to the difficultly in isolating the corresponding six-coordinate dihydrides that are related to these monohydrides by a simple protonation reaction, [H2M(diphosphine)2]+ and [H2M'(diphosphine)2]2+, have received much less attention despite their relevance as potential reaction intermediates. For a few systems however, the homolytic bond dissociation free energies for both the five-coordinate hydride and the corresponding six-coordinate dihydride have been determined. For these species, the homolytic single-bond dissociation free energies (SBDFE's) of the dihydrides are 1-3 kcal/mol less than those of the corresponding monohydrides15, 18, 21. This begs the question as to weather this is a ubiquitous phenomenon. We postulate that one could use the homolytic SBDFEs of the more extensively studied five-coordinate monohydride species to estimate those of the corresponding six-coordinate dihydride, and hence further extend our


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quantitative understanding of M-H bond energies to a class of compounds where experimental data are difficult to come by. Inspired by the seminal work of Prof. Mark Gordon on the accurate computation of M-H bond dissociation potentials31-34, we have chosen to examine homolytic M-H bond strengths in transition metal complexes of Co, Rh and Ir. This work is further motivated by the necessity to obtain reliable thermodynamic data regarding transition metal organometallic catalysts14, 35. We specifically focus on homolytic M-H bond dissociation energies, as these quantities are very hard to measure experimentally, yet they exhibit the least-solvent dependency and as such are the most transferable between different operating conditions. We use theoretical calculations to explore: (1) the structural changes anticipated upon removal of a hydrogen atom from HM(L)2 and [H2M(L)2]+ complexes (where L is a diphosphine ligand) and upon oxidation of HM(L)2 to [HM(L)2]+, (2) the relationship of the homolytic bond dissociation energies of HM(L)2 and [H2M(L)2]+ complexes, (3) the role of the polarity of the metal-hydrogen bond in homolytic M-H bond cleavage reactions, and (4) what would be the minimal requirements for modeling complicated systems, in order to obtain reliable predictions through economic computations. The systems for which there are experimental data for both the mono- and di-hydride are: Co(H)2(dppe)2+ and CoH(dppe)2, Rh(H)2(depx)2+ and RhH(depx)2, and Pt(H)2(Xanthphos)22+ and PtH(Xanthphos)2+,

where

dppe=bis(diphenylphosphino)-ethane,

DEPX=´-

bis(diethylphosphino)xylene and Xanthphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene. Formal oxidation states and thermodynamic relationships of the computed energies between species are summarized in Figure 1, showing an extended array of potential intermediates which have been invoked in the chemistry of these complexes. 12-14 We obtained theoretical values of the homolytic bond dissociation energies for these systems through the computed heats of reaction, along with estimates for Co, Rh and Ir complexes with simplified model ligands.



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Figure 1. Schematic of hydrogen abstraction in metal hydrides. Black arrows denote a Hydrogen atom transfer, Green a proton transfer and red a hydride transfer. Maroon arrows connect charge transfer and redox changes within the same coordination complexes.

Computational details Geometry optimizations and frequency calculations for all systems were computed with the hybrid functional B3LYP within the spin unrestricted formalism, which has been shown to predict bond lengths within 0.04Å and angles within 1-2 deg36,

37

as well as describe reaction

energetics satisfactorily38-40. We note, that spin contamination in these complexes was found to be minimal, ~0.77 (0.75 for the pure states) at the most for doublet states such that both the current approach and the unrestricted open shell formalism produce the M-H bond dissociation energies to within 1-3 kcal/mol. For the metal centers, we used relativistic core potentials SBKJC supplemented with double zeta quality basis41, and on all other atoms (H, C, O and P) 6-311G**for the model ligands, and 6-31G** for the actual DPPE, depz and xanthphos ligands. All optimizations and harmonic frequency analysis at the same level of theory were performed with the NWCHEM suite of codes42 and GAMESS43. The coordinates of the optimized structures of the large systems are listed in the SI. Energy differences at 0 K obtained from our quantum chemical calculations were adjusted to Ho under the standard conditions of T=298 K and P=1 Atm by adding the zero-point 6 ACS Paragon Plus Environment

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vibrational energies from harmonic analysis of the equilibrium structures, as well as thermal contributions from the rotational and translational partition functions. For H•, the value of -0.5 au was used to circumvent the known problem of self-interaction overestimation of the atomic energy at 0 K and adjusted for thermal corrections due to translational partition function. For a select subset of these model systems, we performed a natural bond orbital charge analysis (NBO)44, 45 on the optimized structures to monitor the redistribution of the charges on the metal center and the first-shell atoms that accompany the H abstraction and the different oxidation states.

Results and Discussion 1. Comparison of calculated and experimental structures In order to examine a wide variation of ligand sets for up to three transition metals, we have chosen to make the structural approximation of dealing with a homologous series of diphosphine ligands (H2P(CxHy))PH2) and thereby reduce both the computational and conformational complexity of our models relative to compounds used experimentally. One the one hand, this allows us to focus our attention on trends in MH bond strength, but with the caveat that substitution of H for an organic R groups on the phosphine can impact the electron donating strength of the ligand as well as impose a slightly different geometry about the first coordination sphere of the metal. Both factors are known to have significant impact on heterolytic M-H bond dissociation energies,28 but have a lesser influence on homolytic ones. Nonetheless, a measure of how impactful this substitution may be can be assessed by a careful evaluation of the geometrical parameters, which are highly sensitive to changes in electronic structure about the metal site26, 28. Towards this end, Tables 1 and 2 and 3 list selected bond distances and bond angles for the structurally optimized complexes [H2Co(H)2(H2PCH2CH2PH2)2]+, [HCo(H2PCH2CH2PH2)2]+,



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HCo(H2PCH2CH2PH2)2, and Co(H2PCH2CH2PH2)2, where LL= H2PCH2CH2PH2 for Co and LL= H2PCH2CH=CHCH2PH2 for Rh and Ir. For metals Co and Rh, we list the corresponding parameters from the calculations on the systems with the experimental ligands (Co/dppe and Rh/depx). These structures and the atom numbering scheme are shown in Figures 2 and 3. For Co, values determined by single crystal Xray diffraction studies are also shown in parentheses for [Co(H)2(Ph2PCH2CH2PPh2)2]+ and for Co(H)(Ph2PCH2CH2PPh2)2 18 for comparison.

Figure 2. Changes in local Cobalt coordination sphere as a result of hydrogen abstraction and redox changes. M=Co, L=dppe, bis-(diphenylphosphino)-ethane. (a) Co(L)2H2+. (b) Co(L)2H+*. (c) Co(L)2H. (d) Co(L)2H*,

L=


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Figure 3. Changes in local Rhodium coordination sphere as a result of hydrogen abstraction and redox changes. M=Rh, L=depx, a,a′ ′-bis-(diethylylphosphino)-xylene. (a) Rh(L)2H2+. (b) Rh(L)2H+*. * (c) Rh(L)2H. (d) Rh(L)2

L=

From Table 1, we can see that the calculated M-P and M-H bond distances are slightly longer than the experimentally determined bond distances, but the overall agreement is reasonably good. All H2M(LL)2+ and HM(LL)2 complexes show a distorted octahedron and trigonal bipyramidal structure respectively, with the hydride in the axial position. These geometries are consistent with a formal M(III)/M(I) oxidation state (closed shell d6/d8 configuration). In particular, for M=Co complexes (Table 1), the M-P and M-H bonds show a maximal deviation from the X-ray values of 0.03 and 0.05Å respectively, in the same order as the experimental values with change in ligand structure. In addition, for the dihydride species the Co-



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P distances for the phosphorus atoms trans to the hydride ligands are calculated to be approximately 0.05 Å longer than those trans to each other. The experimental difference is approximately 0.04 Å, also in good agreement with theory. The shortening of the Co-P bonds observed experimentally upon protonation of the five-coordinate monohydride complexes to form six-coordinate dihydride complexes is also reproduced by theoretical calculations. The agreement between experiment and theory for the P2-Co-P5 and the P3-Co-P4 bond angles of the diphosphine ligands is typically 5o or better. Poorer agreement is observed for the other angles as expected, due the use of H atoms in the theoretical calculations that cannot represent the steric interactions of the phenyl groups used in the experimentally studied Co complexes . This was further proven by the marked improvement in P-M-P bond angles as obtained from the calculations obtained with the full ligand set relative with those of the reduced ones. Overall, this evaluation gives us confidence that, to first order, our reduced models accurately reflect the local coordination environment about Co and Rh diphosphine complexes. Moreover, deviations in the energetics anticipated from ligand inductive effects and geometrical distortions can be approximately accounted for by comparing the H-substituted model ligand with the calculations based on the full parent species. 2. Structural changes upon removal of a hydrogen atom or redox state changes. Upon removal of a hydrogen atom from the H2M(LL)2+, and/or HM(LL)2 species results in a change in formal oxidation state, as the H ligand by definition is counted as H- by convention (see Figure 1). For the H2M(LL)2+ species, this is consistent with the transition from M(III) to M(II) and a change from a closed shell d6 species to an open shell d7. Likewise, for the neutral species, HM(LL)2, the change in oxidation is M(I) to M(0), i.e. d8 to d9. Thus a reliable representation of the geometric and energetic changes associated with these M-H bond cleavages requires that we model these changes across four different formal oxidation states that are often difficult to distinguish experimentally. 10 ACS Paragon Plus Environment

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In the first case, the H2M(LL)2+ complex undergoes distortion from an octahedral structure to a distorted square pyramidal complex HM(LL)2+ . The Co-P bond trans to the hydrogen atom being removed (Co-P4) lengthens by approximately 0.10 Å, see Table 1, while the other three Co-P bond distances lengthen by 0.03-0.06 Å. This effect is less pronounced for the rhodium and iridium dihydride complexes, see Tables 2 and 3, where the M-P4 distances shorten only by 0.03 to 0.05 Å, and the other three bonds lengthen by 0.01-0.03 Å. Changes in the P-M-P bond angles upon removal of a hydrogen atom are small, less than five or six degrees. Overall, other than the M-H bond, the structural changes observed upon removal of a hydrogen atom from the eighteen electron dihydride complexes are small. This is consistent with experimental observations, see Tables 1 and 2, providing confidence that the DFT approach used is adequately representing the electronic structure effects. Removal of a hydrogen atom from the HM(LL)2 trigonal bipyramidal species results in a distorted tetrahedral M(LL)2 structure consistent with the open shell d9 configuration. For the cobalt monohydride complexes, all four Co-P bond distances increase by 0.04 to 0.06 Å, whereas, the corresponding rhodium and iridium complexes, the M-P bonds trans to the position of the hydride ligand decrease by 0.03-0.06 Å, and the other three M-P bond lengths increase by up to 0.04 Å. The P-M-P bond angles generally changed by less than 5 degrees upon removal of a hydrogen atom from these monohydride complexes. These results are again consistent with a relatively small, but measureable, change in structure upon hydrogen atom removal from the monohydride complexes observed experimentally, see Table 1. Conversely, large structural changes are seen upon oxidation of the neutral HM(LL)2 complexes to the corresponding cationic [HM(LL)2]+ complexes. This is accompanied by a change in the electronic configuration for the closed shell d8 to an open shell d7-i.e. M(I) to M(II) formal oxidation state change. For cobalt complexes, this oxidation leads to a lengthening of the Co-P bonds by 0.09 to 0.20 Å, whereas rhodium and iridium this lengthening is somewhat smaller, 0.04-0.10 Å. Unlike the homolytic M-



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H cleavage, here large changes in bond angles are observed. In particular, the P2-M-P3 bond angle increases by 30-35 degrees upon oxidation. This reflects a change in geometry from the d8 trigonal bipyramidal geometry, in which P2 and P3 occupy equatorial positions with an idealized P-M-P angle of 120 degrees, to a d7 square pyramidal geometry in which P2 and P3 occupy trans positions in the basal plane. As a result, oxidation of HM(LL)2 complexes is expected to invoke a large structural change. Experimentally, these oxidation changes are followed electrochemically by cyclic voltammetry18. Large structural re-arrangement are potentially indicative of nonreversible redox chemistry. Overall, the current DFT approach captures all these changes in geometry, either subtle or large, upon hydrogen removal and/or redox processes for species spanning four different oxidation states, as is evident from the structural data provided in Tables 1 and 2. 3. Enthalpies of Reaction: homolytic M-H bond dissociation Now that we have described the coordination chemistry and structure of the HM(LL)2 species, we consider the thermodynamics of homolytic M-H bond scission and its dependence on the bite angle of the diphosphine ligand. DFT calculations were used to compute the gas-phase, homolytic, metal-hydrogen, bond dissociation enthalpies for five-coordinate monohydrides of the formula

HM(PH3)4,

HM(H2PCH2CH2PPH2)2,

HM((H2PCH=CHPH2)2,

and

HM(H2PCH2CH=CHCH2PH2)2 where M = Co, Rh, and Ir. Similar bond dissociation enthalpies were also calculated for the corresponding six-coordinate dihydride complexes, [H2M(PH3)4]+, [H2M(H2PCH2CH2PH2)2]+, [H2M(H2PCH=CHPH2)2]+, and [H2M(H2PCH2CH=CHCH2PH2)2]+. The results of these calculations are summarized in Table 3. The calculated gas phase homolytic bond dissociation enthalpies are summarized in Table 4 and range from 52 to 80 kcal/mol, and they increase from Co to Rh to Ir for both the


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monohydride and dihydride species. This is consistent with the observed trends on homolytic solution bond dissociation free energies observed experimentally16, 18, 20-22. For the Co complexes, the M-H bond dissociation enthalpies for the monohydrides range from 56 -60 kcal/mol with the general trend that the HCo(LL)2 species have on average 3 kcal/mol larger M-H bond dissociation enthalpy than the H2Co(LL)2+ complexes. This is consistent with the experimentally measured homolytic solution bond dissociation free energy of HCo(dppe)2 of 50 kcal/mol which was found to be 1 kcal/mol larger than that of [H2Co(dppe)2]+18. It is noted however that our calculations show an interesting dependence on the bite angle such that the highly strained complexes with LL = H2PCH=CHPH2 have a higher M-H bond dissociation enthalpy for the dihydride complex. Otherwise, complexes containing four atom backbones or only monodentate phosphine ligands, subtracting 2-5 kcal/mol from the bond energy of the monohydride, appear to provide a reasonable estimate of the energy for the bond energy of the dihydride. Finally, a similar small difference in M-H bond disassociation enthalpies can be inferred from the thermodynamic data computed on HNi(LL)2 complexes indicating a similarity between the d9 species discussed here and these d9 analogues28, 46. For rhodium complexes, the calculated enthalpies of the monohydrides and the corresponding dihydrides are nearly the same with all values falling within the range of 69.5±2.0 kcal/mol. As the number of carbon atoms in the ligand backbone increases from two to four carbons, the bond energies of the monohydrides increase by about 2 kcal/mol while those of the dihydrides decrease by a similar amount. Similar to the Co case, Rh complexes containing LL= H2PCH=CHPH2 with two carbon atoms in the ligand backbone, show an Rh-H bond energy of the dihydride complex 2 kcal/mol higher than the value observed for the monohydride. For complexes with ligands containing four carbon atoms, subtracting 2 kcal/mol from the monohydride bond energy would also provide a reasonable estimate of the bond energy of the corresponding dihydride. For comparison HRh(depx)2 and [H2Rh(depx)2]+ (where depx is ,'-bis(diethylphosphino)xylene 13 ACS Paragon Plus Environment


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which has a four carbon backbone), the experimentally measured M-H bond dissociation free energy in ~70 kcal/mol for the monohydride and 67 kcal/mol for the dihydride15. Like the rhodium case, for iridium complexes the calculated enthalpies of the monohydrides and the corresponding dihydrides are nearly the same, ranging from 75-81 kcal/mol for diphosphine ligands with a typical difference of only 3 kcal/mol between mono and di hydride species. To the best of our knowledge no measurements currently exist for Ir complexes despite their relevance as hydrogenation complexes. Hence the current calculations may serve as very useful predictions.

Nonetheless, some validation can be gleamed from the analogous Pt

complexes. Here, it is interesting to point out that the solution homolytic bond dissociation free energy for [HPt(EtXanthphos)2]+, containing a five-atom ligand backbone, is 69.8 kcal/mol, which is 2 kcal/mol larger than for [H2Pt(EtXanthphos)2]2+

21

. As in the case of Co/Ni

comparison, the Ir/Pt comparison implies a more universal trend for estimating MH/MH2+ bond energies for neighboring metals. These empirical rules allow the values of homolytic solution bond dissociation free energies in acetonitrile that are now known for a large number of monohydride complexes to be extended to dihydride complexes. Some of these complexes are listed in Table 5 along with the values estimated for the corresponding dihydride complexes. In this table, the empirical rules discussed above for cobalt complexes are applied to nickel complexes, those for rhodium complexes are applied to palladium complexes, and those for iridium are applied to platinum complexes. These relationships can be refined as additional data becomes available either from computations or from experiment. 4. NBO Charges Transition metal hydrides can act as hydride donors, and this ability to act in this manner is often thought to be associated with a polarization of the M-H bond, with significantly larger


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negative charges on the hydride ligands correlating with better hydride donor abilities. Tables 6 and 7 list the charges on the hydride ligands and the metal from the NBO analysis for the dihydride and monohydride species with LL=H2PCH2CH2PH2 and LL= H2PCH2CH=CHCH2PH2 respectively. Based on the criterion of increased negative charge on the hydride ligand, the hydride donor abilities of the monohydride species should follow the order Co > Rh > Ir. A similar conclusion may be drawn from the difference in charge of the hydride ligand and the metal, or the polarity of the M-H bond. However, the actual order of the hydride donor abilities for these complexes in solution is Rh > Ir > Co. Clearly using bond polarity or the charge on the hydride ligand as an indicator of hydride donor abilities is not useful. Similar conclusions can be reached regarding acidities of these transition metal hydride complexes and the charge on the hydride ligand. Based on the charges on the hydride ligand, the order of increasing acidity should be Co < Rh < Ir. The experimentally observed order of acidity in acetonitrile solutions is Ir < Co < Rh. Correlations of molecular charge distributions with either acidity or hydricity appear to be of little value. Instead, the order in polarity Co> Rh> Ir is very much reflected in the homolytic M-H bond energies.

Conclusions The homolytic cleavage of the M-H bond in [H2M(LL)2]+ and HM(LL)2 complexes for M=Co, Rh and Ir has been studied using density functional theory. We set out to address four fundamental questions: (1) What are the structural changes expected upon removal of a hydrogen atom from HM(L)2 and [H2M(L)2] + complexes (where L is a diphosphine ligand) and upon oxidation of HM(L)2 to [HM(L)2] +? We found that bond cleavage reaction results in small changes in bond distances and angles, but major structural changes are not



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observed. For Co and Rh where experimental structural data are available, we note that the structure is predicted quite well, even with a much simpler ligand, as long as the bite angle is reasonably represented. In contrast, oxidation of HM(LL)2 complexes to form [HM(LL)2]+ complexes, results in significant structural reorganization. In particular there is a lengthening of all of the M-P bonds by 0.1 to 0.2 Å and an opening of one of the equatorial angles of the trigonal bipyramid to form the transphosphines of a distorted square pyramid. Nonetheless, both large and small structural distortions are adequately reproduced by a hybrid functional, such as B3LYP, over a variation of four formal oxidation states. (2) What are the relationship of the homolytic bond dissociation energies of HM(L)2 and [H2M(L)2] + complexes? We find that our DFT results are within only a few kcal/mol of the available experimentally measured results and that for the majority of compounds the M-H bond dissociation enthalpy is consistently 1-2 kcal/mol lower in [H2M(L)2]+ complexes than their HM(L)2 analogues. This allows us to predict the M-H bond energies for a wide number of [H2M(L)2]+ species as provided in Table 5. (3) What is the role of the polarity of the metal-hydrogen bond in homolytic M-H bond cleavage reactions? We found that ordering of homolytic M-H bond dissociation enthalpies is consistent with the order in their bond polarity though this is not the only electronic component in play. (4) What would be the minimal requirements for modeling complicated systems, in order to obtain reliable predictions through economic computations? The model complexes employed here for homolytic M-H bond energies have adequately recovered


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both the structure and energetics of M-H bonds. Clearly, this is in stark contrast to the heterolytic M-H cleavage energies, which rely both on M-H bond strengths as well as redox state changes at the metal28. For these properties, ligand inductive and bite angle effects are more pronounced. Nonetheless, judicious use of hybrid DFT functionals is adequate for reliable estimates of thermodynamic parameters for these complexes.

Acknowledgments This paper is dedicated to Professor Mark S. Gordon on the occasion of his 75th birthday. It represents a small contribution to his legacy as one of the most respected scientists in the field of Quantum Chemistry. V.-A. G. will always be grateful for having been one of his students. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences under grant KC0301050-47319 and computer time was provided through a NERSC computer allocation. PNNL is a multiprogram national laboratory operated for DOE by Battelle.

Supplementary Information The coordinates of the optimized structures of the large systems [(H)2Co(dppe)2]+, [(H)2Rh(depx)2]+ and [(H2)Ir(PH2CH2CH=CHCH2PH2)2]+ are given in SI. Table 1. Selected geometric parameters of Co complexes where LL corresponds to PH2CH2CH2PH2. Values in square brackets are from calculations on the system with for LL = dppe and in parentheses the experimentally determined values (crystallographic data). +

+*

*

H2Co(LL)2

HCo(LL)2

HCo(LL)2

Co(LL)2

Co-H1

1.476 [1.465](1.42)

1.493 [1.487]

1.497 [1.485](1.46)

--

Co-H2

1.473 [1.461](1.47)

--

--

--

Co-P2

2.191 [2.240](2.18)

2.233 [2.271]

2.142 [2.164](2.12)

2.201 [2.219]



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

Co-P3

2.187 [2.240](2.17)

2.245 [2.309]

2.158 [2.212](2.15)

2.200 [2.226]

Co-P4

2.253 [2.236](2.21)

2.361 [2.430]

2.165 [2.211](2.15)

2.216 [2.240]

Co-P5

2.247 [2.322](2.21)

2.285 [2.322]

2.175 [2.230](2.16)

2.234 [2.278]

Trends in Homolytic Bond Dissociation Energies of Five- and Six

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