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Thermodynamics of phosphine coordination to the [PNP]Rh-I fragment : an example of the importance of reorganization energies in the assessment of metal-ligand 'Bond Strengths'

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Organization
Abstract
Reaction enthalpies of the complexes [RPNP]Rh(COE) ([RPNP] = N(SiMe2CH2PPh2)(2), N(SiMe2-(CH2PPr2)-Pr-i)(2); COE = cyclooctene) with a series of phosphine ligands and CO have been measured by solution calorimetry. The measured enthalpies span range of ca. 40 kcal/mol. These systems favor coordination of strong pi-acceptor/weak sigma-donor ligands as shown by the trend in Delta H-rxn: CO much greater than Ppyrl(3)' > Ppyrl(3) > PPhpyrl(2) > PPh(2)pyrl > PPh3. This trend is exactly the opposite of that observed in another square planar rhodium(I); system, trans-RhCl(CO)(PZ(3))(2). With the exception of CO, the ligands investigated are isosteric, and so the observed trends are electronic in nature. Single-crystal X-ray diffraction studies on several of theses complexes ([RPNP]RhL where R, L = Ph, PPh3; Ph, Ppyrl(3); Ph, CO; Pr-i, PPh3; Pr-i, Ppyrl(3); Pr-i, CO; Pr-i, COE) have been performed. Although the structural trends are readily understood in:terms of the electronic (donor/acceptor) nature of each ligand array, it is not obvious that the structural data predict the trends or, in particular, the trend reversal in Delta H-rxn in the two Rh(I) systems. Rather, these results illustrate the importance of reorganization energies in thermodynamic analyses of metal-ligand bonding, especially in the presence of synergistic bonding involving sigma-donor, pi-donor, and pi-acceptor ligands, interacting through shared metal orbitals (electron push-pull). In such cases the interpretation of a metal-ligand bond dissociation enthalpy (D) as an intrinsic, universal, and transferable property of that bond (e.g., a ''bond strength") is an invalid proposition.
Keywords
N-PYRROLYL PHOSPHINES, PI-ACCEPTOR CHARACTER, RU-X BOND, PHOSPHORUS LIGANDS, ORGANORUTHENIUM THERMOCHEMISTRY, QUANTITATIVE-ANALYSIS, L(2)FE(CO)(3) SYSTEM, RHODIUM COMPLEXES, EFFECTS QALE, ENTHALPIES

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MLA
Huang, JK, et al. “Thermodynamics of Phosphine Coordination to the [PNP]Rh-I Fragment : An Example of the Importance of Reorganization Energies in the Assessment of Metal-Ligand ‘Bond Strengths.’” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, vol. 120, no. 31, 1998, pp. 7806–15.
APA
Huang, J., Haar, C., Nolan, S., Marshall, W., & Moloy, K. (1998). Thermodynamics of phosphine coordination to the [PNP]Rh-I fragment : an example of the importance of reorganization energies in the assessment of metal-ligand “Bond Strengths.” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 120(31), 7806–7815.
Chicago author-date
Huang, JK, CM Haar, Steven Nolan, WJ Marshall, and KG Moloy. 1998. “Thermodynamics of Phosphine Coordination to the [PNP]Rh-I Fragment : An Example of the Importance of Reorganization Energies in the Assessment of Metal-Ligand ‘Bond Strengths.’” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 120 (31): 7806–15.
Chicago author-date (all authors)
Huang, JK, CM Haar, Steven Nolan, WJ Marshall, and KG Moloy. 1998. “Thermodynamics of Phosphine Coordination to the [PNP]Rh-I Fragment : An Example of the Importance of Reorganization Energies in the Assessment of Metal-Ligand ‘Bond Strengths.’” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 120 (31): 7806–7815.
Vancouver
1.
Huang J, Haar C, Nolan S, Marshall W, Moloy K. Thermodynamics of phosphine coordination to the [PNP]Rh-I fragment : an example of the importance of reorganization energies in the assessment of metal-ligand “Bond Strengths.” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. 1998;120(31):7806–15.
IEEE
[1]
J. Huang, C. Haar, S. Nolan, W. Marshall, and K. Moloy, “Thermodynamics of phosphine coordination to the [PNP]Rh-I fragment : an example of the importance of reorganization energies in the assessment of metal-ligand ‘Bond Strengths,’” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, vol. 120, no. 31, pp. 7806–7815, 1998.
@article{8171061,
  abstract     = {Reaction enthalpies of the complexes [RPNP]Rh(COE) ([RPNP] = N(SiMe2CH2PPh2)(2), N(SiMe2-(CH2PPr2)-Pr-i)(2); COE = cyclooctene) with a series of phosphine ligands and CO have been measured by solution calorimetry. The measured enthalpies span range of ca. 40 kcal/mol. These systems favor coordination of strong pi-acceptor/weak sigma-donor ligands as shown by the trend in Delta H-rxn: CO much greater than Ppyrl(3)' > Ppyrl(3) > PPhpyrl(2) > PPh(2)pyrl > PPh3. This trend is exactly the opposite of that observed in another square planar rhodium(I); system, trans-RhCl(CO)(PZ(3))(2). With the exception of CO, the ligands investigated are isosteric, and so the observed trends are electronic in nature. Single-crystal X-ray diffraction studies on several of theses complexes ([RPNP]RhL where R, L = Ph, PPh3; Ph, Ppyrl(3); Ph, CO; Pr-i, PPh3; Pr-i, Ppyrl(3); Pr-i, CO; Pr-i, COE) have been performed. Although the structural trends are readily understood in:terms of the electronic (donor/acceptor) nature of each ligand array, it is not obvious that the structural data predict the trends or, in particular, the trend reversal in Delta H-rxn in the two Rh(I) systems. Rather, these results illustrate the importance of reorganization energies in thermodynamic analyses of metal-ligand bonding, especially in the presence of synergistic bonding involving sigma-donor, pi-donor, and pi-acceptor ligands, interacting through shared metal orbitals (electron push-pull). In such cases the interpretation of a metal-ligand bond dissociation enthalpy (D) as an intrinsic, universal, and transferable property of that bond (e.g., a ''bond strength") is an invalid proposition.},
  author       = {Huang, JK and Haar, CM and Nolan, Steven and Marshall, WJ and Moloy, KG},
  issn         = {0002-7863},
  journal      = {JOURNAL OF THE AMERICAN CHEMICAL SOCIETY},
  keywords     = {N-PYRROLYL PHOSPHINES,PI-ACCEPTOR CHARACTER,RU-X BOND,PHOSPHORUS LIGANDS,ORGANORUTHENIUM THERMOCHEMISTRY,QUANTITATIVE-ANALYSIS,L(2)FE(CO)(3) SYSTEM,RHODIUM COMPLEXES,EFFECTS QALE,ENTHALPIES},
  language     = {eng},
  number       = {31},
  pages        = {7806--7815},
  title        = {Thermodynamics of phosphine coordination to the [PNP]Rh-I fragment : an example of the importance of reorganization energies in the assessment of metal-ligand 'Bond Strengths'},
  url          = {http://dx.doi.org/10.1021/ja974200v},
  volume       = {120},
  year         = {1998},
}

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