NDLI: Electronic Structure and Ligand Vibrations in FeNO, CoNO and FeOO Porphyrin Adducts

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Electronic Structure and Ligand Vibrations in FeNO, CoNO and FeOO Porphyrin Adducts

Content Provider	PubMed Central
Author	Soldatova, Alexandra V. Ibrahim, Md Spiro, Thomas G.
Abstract	The gaseous ligands, CO, NO and O2 interact with the Fe ion in heme proteins largely via backbonding of Fe electrons to the π* orbitals of the XO (X = C, N, O) ligands. In these FeXO adducts, the Fe–X stretching frequency varies inversely with the X–O stretching frequency, since increased backbonding strengthens the Fe–X bond while weakening the X–O bond. Inverse frequency correlations have been observed for all three ligands, despite differing electronic and geometric structures, and despite variable composition of the `FeX' vibrational mode, in which Fe–X stretching and Fe–X–O coordinates are mixed for bent FeXO adducts. We report experimental data for 5-coordinate CoII(NO) porphyrin adducts (isoelectronic with FeII(OO) adducts), and the results of DFT modeling for 5-coordinate FeII(NO), CoII(NO) and FeII(OO) adducts. Inverse ν(MX)/ν(XO) correlations are obtained computationally, using model porphyrins with graded electron-donating and -withdrawing substituents to modulate the backbonding. Computed slopes agree satisfactorily with experiment, provided non-hybrid functionals are used, which avoid overemphasizing high-spin states. The BP86 functional gives correct ground states, a closed-shell singlet for CoII(NO) and an open-shell singlet for the isoelectronic FeII(OO), as corroborated by structural data for CoII(NO), and the ν(MX)/ν(XO) slope agreement with experiment for both adducts. However, for FeII(OO) adducts, the computed inverse ν(MX)/ν(XO) correlation applies only to porphyrins with electron-donating and withdrawing substituents of moderate strength. For substituents more donating than –CH3, a direct correlation is obtained, the Fe–O and O–O bonds weakening in concert. This effect is ascribed to the dominance of σ bonding via the in-plane dxz(+dz2)-π* orbital, when electron-donating substituents raise the d orbital energies sufficiently to render backbonding (dyz-π*) unimportant.
Related Links	http://dx.doi.org/10.1021/ic400364x
Ending Page	7486
Page Count	9
Starting Page	7478
File Format	PDF
ISSN	00201669
e-ISSN	1520510X
Journal	Inorganic chemistry
Issue Number	13
Volume Number	52
Language	English
Publisher Date	2013-07-01
Access Restriction	Open
Subject Keyword	Physical and Theoretical Chemistry Inorganic Chemistry Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Physical and Theoretical Chemistry Inorganic Chemistry

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