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Issue 2, 2016
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How π back-donation quantitatively controls the CO stretching response in classical and non-classical metal carbonyl complexes

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Abstract

The CO stretching response upon coordination to a metal M to form [(L)nM(CO)]m complexes (L is an auxiliary ligand) is investigated in relation to the σ donation and π back-donation components of the M–CO bond and to the electrostatic effect exerted by the ligand–metal fragment. Our analysis encompasses over 30 carbonyls, in which the relative importance of donation, back-donation and electrostatics are varied either through the ligand in a series of [(L)Au(CO)]0/+ gold(I) complexes, or through the metal in a series of anionic, neutral and cationic homoleptic carbonyls. Charge-displacement analysis is used to obtain well-defined, consistent measures of σ donation and π back-donation charges, as well as to quantify the σ and π components of CO polarization. It is found that all complexes feature a comparable charge flow of σ symmetry (both in the M–CO bonding region and in the CO fragment itself), which is therefore largely uncorrelated to CO response. By contrast, π back-donation is exceptionally variable and is found to correlate tightly with the change in CO bond distance, with the shift in CO stretching frequency, and with the extent and direction (C → O or C ← O) of the CO π polarization. As a result, we conclusively show that π back-donation can be an important bond component also in non-classical carbonyls and we provide the framework in which the spectroscopic data on coordinated CO can be used to extract quantitative information on the π donor properties of metal–ligand moieties.

Graphical abstract: How π back-donation quantitatively controls the CO stretching response in classical and non-classical metal carbonyl complexes

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Supplementary files

Article information


Submitted
11 Aug 2015
Accepted
23 Oct 2015
First published
26 Oct 2015

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2016,7, 1174-1184
Article type
Edge Article
Author version available

How π back-donation quantitatively controls the CO stretching response in classical and non-classical metal carbonyl complexes

G. Bistoni, S. Rampino, N. Scafuri, G. Ciancaleoni, D. Zuccaccia, L. Belpassi and F. Tarantelli, Chem. Sci., 2016, 7, 1174
DOI: 10.1039/C5SC02971F

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