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Carbonyls and Nitrosyls


(d) Carbonyls and Nitrosyls

(i) Carbonyls

Carbon monoxide is a very good ligand, and is widely used in organometallic chemistry. It always coordinates to the metal via the C atom, but can act either as a terminal or bridging ligand, and these two types are often readily identifiable from the IR spectra.


The s-bonding involves donation of electron density from a s-(slightly) anti-bonding orbital localised on the carbon to the metal dz2 or dx2-y2 orbitals. This has the effect of increasing the vCO stretching frequency slightly from the gas phase value (2143 cm-1). The majority of carbonyl complexes have metals in low oxidation states, therefore the CO ligand can act as a p-acid, with electron density being donated from the metal dxy, dxz or dyz orbitals into the p* anti-bonding molecular orbital on the CO. This has the effect of reducing the vCO stretching frequency as the C-O bond order is reduced. In general, this effect is greater than the increase due to s bonding, therefore vCO is usually lower than that for ‘free’ CO.


(ii) Nitrosyls

Nitric oxide (NO) can act as a s-donor and p-acceptor much like CO, except that it now has 11 valence electrons.
When bound as a ligand it can be thought of as a two electron donor as NO (linear, isoelectronic with CO) or NO (bent, isolectronic with O2). The other way of think of NO is to regard it as neutral but as a 3 electron donor when linear, or 1 electron donor when bent. The choice of model adopted obviously has important connotations on the formal oxidation state of the metal, but the total number of valence electrons should always be accounted for. As the formal NO bond order is different for the two the linear and bent forms, IR spectroscopy can be used to distinguish between them.


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