CO stretch in metal carbonyl complexes

In the previous examples for molecules with relatively few atoms, we studied and presented the results of all 31V-6 vibrations of the molecules. However, for molecules composed of a large number of atoms, often it is convenient to concentrate on a certain type of vibration. The prime example of this kind of investigation is the enumeration of the CO stretching modes in metal carbonyl complexes. 

Most metal carbonyl complexes exhibit sharp and intense CO bands in the range 1800-2100 cm-1. Since the CO stretch motions are rarely coupled with other modes and CO absorption bands are not obscured by other vibrations, measurement of the CO stretch bands alone often provides valuable information about the geometric and electronic structures of the carbonyl complexes. As we may recall, free CO absorbs strongly at 2155 cm-1, which corresponds to the stretching motion of a C=0 triple bond. On the other hand, most ketones and aldehyde exhibit bands near 1715 cm-1, which corresponds formally to 

The o and n bonding in metal carbonyls. Note that electrons flow from the shaded orbital to the unshaded orbital in both instances. 

The M-C a bond is formed by donating the lone electrons on C to the empty d,2 orbital on M (upper portion of Fig. 7.3.10). The it bond is formed by back donation of the metal d7r electrons to the it orbital (introduced in Chapter 3) of CO. Populating the it orbital of CO tends to decrease the CO bond order, thus lowering the CO stretch frequency (lower portion of Fig. 7.3.10). These two components of metal-carbonyl bonding may be expressed by the two resonance structures 

Thus structure (I), with higher CO frequencies, tends to build up electron density on M and is thus favored by systems with positive charges accumulated on the metal. On the other hand, structure (II), with lower CO frequencies, is favored by those with negative charge (which enhances back donation) on metal. In other words, net charges on carbonyl compounds have a profound effect on the CO stretch frequencies, as illustrated by the following two isoelectronic series  

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