_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Transition metal carbonyls
2400
Fe=•
CO=•
2400
1
1600 (cm- 1 )
Fe=•
CO=•
(a )
( b)
Ru=•
CO=•
2400 1600 (cm- 1 )
( c)
Fig. 5.9 Infrared spectra of (a) Fe 2 (C0) 9 (b) Fe,(C0) 12 and (c) Ru,(C0) 12 in Nujol mulls.
molecular orbita! parlance, high electron density at the metal means that the
metal n-orbitals are of relatively high energy. There is therefore an especially
strong interaction with the n* orbitals of CO leading to considerable electron
delocalization into these orbitals.
The effect ofM -CO a-donation on the CO bond order and frequency is less easy
to predict. Nevertheless it is agreed that it is the back donation which exerts the
dominant effect and trends in v(CO) frequencies can be understood in terms of it
alone. A good demonstration of the relationship between electron density, the
extent of back donation and v(C-0) frequency is provided by the positions of
corresponding bands in infrared spectra of two isoelectronic series (Table 5. 3 ).
As negative charge is added, ~'CCO(n*) back donation increases and v(CO)
decreases. The greater the back donation, however. the stronger does the M-C
bond become, leading to an increase in frequency of the M-C stretching
vibrations.
A phosphorus ligand, PR,. can, like carbon monoxide. function both as a a­
donor through the lone pair on phosphorus and also as a n-acceptor. Where the
substituents Rare strongly electron withdrawing, the phosphine is only a poor a­
donor. It appears. however, that such ligands are quite good n-acceptors. Until
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