(d) Carbonyls and
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.
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
bonding, therefore vCO
is usually lower than that for ‘free’ CO.
oxide (NO) can act as a s-donor
much like CO, except that it now has 11 valence electrons.
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
to Predicting Vibrational Spectra>