= Solution
<triphenylphosphine> is a good sigma donor and a substantially poorer pi acceptor than CO. Replacing CO by this <phosphine ligand> increases <electron> density at <chromium> and reduces competition among <carbonyl ligands> for <pi backbonding>. Backbonding into each remaining CO therefore strengthens, their internal <bond force constants> fall, and \b[both remaining carbonyl stretching frequencies are expected to shift downward] relative to the parent tricarbonyl.
The two remaining <carbonyl ligands> have coupled approximately symmetric and antisymmetric stretches; both are generally infrared active in this geometry. One should not treat the spectral change as simply deleting one of the original two bands. The substituted complex still has $6+6+2(2)+2=18$ valence <electrons>. No numerical frequency shift follows from <electron counting> alone.
Back to article page
