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

Allyl and diene complexes
signals arising from H' and H 4 can be assigned on this hasis. H 2 gives rise ta a
multiplet.
The spectrum of ( IJ 3 -C 3 H 5 )Mn( CO) 4 can be interpreted in terms of a symmetrical
1] 3 -group. There are three different proton environments; Hm (meso) attached ta
the central carbon, H' (syn) and Ha (an ti). The an ti protons are nearer to the metal
centre than the syn protons and hence are more strongly shielded. The signal to
highest field is therefore assigned to Ha (doublet). This is confirmed from the
coupling constants. The trans coupling 'J(H"Hm) is greater (12.2Hz) than the
cis coupling 'J(H'Hm) (7.4Hz). Small geminal 2 ](H.H') and W 4 ](H'H") couplings
are often resolved in these spectra. Hm appears at low field as a multiplet,
a triplet of triplets. (If the difference in chemical shift between two resonances is of
the same order of magnitude as the coupling constant, second order spectra,
which do not conform to the first order treatment given here, are obtained. This is
a general effect which is more likely to be observed using smaller spectrometers
operating at low frequency (e.g. 60 or 100 MHz) than with modern high
frequency instruments (e.g. 2 50 or 400 MHz). Chemical shifts, measured in Hz,
are proportional to operating frequency, whereas coupling constants are
independent of frequency.)
Protons on opposite sides of the allyl group are distinguished in the spectra of
some complexes. This can reflect actual asymmetry in bonding as in
(Ph 3 P)PdCl(IJ 3 -2-MeC,H 4 ). It can, however, merely be caused by slightly different
environments of the two syn or an ti protons in a molecule of low symmetry. As a
general guide ta assignment of spectra of 17-bonded ligands, the proton resonances
normally lie progressively to lower field on going from the front (i.e. H•, H') ta the
back of the ligand.
The spectrum of (IJ 3 -C 3 H 5 )Mn(C0) 4 conforms to the A 2 B 2 X pattern. Some allyl
complexes show this pattern at low temperatures, but as the tempera ture is raised
the resonances broaden, finally coalescing to an A 4 X spectrum in which the syn
and anti protons show an averaged signal. Such observations indicate that
dynamic processes are proceeding at a rate comparable with the lifetime of the
nuclear spin states.
The main mechanism responsible for such changes involves IJ 3 ~1J 1 ~1J'
interconversions. Formation ofthe 1] 1 -allylleaves a vacant coordination site. This
can be occupied by a solvent molecule or other ligand. 17' ~'7 1 interchange is often
induced by the addition of ligands or coordinating solvents which stabilize the IJ 1
intermediate (Fig. 8.2).
Another type of dynamic process, rotation of the allyl group about the metal
ligand axis, bas also been observed in certain complexes. In this case interchange
of syn and anti protons does not occur. The 1 H n.m.r. spectrum of
tris(allyl)rhodium shows three separate A 2 B 2 X patterns at - 7 4 °C, o ne from each
ofthree inequivalent allyl groups. On warming to - lOoC two ofthese coalesce. It
is suggested that rotation of o ne of the allyl groups a baut the allylrhodium axis
occurs making the other two allylligands equivalent (Fig. 8.3).
Bis(allyl)nickel and the corresponding palladium and platinum compounds
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