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

Allyl and diene complexes
doublets. 33% ofthe molecules contain 195 Pt (I =il which couples to the adjacent
13 C nucleus. Note that the meso carbon C 2 resonates to low field of the terminal
carbon nuclei C 1 , C'.
Trialkylphosphines add to M(C 3 H 5 ) 2 (M =Ni, Pd, Pt) giving 1:1 adducts in
which the two allyl groups are cis to each other. The n.m.r. spectra reveal
dynamic IJ 3 ~1J 1 interconversions between -100° and 0°C. Addition of excess
L(L = PR,, CO) cleaves the allyl groups giving NiL 4 • This property has been
exploited in catalysis (p. 3 6 7).
8.1.2 Preparation of 11'-allyl complexes
The methods ofpreparation of allyl complexes are in general similar to those used
for 1] 1 -alkyls. If a free coordination site is available, however, any 1] 1 -allyl which
may form initially is converted into the 1] 3 -allyl. Binary allyls of most of the
transition elements have been prepared by reacting allyl Grignard reagents with
the anhydrous metal halide in ether at low temperatures.
MXn + n al!MgX-----> Mall"+ n MgX 2
(ali= C,H, or substituted allyl)
They include Mall 4 (M = Zr, Hf. Th, U, Mo, W), Mall, (M =V. Cr, Fe, Co, Rh, Ir)
and Mall 2 (M =Ni, Pd, Pt). Binary allyls are generally very air sensitive, ha ve low
thermal stability and are labile towards displacement of allyl groups.
Bis(allyl)nickel forms orange yellow crystals, m.p. ooc after recrystallization
from pentane. Its molecular structure has been determined by neutron diffraction
at 100 K, which locates the hydrogen atoms precisely, as well as by high
resolution X-ray methods. It has a sandwich structure in which the allyl groups
are arranged trans to each other, that is, with the arrowheads pointing in opposite
directions. The syn hydrogen atoms are bent 8.9° out ofthe allyl plane towards
the metal and the anti hydrogens 29.4° away from the metal. This distortion
reduces non-bonded interactions between the two an ti hydrogens which would
arise in a planar allyl group. Moreover the C(2p)n orbitals are better directed
Ni(C0)4
l all Br
Fig. 8.5 Preparation and reactions of IJ'-allylnickel halides.
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