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

Molecular orbital theory
n overlap whatever the orientation of the alkene. This is predicted for
(alkene)M(C0) 4 , for which rotational barriers are small ( < 40 kJ mol- 1 ) arising
essentially from steric effects. Usually, however, dzx and dvz are not of equal energy
and their respective overlap with Pxn* and pyn* differs. This gives rise to a definite
preference for one of the two mutually perpendicular conformations in which
overlap is maximized.
(a) Square plenar ML 3 (alkene):(d8)
Alkene lr !o plane of PtL 3
( b) TrigonaL planor ML 2 (olkene): (diO)
ALkene in pLane of PtL 2
Evidence for restricted rotation about the metal-alkene axis in complexes is
derived from measurements of n.m.r. spectra over a range of temperature. The
1 Hn.m.r. spectra of [PtCl(C 2 H 4 )(acac)] at -4SoC and at 3SoC are shown in
Fig. 6.6. At -- 4Soc the ethene gives rise to an AA'BB' pattern, as expected from
the complex in its 'frozen' conformation similar to (a). At - 28'C the AA'BB'
multiplet coalesces and reemerges at higher temperatures as a sharp singlet. The
environment of the ethene protons is averaged on rotation through the
metastable conformation similar to (b ). The activation energy barrier to rotation
(.~Gt) is about SOkJmol- 1 • Platinum has an isotope 195 Pt ( B% natural
abundance) which has nuclear spin I = 1· The mPt- 1 H coupling appears as
satellites S. These satellites are present over the complete temperature range of
the experiment. This indicates an intramolecular process in which the ethene
remains bound to platinum throughout. If the alkene were to dissociate. satellite
structure would be lost above the coalescence temperature.
6.2.2 Bonding between other unsaturated hydrocarbons and transition
elenzents
The bonding between other hydrocarbon ligands and transition metals can be
described in similar m.o. terms. In the discussion which follows it is assumed that
the hydrocarbon O"-skeleton does not contribute to bonding with the metal. After
formation of this skeleton each carbon atom has an unused 2p, orbita!, which
combines with the other 2p, orbitals of the delocalized system to form an equal
number of n-molecular orbitals. The shapes of these m.o.s and their energies,
calculated using the simple Hiickel approximation, are given in Figs. 6. 7 and 6.8.
These m.o.s can be classified as of O", n or 6 symmetry with respect to rotation
about the z-axis. These ligand orbitals can combine only with metal orbitals of the
same symmetry.
From Fig. n.8 it can be seen that the energies ofthe n- and 3-m.o.s ofthe ligand
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