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

Complexes of alkynes
alkoxy group. On protonation the product loses alcohol to give a cationic complex
in which the ether group has been replaced by the nucleophile. The new vinyl
derivative is released from iron by treatment with Nai in acetone. IfNuH = R'OH,
the alkyl groups in a vinyl ether can be exchanged. Less trivially. the method has
been used to make vinyl derivatives of enolates
# a
The molecular structure of Fp(H 2 C=CH0Et)+. BF,;. determined by X-ray
diffraction, shows unsymmetrical bonding ofthe vinyl group to iron. The distance
Fe-C, (2.32 Ă) is significantly longer than Fe-C~ (2.09 Ă). This, together with
the rather long C-C bond (1.42 Ă), suggests that the charge distribution may be
described in terms of contributions from the canonica! structures Fp-CH 2 CHOEt
and Fp-CH 2 CH=0Et.
+
+
7.4 Complexes of alkynes
An alkyne possesses two pairs of orthogonal n and n* molecular orbitals. In many
alkyne complexes, the major interactions are analogous to those found in alkene
derivatives, viz. O"-donation from a filled bonding (n ) m.o. on the ligand to the
metal and n-donation from filled metal orbitals. In addition there can be
interaction between the filled n j_ orbital ofthe ligand and M(dxy), so that an alkyne
can function as an-donor as well as a n-acceptor. b-Overlap between M(dyz) and
n: is also permitted on symmetry grounds, although it is thought not to be
energetically very significant (Fig. 7.10).
As with alkenes, two extreme representations of the bonding in alkyne
complexes can be drawn, in which the alkyne is formally a two-electron donor
(p. 19 7). Arylalkynes form a series of stable complexes with platinum of
stoichiometry (Alkyne)Pt(PPh,) 2 • The infrared spectrum of the diphenylethyne
complex shows a C-C stretching band at 1750cm- 1 (cf. vC C in C 2 H 2 ,
2100cm - 1 and vC=C in C 2 H 4 • 162 3 cm - 1 • Raman). The X-ray crystal structure
reveals a planar P 2 PtC 2 skeleton. The phenyl groups are bent back considerably
from linearity. This observation, together with the fact that the C -C bond length
is close to that of a typical double bond (1.33 Â) suggests that the structure can
243