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

Complexes of alkynes
Here the alkyne is attacked by nucleophiles, like the alkenes in Pp( alkene) + or the
alkynes in the analogous [CpFe(CO)(PPh 3 )(RC-CR)] +.
When both n-systems of the alkyne are significantly involved in bonding to a
metal. simple electron counting is no longer helpful. Metal dn orbitals which
interact with strongly n-donating ligands can be raised in energy to such an
extent that they remain empty. In some cases e.g. [CpMoL 2 (RC=CR)] +, an
apparently correct 18-electron count can be obtained by including the n­
donating electron pair, making the alkyne a formal four-electron donor. In others
e.g. [CpMo(CO)(RC=CR) 2 ] + this is confusing, because the n-donor orbitals of
both alkyne ligands interact with the same metal d-orbital.
The preparation ofthe complexes just cited is given below. They undergo some
unusual reactions with nucleophiles.
(?D
L=P(OMe)3
R'-... 9. /R
\II-Mo-111
R'/ L "-R'
c
o
(?D
~~Ph
L/J
[ 19
There are also complexes in which an alkyne forms a bridge between two metal
atoms. To a first approximation, one ofthe n-systems interacts with the first metal
atom and the other with the second (Fig. 7.11 ). The cobalt compounds are
prepared in high yield by reacting alkynes with Co 2 (C0) 8 • The cobalt can be
removed cleanly by mild oxidation. This provides a useful procedure for
protecting an alkyne during organic synthesis.
Ph
Me
245