In
inorganic chemistry, the
cis effect is defined as the labilization (making unstable) of
CO ligands that are
cis to other ligands. CO is a well-known strong
pi-accepting ligand in
organometallic chemistry that will labilize in the
cis position when adjacent to ligands due to
steric and
electronic effects. The system most often studied for the
cis effect is an
octahedral complex M(CO)
5X where X is the ligand that will labilize a CO ligand
cis to it. Unlike
trans effect, where this property is most often observed in 4-coordinate
square planar complexes, the
cis effect is observed in 6-coordinate octahedral
transition metal complexes. It has been determined that ligands that are weak
sigma donors and non-pi acceptors seem to have the strongest
cis-labilizing effects. Therefore, the
cis effect has the opposite trend of the
trans-effect, which effectively labilizes ligands that are
trans to strong pi accepting and sigma donating ligands.