12.5.2: Substitution in cis-en octahedral complexes

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The \(D\) substitution reactions of cis complexes occur through the same intermediate structures described above for trans complexes. For example, the reaction,

\[\ce{cis{-}[M(en)2LX]^+ + Y^- -> [Co(en)2LY]^2+ + X^-} \nonumber \]

can proceed through square planar or trigonal bipyramidal intermediates. Complete the exercise below.

Exercise \(\PageIndex{1}\)

Follow the example of Figure 12.5.1.1 to sketch the possible reaction intermediates and products for \(\ce{cis{-}[M(en)2LX]^+ + Y^-}\). Which intermediates are most likely? In the case of each trigonal bipyramid, what would be the ratios of products if ligand attack from the three angles along the trigonal plane are equally likely?

Answer: Add texts here. Do not delete this text first.

Table \(\PageIndex{1}\) gives the % of cis stereochemistry in the displacement of \(\ce{Cl^-}\) by either \(\ce{H2O}\) or \(\ce{OH^-}\) in a series of \(\ce{cis-[Co(en)2LCl]^+}\) complexes where the identity of the trans ligand, \(\ce{L^-}\), is varied.

The reactions run under acidic conditions is shown below, and its data is shown in the middle column in Table \(\PageIndex{1}\) :

\[\pm \ce{cis-[Co(en)2LCl]^+ + H2O <==> [Co(en)2L(H2O)]^2+ + Cl^-} \nonumber \]

The reactions run under basic conditions is shown below, and its data is shown in the right column in Table \(\PageIndex{1}\). Note that in all but one case, the reactions run under basic conditions were done so using optically pure starting material, and gave an optically active product.

\[\Delta-\ce{cis-[Co(en)2LCl]^+ + OH^- <==> [Co(en)2L(OH)]^+ + Cl^-} \nonumber \]

Table \(\PageIndex{1}\): % cis product under different conditions
\(\ce{L^-}\)	% cis product under acidic conditions from racemic cis reactant (stereochemistry of reactant is retained)	% cis and trans product under basic conditions from \(\Delta-cis\) starting material (stereochemistry of reactant is retained)
\(\ce{OH^-}\)	100	61% \(\Delta-cis\) \| 36% \(\Lambda-cis\) \| 3% trans
\(\ce{NCS^-}\)	100	56% \(\Delta-cis\) \| 24% \(\Lambda-cis\) \| 20% trans
\(\ce{Br^-}\)	100	-
\(\ce{Cl^-}\)	100	21% \(\Delta-cis\) \| 16% \(\Lambda-cis\) \| 63% trans
\(\ce{NH3^-}\)	-	60% \(\Delta-cis\) \| 24% \(\Lambda-cis\) \| 16% trans
\(\ce{NO2^-}\)	100	46% \(\Delta-cis\) \| 20% \(\Lambda-cis\) \| 34% trans
Data from Miessler, G. L.; Fischer, P. J.; Tarr, D. A. Inorganic Chemistry, 5th Ed. Pearson: Boston, 2014, p. 459.

Under acidic conditions, and when the conjugate base mechanism is unlikely, the product specifically retains the stereochemistry of the starting material. This data (middle column above) indicates that the \(D\) reaction of the cis isomer, in contrast to trans, reacts exclusively through the square pyramidal intermediate under acidic conditions. This is because in every case, the ligand trans to the X leaving group is an amine from the en ligand, which is not a \(\pi\) donor. The ligand cis to the leaving group (L) has no influence on the reaction outcome under acidic conditions.

Under basic conditions, where the conjugate base mechanism is possible, cis stereochemistry is not entirely retained, and the cis products are produced in approximately a 2:1 ration of \(Delta:\Lambda\) from a \(Delta\) reactant.

Exercise \(\PageIndex{1}\)

Consider the data in the right colum of Table \(\PageIndex{1}\). What does the presence of trans product and a 2:1 ratio of the two cis isomers indicate about the intermediates of reaction under basic conditions?

Answer: Add texts here. Do not delete this text first.