Chemistry · Ch 5 — Coordination Compounds

Geometric Isomerism

5.4.1

Geometric Isomerism

Geometric (geometrical) isomerism arises in heteroleptic complexes — complexes bonded to more than one kind of ligand — because of the different geometric arrangements the ligands can adopt around the central metal. It is important chiefly for coordination numbers 4 and 6.

Square planar complexes

Consider a square planar complex of the general formula $[MX_2L_2]$ , where $X$ and $L$ are unidentate ligands. The two $X$ ligands can be arranged:

adjacent to one another — the cis isomer, or
opposite to one another — the trans isomer.

This is shown for $[Pt(NH_3)_2Cl_2]$ (Fig. 5.2), which exists as cis- and trans- $[Pt(NH_3)_2Cl_2]$ .

Figure 5.2Geometrical isomers (cis and trans) of [Pt(NH₃)₂Cl₂]

Fig. 5.2 — Geometrical isomers (cis and trans) of [Pt(NH₃)₂Cl₂]

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

The figure shows two square-planar structures of the complex $\ce{[Pt(NH3)2Cl2]}$ , drawn side by side to illustrate geometrical isomerism.

Cis isomer (left): The two chloride ligands ( $\ce{Cl}$ ) are placed on adjacent corners of the square, separated by a $90^\circ$ angle. The two ammine ligands ( $\ce{NH3}$ ) are also adjacent to each other.
Trans isomer (right): The two chloride ligands are placed on opposite corners, separated by a $180^\circ$ angle. The two ammine ligands are also opposite each other.

The physical idea is that in square-planar complexes with the formula $\ce{[Ma2b2]}$ (where a and b are different unidentate ligands), the ligands can occupy positions that are either cis (same ligands next to each other) or trans (same ligands across from each other). These are distinct compounds with different physical and chemical properties, even though they have the same molecular formula.

The key formula the textbook develops with this figure is the general condition for geometrical isomerism in square-planar complexes:

\text{For } \ce{[Ma2b2]} \text{ (square planar): cis and trans isomers exist.}

Here:

$\ce{M}$ = central metal ion (here, $\ce{Pt^{2+}}$ )
$\ce{a}$ and $\ce{b}$ = two different unidentate ligands (here, $\ce{NH3}$ and $\ce{Cl}$ ) …

A more substituted square planar complex of the type $MABXL$ (where $A$ , $B$ , $X$ , $L$ are all different unidentate ligands) shows three isomers — two cis forms and one trans form.

Geometrical isomerism of this kind is not possible for a tetrahedral geometry, because in a tetrahedral arrangement the relative positions of the unidentate ligands attached to the central atom are the same with respect to one another — there is no distinct "adjacent" versus "opposite" arrangement to distinguish.

Octahedral complexes

Similar cis–trans behaviour occurs in octahedral complexes of formula $[MX_2L_4]$ , where the two $X$ ligands may be oriented cis or trans to each other — illustrated for the ion $[Co(NH_3)_4Cl_2]^+$ (Fig. 5.3).

Figure 5.3Geometrical isomers (cis and trans) of [Co(NH₃)₄Cl₂]⁺

Fig. 5.3 — Geometrical isomers (cis and trans) of [Co(NH₃)₄Cl₂]⁺

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

The figure shows two octahedral complexes of cobalt(III) with the formula $[\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+$ . Each complex has a central $\text{Co}^{3+}$ ion surrounded by six ligands: four neutral ammonia molecules ( $\text{NH}_3$ ) and two chloride ions ( $\text{Cl}^-$ ). The overall charge is $+1$ .

The two diagrams represent the cis and trans isomers. In the cis isomer, the two chloride ligands occupy adjacent positions on the octahedron, meaning the $\text{Cl–Co–Cl}$ bond angle is $90^\circ$ . In the trans isomer, the two chloride ligands are opposite each other, with a $\text{Cl–Co–Cl}$ bond angle of $180^\circ$ . The four $\text{NH}_3$ ligands fill the remaining coordination sites.

The physical idea taught is geometric isomerism in octahedral complexes of the type $[\text{Ma}_4\text{b}_2]$ , where a and b are different unidentate ligands. The arrangement of the two b ligands determines the isomer: adjacent (cis) or opposite (trans). These isomers have different physical and chemical properties (e.g., polarity, reactivity) but the same molecular formula.

The key formula the textbook develops with this figure is the general condition for geometric isomerism in octahedral complexes: for $[\text{Ma}_4\text{b}_2]$ , two isomers exist (cis and trans). For $[\text{Ma}_3\text{b}_3]$ , two isomers also exist — facial (fac) and meridional (mer) — as shown in Fig. 5.5. No formula is explicitly given, but the concept is summarised by the notation:

[\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+ \quad \text{cis and trans}

where:

$\text{Co}$ = cobalt(III) central metal ion
$\text{NH}_3$ = ammonia (neutral ligand)
$\text{Cl}$ = chloride ion (anionic ligand) …

The same cis–trans possibility exists when the complex carries didentate ligands $L\text{–}L$ — for example ethane-1,2-diamine, $H_2N\text{–}CH_2\text{–}CH_2\text{–}NH_2$ (abbreviated en) — present in a complex of formula $[MX_2(L\text{–}L)_2]$ , as shown for $[CoCl_2(en)_2]$ (Fig. 5.4).

Figure 5.4Geometrical isomers (cis and trans) of [CoCl₂(en)₂]

Fig. 5.4 — Geometrical isomers (cis and trans) of [CoCl₂(en)₂]

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

The figure shows two octahedral complexes of the formula $[CoCl_2(en)_2]^+$ , where en is the bidentate ligand ethylenediamine ( $NH_2CH_2CH_2NH_2$ ). Each complex has a central cobalt ion (likely $Co^{3+}$ ) surrounded by six donor atoms: two chloride ions ( $Cl^-$ ) and four nitrogen atoms from two en ligands. The en ligands are drawn as arcs connecting two nitrogen donor sites, indicating that each en molecule occupies two adjacent coordination positions (a chelate ring).

The two complexes are labelled cis and trans, referring to the relative positions of the two chloride ligands:

In the cis isomer, the two Cl atoms are adjacent (at 90° to each other), occupying neighbouring vertices of the octahedron.
In the trans isomer, the two Cl atoms are opposite each other (at 180°), at opposite ends of the octahedron.

The physical idea taught is geometrical isomerism in octahedral complexes with bidentate ligands. Because the en ligand is bidentate and forms a chelate ring, it forces the two nitrogen atoms to be cis to each other. This restricts the possible arrangements of the two Cl atoms, leading to only two distinct geometrical isomers: cis and trans. The figure illustrates that even though the formula is the same, the spatial arrangement of ligands differs, giving isomers with different physical and chemical properties.

The key formula the textbook develops with this figure is the general formula for complexes showing this type of isomerism:

[MX_2(L-L)_2]

where:

$M$ = central metal ion (here $Co$ )
$X$ = unidentate ligand (here $Cl$ )
$L-L$ = bidentate ligand (here $en$ ) …

Facial and meridional isomers

A further, distinct type of geometrical isomerism occurs in octahedral coordination entities of the type $[Ma_3b_3]$ , such as $[Co(NH_3)_3(NO_2)_3]$ , where three ligands of one kind and three of another are present. …

Figure 5.5The facial (fac) and meridional (mer) isomers of [Co(NH₃)₃(NO₂)₃]

Fig. 5.5 — The facial (fac) and meridional (mer) isomers of [Co(NH₃)₃(NO₂)₃]

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

The figure shows two octahedral complexes of the type $\ce{[Co(NH3)3(NO2)3]}$ , where the central cobalt ion is surrounded by three ammine ( $\ce{NH3}$ ) ligands and three nitrito ( $\ce{NO2}$ ) ligands. The diagram presents the two possible geometric arrangements of these six ligands around the octahedron.

In the facial (fac) isomer, all three $\ce{NO2}$ ligands occupy three positions that form a triangular face of the octahedron. The three $\ce{NH3}$ ligands occupy the opposite triangular face. This arrangement gives the complex a $C_{3v}$ symmetry — the three identical ligands are all adjacent to each other, sharing a common face.

In the meridional (mer) isomer, the three $\ce{NO2}$ ligands lie along a meridian of the octahedron: two are trans to each other (opposite positions), and the third is at a position perpendicular to the line joining them. The three $\ce{NH3}$ ligands similarly occupy the remaining three positions, also lying along a meridian. This arrangement has $C_{2v}$ symmetry.

The physical idea taught is that in an octahedral complex of the type $\ce{[Ma3b3]}$ (where a and b are different unidentate ligands), the three identical ligands can be placed either all on one face (fac) or along a great circle (meridian) of the octahedron (mer). These are distinct geometric isomers — they have the same chemical formula but different spatial arrangements, leading to different physical and chemical properties (e.g., dipole moments, reactivity). …

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