Isomerism– Real 3D Effects

Given a large pile of bricks, there exists an almost infinite number of ways in which they can be assembled into a three-dimensional shape. Likewise, on the molecular scale, one of the inevitable consequences of assembling three-dimensional molecules with components arranged around a central core atom is the existence of options for arrangement of those components– a phenomenon that we call isomerism. Because of the designation of a core shape for the complex limitations on ligand shapes, and connectivity and bonding rules, the number of options is far from infinite– yet not much less concerning for that. Defining the basic shape of the complex is the first stage with next the task of defining options for ligand attachment needing to be examined. In very few cases is the exercise trivial, because we need to think in three dimensions to identify possibilities. This can be illustrated in the case of a tetrahedral MA3B complex where there is only one possible outcome, yet this is not immediately obvious (Figure 4.23). Only in the case where all ligands are identical monodentates with a single common donor atom, is the situation simple and the prospect of positional isomers removed for

Figure 4.23 Despite first appearances, the molecules at the left and right are identical, since simple rotation by 120◦ around the marked axis as shown in the central view ‘converts’ left view into right view. They are not isomers, merely different orientations of the same molecule in space. The need to consider orientation issues in three dimensions complicates decisions on isomers.

a particular stereochemistry; this occurs for [CoCl₄]^2- and [Ni(OH₂)₆]²⁺ for example. Wherever more than one type of ligand is bound and even where either one type of ligand with a set of more donor atoms than required, or a number of identical chelating ligands bind, the possibility of isomers needs to be considered, although it is not the case that all those possible will always exist.

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مواضيع ذات صلة

Constitutional (Structural) Isomerism

Chameleon Complexes

Moulding a Relationship - Ligand Influences

Metallic Genetics– Metal Ion Influences

Higher Coordination Numbers (ML7 to ML9)

Six Coordination (ML6)

Five Coordination (ML5)

Four Coordination (ML4)

Three Coordination (ML3)

Two Coordination (ML2)

Complex Lifetimes– Together Forever

?Preferences– Do You Like What I Like