Moulding a Relationship - Ligand Influences

If we focus next on the bound donor molecules or ions there are also two of the four key factors mentioned earlier that we can consider ligand-centric. These are:

⚫ ligand-ligand repulsion forces; and

⚫ ligand rigidity or geometry.

Obviously, each ligand is unique in its shape and size. The effect of repulsion between ligands, which we can term nonbonding interactions, can usually be recognized readily in the solid state from distortions in shape as defined by crystal structure analysis. Two PH₃ molecules may bind in a square planar shaped complex with little preference for trans over cis geometry, whereas two very bulky P(C₆H₅)₃ molecules may exhibit strong preference for coordination in a trans geometry, where they are much further apart. Another way that two cis-disposed bulky ligands can relieve ligand-ligand repulsion is for the complex to undergo distortion from square planar towards tetrahedral, which leads to the two ligands moving further apart in space; this is also observable in the solid state and, from spectroscopic behaviour, in solution. However, there are also weak structure-making effects arising from favourable hydrogen bonding interactions that can assist in stabilizing a particular shape. This might occur where a carboxylate group oxygen participates in a hydrogen bond with

an amine hydrogen atom arising from the interaction

Such directed close contacts can actually be observed in the crystal structure also, but distances are longer than formal covalent bonding distances and the bonds much weaker. Some ligands are structurally so rigid that they can bind to a metal ion in only one manner. The aromatic porphyrin molecule is completely flat, and large amounts of energy are required to distort it. Therefore, when it binds to a metal, it seeks to retain this shape, and will simply use the square-shaped array of four N-donors to wrap around a metal ion in a planar manner. Not only aromatic ligands are rigid; some polycyclic fused-ring aliphatic molecules may be sufficiently rigid and require a particular shape. An example of a cyclic rigid four nitrogen donor ligand that can bind effectively only with the square-shaped array of four N-donors in the plane about the metal ion (Figure 4.22) contrasts with the flexible aliphatic ligand also shown which can bind in a flat or ‘bent’ arrangement. Even ligands where the donors do not form part of a ring are affected by rigidity; the tridentate ligand at the right in the figure is conjugated and must preferentially remain flat, limiting the way

Figure 4.22

Rigid and flexible ligands influence the way they bind to metal ions. The more rigid ligands on the right do not permit folding, whereas the more flexible ones on the left can accommodate folding and thus offer options when coordinating to an octahedral metal ion. it can bind all donors, whereas the analogue on the left with a flexible saturated pendant chain, can adjust to flat or bent coordination modes.

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Constitutional (Structural) Isomerism

Isomerism– Real 3D Effects

Chameleon Complexes

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