Metallic Genetics– Metal Ion Influences

Although a coordination complex is an assembly of central atom and donor molecules or atoms, and as such is best viewed holistically, it is still instructive to identify factors influencing shape that depend on each component. If we focus first on the central atom or ion, there are two of the four key factors mentioned earlier that we can consider metal centric. These are: the number of d electrons on the metal ion; and metal ion size and preferred metal ion–ligand donor group bond length.

Each metal in a particular oxidation state brings a unique character, almost like a gene, to play in its complexes. Examples of the way the size and bond distances vary across the d block are given in Table 4.1 reporting data for N-donor O-donor and Cl− ligands. The M-L distances are averages only as distances vary over a range of at least20pm influenced by the specific type of donor group for a particular type of donor atom, the ligand shape and associated strain energy, as well as influences of other donors in the coordination sphere (through what are trans or cis effects of an electronic nature). Moreover, the spin state of the central metal plays a role (e.g. M O distances for high spin Mn (III) are typically 20 pmlonger than for low-spin compounds).

As mentioned above, distances vary for any particular metal ion depending on the character of the donor (carboxylate-O or alcohol-O, for example, or else amine-N versus amido-N), influences from the ligand framework itself, and influences of other ligands bound to the same complex. Some modest trends are apparent, such as the fall in M L distance with increasing charge on central metal ions with the same number of d electrons. There are some more consistent trends, nevertheless; for example, the variation in bond length M-Cl>M-N>M-O is almost universally observed. Further, there is a modest relationship between bond distances and the size of the metal ion.

The increase in metal cation size from the first to the second and third row of the Periodic Table is accompanied by usually longer M-L distances, as exemplified for Co (III) Rh (III) and Ir (III) in Table 4.1. Overall, metal–donor distances fall within a range of ∼160–260 pm, with the smaller distances found where highly charged metal ions, small anionic ligands and/or multiple bonding operate. For example, the VIV O distance of ∼160 pm is markedly shorter than usual VIV–O distances of ∼185 pm. Experimentally, every metal ion–donor group unit

Figure4.21 A summary of ideal polyhedral shapes for the limiting structures for coordination numbers 2to9. More practical metal bonding representations for complexes are also shown.

Table4.1 Variation of ion size and average bond distances found for first-row d-block metal ions in common oxidation states and with six-coordinate octahedral geometry. Entries for a second and third row element to exemplify differences down a column of the Periodic Table d block appear in

Displays bond distances that vary slightly across a usually large number of ligand systems examined indicating that each assembly does have a preferred metal–donor distance.

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Constitutional (Structural) Isomerism

Isomerism– Real 3D Effects

Chameleon Complexes

Moulding a Relationship - Ligand 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

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