Two Coordination (ML2)

Two coordination is the lowest stable coordination number that is well reported. It also presents the first opportunity for variation from the shape predicted in Figure 3.3. We expect a ML2 molecule to be linear, with the two donor groups disposed as far away from each other as possible on opposite ends of a line joining them and passing through the metal centre. Deviation from prediction may arise in this case simply by bond angle deformation, with the usual 180◦ L-M-L bond angle reduced to 180◦ through bending (Figure 4.4). Experimentally ML2 complexes are overwhelmingly linear.Both electron pair repulsion and simple steric arguments favour this shape. If bending occurs, it brings the two ligands closer towards each other, providing greater opportunity for repulsive interaction between the ligands; this would seem both unreasonable and unlikely, yet bent molecules do occur. ‘Bent’ geometries are well known in p-block chemistry, of course, where lone pairs play an important directional role. Water is the classical example, with its two lone pairs and two bond pairs around the oxygen centre leading to a bent H-O-H as a result of its inherently tetrahedral shape (on including lone pairs) as well as additional effects due to differing repulsions between the lone pairs and bond pairs. Where such bending is seen in metal complexes, it can often be assigned to a higher pseudo-coordination number shape with nonbonding orbitals present contributing and occupying some region of space. X-ray crystallography defines atomic centres but cannot readily detect regions of electron density that do not involve atoms. Lone pairs are effectively not observable directly; the presence of directed nonbonding electron density can only be inferred, as a result of an influence on structure through repulsive terms, seen experimentally as changes in bond angles. The ML2 geometry is rare for all but metal ions rich in d electrons, particularly d10 and d9 metal ions. This is a recurring theme in coordination number for transition metal

Figure 4.4

Possible shapes for two-coordination and (at right) an example of a linear complex cation [Au (PR3)2] + (where R = CH3).

 Figure 4.5

Examples of complexes with two-coordination including both linear and bent species.

complexes– as a very rough rule, the more electrons in the valence shell, the lower the coordination number. Complexes which are two-coordinate include those of the d10 cations Ag(I) and Au(I), for example the [Ag (NH₃)2] + and [Au (CN)₂] − complexes, which have linear N Ag N and C Au C cores respectively; a related example with a bulkier PR3 ligand is drawn in Figure 4.4. It is also believed that the simple dihalides (MX2) of most d-block metal ions in the gas phase tend to be linear two-coordinate species, although this is not generally the case in the solid or solution state. Examples of bent complexes are few in number. One of the simplest is Ag(SCN) which in the solid state forms a polymer with. . .SCN Ag SCN . . . character, and a S Ag N angle of 165◦.TheAg N CS linkage is driventowardslinearitybythemultiply-bondednitrogen,withalonepairdirected towards the metal directly opposite the multiple bond, whereas for the Ag S CN linkage the set of lone pairs on the pseudo-tetrahedral S atom influence the coordination and lead to a bent arrangement (Figure 4.5). Occasionally, very bulky ligands enforce two-coordination on metals with fewer d electrons that would usually prefer other higher coordination numbers. One of very few such compounds is the d6 Fe (II) complex of the amido anion −N(SiR3)2. The particular two coordinate example [Fe{N(Si(CH₃)(C6H5)₂)₂}₂] approaches linear shape (Figure 4.5), but the N Fe Nangleisonly∼170◦.Asthebulkofthesubstituents on the Si atoms decrease, there is a switch to higher coordination number; the anion −N(Si(CH₃)₃)₂, for example forms a three-coordinate [Fe{N(Si(CH₃)₃)₂}₃] complex.

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مواضيع ذات صلة

Constitutional (Structural) Isomerism

Isomerism– Real 3D Effects

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)

Complex Lifetimes– Together Forever

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