KEY POINT The stability of chelate complexes of d metals involving diimine ligands is a result of the chelate effect in conjunction with the ability of the ligands to act as π acceptors as well as σ donors.
Steric effects have an important influence on formation con- stants. They are particularly important in chelate formation because ring completion may be difficult geometrically.
Chelate rings with five members are generally very stable because their bond angles are near ideal in the sense of there being no ring strain. Six-membered rings are reasonably stable and may be favoured if their formation results in elec- tron delocalization. Three-, four-, and seven-membered (and larger) chelate rings are found only rarely because they nor- mally result in distortions of bond angles and unfavourable steric interactions.
Complexes containing chelating ligands with delocalized electronic structures may be stabilized by electronic effects in addition to the entropy advantages of chelation. For example, diimine ligands (81), such as bipyridine (82) and phenanthroline (83), are constrained to form five-membered rings with the metal atom. The great stability of their com- plexes with d metals is probably a result of their ability to act as π acceptors as well as σ donors and to form π bonds by overlap of the full metal d orbitals and the empty ring π*
orbitals (Section 20.2). This bond formation is favoured by electron population in the metal t2g orbitals, which allows the metal atom to act as a π donor and transfer electron density to the ligand rings. An example is the complex
[Ru(bpy)3]2+ (84). In some cases the chelate ring that forms can have appreciable aromatic character, which stabilizes the chelate ring even more.
N N
M
N N
81 82 bpy diimine metal complex
N N
83 phen
Ru
bipy
2+
84 [Ru(bpy)3]2+
Box 7.3 describes how complicated chelating and macro- cyclic ligands might be synthesized.
BOX 7.3 How can we make rings and knots?
A metal ion such as Ni(II) can be used to assemble a group of ligands that then undergo a reaction among themselves to form a macrocyclic ligand, a cyclic molecule with several donor atoms. A simple example is
NH NH
NH2
NH2
O H
H O
Ni2+
N
N N
N H
H Ni
2+
This phenomenon, which is called the template effect, can be applied to produce a surprising variety of macrocyclic ligands.
The reaction shown above is an example of a condensation reaction, in which a bond is formed between two molecules, and a small molecule (in this case H2O) is eliminated. If the metal ion had not been present, the condensation reaction of the
component ligands would have been an ill-defined polymeric mixture, not a macrocycle. Once the macrocycle has been formed, it is normally stable on its own, and the metal ion may be removed to leave a multidentate ligand that can be used to complex other metal ions.
A wide variety of macrocyclic ligands can be synthesized by the template approach. Two more complicated ligands are shown.
Zn2+
N H
N N
N
N O
H
Zn
CN
CN
N
N N
N N
N N
N Cu
Cu2+
The origin of the template effect may be either kinetic or thermodynamic. For example, the condensation may stem either from the increase in the rate of the reaction between coordinated ligands (on account of their proximity or electronic effects) or from the added stability of the chelated ring product.
More complicated template syntheses can be used to construct topologically complex molecules, such as the chain- like catenanes, molecules that consist of interlinked rings. An example of the synthesis of a catenane containing two rings is shown below.
1 Knotted and linked systems are far from being purely of academic interest and many proteins exist in these forms: see C. Liang and K. Mislow, J. Am. Chem. Soc., 1994, 116, 3588 and 1995, 117, 4201.
N N
OH
OH 2
N N
OH
OH N
N HO
HO
Cu
N N
O
O N
N O
O 2 ICH2(CH2OCH2)4CH2I Cu
O
O O
O
O O O
O
+ +
Cu+
base
Here, two bipyridine-based ligands are coordinated to a copper ion, and then the ends of each ligand are joined by a flexible linkage. The metal ion can then be removed to give a catenand (catenane ligand), which can be used to complex other metal ions.
Even more complicated systems, equivalent to knots and links,1 can be constructed with multiple metals. For instance, the following synthesis gives rise to a single molecular strand tied in a trefoil knot:
2
N N
OH N N
HO
Cu
2 ICH2(CH2OCH2)5CH2I 2+
N N
OH
N N HO
Cu
2+
N N
N
N
OH
OH 2 Cu+
N N
O N N
O
Cu N N
O
N N O
Cu
O O O O O O
O
O
O
O base
Work on these and related systems was rewarded in 2016 with the award of the Nobel Prize for Chemistry to J.-P. Sauvage,
J.F. Stoddart, and B. Feringa ‘for the design and synthesis of molecular machines’.
FURTHER READING
G.B. Kauffman, Inorganic coordination compounds. John Wiley &
Sons (1981). A fascinating account of the history of structural coordination chemistry.
G.B. Kauffman, Classics in coordination chemistry. I. Selected pa- pers of Alfred Werner. Dover (1968). Provides translations of Werner’s key papers.
G.J. Leigh and N. Winterbottom (ed.), Modern coordination chem- istry: the legacy of Joseph Chatt. Royal Society of Chemistry (2002). A readable historical discussion of this area.
A. von Zelewsky, Stereochemistry of coordination compounds.
John Wiley & Sons (1996). A readable book that covers chirality in detail.
J.A. McCleverty, and T.J. Meyer (eds), Comprehensive coordina- tion chemistry II. Elsevier (2004).
N.G. Connelly, T. Damhus, R.M. Hartshorn, and A.T. Hutton, Nomenclature of inorganic chemistry: IUPAC recommendations 2005. Royal Society of Chemistry (2005). Also known as ‘The IUPAC red book’, the definitive guide to naming inorganic com- pounds.
R.A. Marusak, K. Doan, and S.D. Cummings, Integrated approach to coordination chemistry: an inorganic laboratory guide. John Wiley & Sons (2007). This unusual textbook describes the con- cepts of coordination chemistry and illustrates these concepts through well-explained experimental projects.
J.-M. Lehn (ed.), Transition metals in supramolecular chemistry, Volume 5 of Perspectives in supramolecular chemistry. John Wiley & Sons (2007). Inspiring accounts of developments and applications in coordination chemistry.
EXERCISES
7.1 Name and draw structures of the following complexes:
(a) [Ni(CN)4]2−, (b) [CoCl4]2−, (c) [Mn(NH3)6]2+.
7.2 Give formulas for (a) chloridopentaamminecobalt(III) chloride, (b) hexaaquairon(3+) nitrate, (c) cis-dichloridobis(1,2- diaminoethane)ruthenium(II), (d) à-hydroxidobis(pentaammine- chromium(III)) chloride.
7.3 Name the octahedral complex ions (a) cis-[CrCl2(NH3)4]+, (b) trans-[Cr(NH3)2(κN-NCS)4]−, (c) [Co(C2O4)(en)2]+.
7.4 (a) Sketch the two structures that describe most four- coordinate complexes. (b) In which structure are isomers possible for complexes of formula MA2B2?
7.5 Sketch the two structures that describe most five-coordinate complexes. Label the two different sites in each structure.
7.6 (a) Sketch the two structures that describe most six- coordinate complexes. (b) Which one of these is rare?
7.7 Explain the meaning of the terms monodentate, bidentate, and tetradentate.
7.8 What type of isomerism can arise with ambidentate ligands?
Give two examples.
7.9 What is the denticity of the following molecules? Which could act as bridging ligands? Which could act as chelating ligands?
PMe2 Me2P
N N
N N
HN H2N
H2N
NH NH
N H
NH
7.10 Draw the structures of representative complexes that contain the ligands (a) en, (b) ox2−, (c) phen, (d) 12-crown-4, (e) tren, (f) terpy, (g) edta4−.
7.11 The two compounds [RuBr(NH3)5]Cl and [RuCl(NH3)5]Br are what types of isomers?
7.12 For which of the following tetrahedral complexes are isomers possible? Draw all the isomers. [CoBr2Cl2]−, [CoBrCl2(OH2)], [CoBrClI(OH2)].
7.13 For which of the following square-planar complexes are isomers possible? Draw all the isomers. [Pt(NH3)2(ox)], [PdBrCl(PEt3)2], [Ir(CO)H(PR3)2], [Pd(gly)2].
7.14 For which of the following octahedral complexes are isomers possible? Draw all the isomers. [FeCl(OH2)5]2+, [IrCl3(PEt3)3], [Ru(bpy)3]2+, [CoCl2(en)(NH3)2]+, [W(CO)4(py)2].
7.15 How many isomers are possible for an octahedral complex of general formula [MA2BCDE]? Draw all that are possible.
7.16 Which of the following complexes are chiral? (a) [Cr(ox)3]3−, (b) cis-[PtCl2(en)], (c) cis-[RhCl2(NH3)4]+, (d) [Ru(bpy)3]2+, (e) fac-[Co(NO2)3(dien)], (f) mer-[Co(NO2)3(dien)]. Draw the enantiomers of the complexes identified as chiral and identify the plane of symmetry in the structures of the achiral complexes.
7.17 Which isomer is the following tris(acac) complex?
O Mn O O O
O O
7.18 Draw and label both Λ and Δ isomers of the [Ru(en)3]2+
cation.
7.19 The stepwise formation constants for complexes of NH3 with [Cu(OH2)6]2+(aq) are log Kf1 = 4.15, log Kf2 = 3.50, log Kf3 = 2.89, log Kf4 = 2.13, and log Kf5 = −0.52. Suggest a reason why Kf5 is so different.
7.20 The stepwise formation constants for complexes of NH2CH2CH2NH2 (en) with [Cu(OH2)6]2+(aq) are log Kf1 = 10.72 and log Kf2 = 9.31. Compare these values with those of ammonia given in Exercise 7.19 and suggest why they are different.