Chapter 1: Introduction: What Is a Metal Complex?

A metal complex is a neutral or ionic compound in which metal ions or metal atoms are surrounded by so-called ligands such as organic molecules or inorganic ions, and is usually called a coordination compound because ‘metal complex’ is somewhat ambiguous. Many metal complexes were already known before the 18th century, and were called complex salts because they were usually obtained as complicated salts; for example, [Co(NH₃)₆]SO₄, [Cu(NH₃)₄]SO₄·H₂O, [Pt(NH₃)₄]Cl₂, K₄[Fe(CN)₆]·3H₂O (yellow prussiate), K₃[Fe(CN)₆] (red prussiate), K[PtCl₃(C₂H₄)]·H₂O (Zeise’s salt), [Pt(NH₃)₄][PtCl₄] (Magnus’s salt), for which there was no knowledge of their structures at the time and, on the contrary, structures were proposed that are not credible from the view point of modern chemistry. It was Alfred Werner who elucidated the structures of the complex salts (see Box 1.1). He published his coordination theory in 1893 and deduced that in complex salts such as those of Co(iii), Fe(iii) and Pt(iv) the metal is surrounded by six groups of atoms forming an octahedron. He also deduced that complex salts containing Pt(ii) have a four-coordinate planar structure surrounded by four groups of atoms. Subsequently, he carried out meticulous experiments to separate the isomers of complex salts and proved his hypothesis correct, for which he was awarded the Nobel Prize in Chemistry in 1913. Since then, the chemistry of coordination compounds has made great progress, and the research field of coordination chemistry has been established. Metal complexes with typical coordination bonds to NH₃, H₂O, and halide ions, which Werner studied, are sometimes referred to as ‘Werner-type metal complexes’. In Japan, the term ‘chemistry of complex salts’ was used until the second half of the 20th century, but since coordination compounds include not only complex ions but also neutral complexes, ‘metal complex chemistry’ is now used (in English ‘coordination chemistry’ is usually used). The bond between a metal and a ligand in a coordination compound is called a coordination bond, which is formed by the donation of a lone pair of electrons from the ligand to an empty orbital of the metal, and is written as M ← L with an arrow indicating a two-electron donor from the ligand (Figure 1.1a). When NH₄⁺ is formed by the reaction between NH₃ and H⁺, the coordination bond (dative bond) of H⁺ ← NH₃ is considered as a donation of the lone pair of ammonia to the empty orbital of H⁺ (Figure 1.1b). In such a bond, the metal ion can be considered as a Lewis acid (electron pair acceptor) and the ligand as a Lewis base (electron pair donor), based on the Lewis definition of acid and base.

Although Werner’s coordination theory did not provide an understanding of the electronic structure of metal complexes, crystal field theory, in which the ligand is a point charge or electric dipole, and ligand field theory, which incorporates electron repulsion and molecular orbital methods, have since been developed. More recently, advanced theoretical calculations (molecular orbital and density functional theory) have provided a systematic and detailed understanding for the electronic structure of metal complexes. In particular, d- and f-block transition metal complexes can assume a variety of oxidation and energetic states because the valence electrons are not directly involved in the bonding, and as a result a wide range of diverse functions and reactivity have been investigated, including electronic absorption and emission, magnetism, redox-induced electron transfer, energy-transfer systems, and so on. Some of these properties are related to biological functions, and research using such metal complexes has established a new field of research called bioinorganic chemistry. The theories of electron-transfer mechanisms between metal complexes, as elucidated by H. Taube (Nobel Prize in Chemistry 1983) and R. A. Marcus (Nobel Prize in Chemistry 1992), have played an important role in the development of metal complex chemistry. C. J. Pedersen, J.-M. Lehn and D. J. Cram synthesised macrocyclic ligands such as crown ethers, cryptands and spherands, respectively, and discovered the beginnings of molecular recognition and host–guest chemistry through these metal complexes (Nobel Prize in Chemistry 1987). These discoveries quickly led to the creation of a new field of supramolecular chemistry, which has encompassed various interdisciplinary fields and has now led to the research of metal complex-based molecular devices, molecular machines, molecular flasks, nano-spatial technology, etc. The 2016 Nobel Prize in Chemistry was awarded to J.-P. Sauvage, J. F. Stoddart and B. Feringa for their pioneering work in the design and synthesis of molecular machines. In particular, Sauvage used metal complexes to create catenanes, a new molecular motif of interlocked rings. Porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) have also been developed as materials that actively use nano-spatial technology for gas separation and catalytic reactions.

Although metal complex chemistry (coordination chemistry) is still developing, some complex salts are already studied in high school chemistry curricula. For example, when a small amount of aqueous ammonia is added to an aqueous solution containing Ag⁺, Cu²⁺ and Zn²⁺ ions then Ag₂O, Cu(OH)₂ and Zn(OH)₂ are precipitated, respectively, but all are dissolved when excess ammonia water is added. This is due to the formation of complex ions, [Ag(NH₃)₂]⁺, [Cu(NH₃)₄]²⁺, [Zn(NH₃)₄]²⁺, which have two-coordinate linear, four-coordinate planar and tetrahedral structures, respectively. However, it is not explained why such structures are formed. Also, [Ag(NH₃)₂]⁺ and [Zn(NH₃)₄]²⁺ are colourless in solution, whereas [Cu(NH₃)₄]²⁺ is deep blue. We do not even think about why there is a colour difference. K₄[Fe(CN)₆], which is called yellow prussiate, contains an octahedral complex ion [Fe(CN)₆]⁴⁻ and dissolves in water to give a light yellow colour, but when an aqueous solution containing Fe³⁺ ions is added a dark blue precipitate called Prussian blue or Berlin blue is formed. The colour change is a mysterious reaction, but we do not learn what kind of reaction takes place, what compounds are produced and why they turn dark blue. The reason for this is that to understand these reactions you need to know the electronic state of the metal complexes, which is quite difficult for high school students. I would like to say that the rest of the subject should be covered by inorganic chemistry at university, but it is surprisingly often the case that coordination chemistry is only studied in the upper classes, and even then only for a little while. We hope that you will read this book and clear up any doubts you may have.

Organometallic compounds are metal complexes with metal–carbon bonds, where the donor carbon atom is part of an organic molecule or organic group. However, although carbon monoxide is not an organic compound, it has often been used in reactions of organometallic compounds, so metal complexes containing carbon monoxide (called carbonyl complexes) are also included in organometallic compounds (carbonyl complexes often have similar properties to organometallic compounds). Cyanide complexes are not included in organometallic compounds, although the carbon of CN⁻ is bonded to a metal. Organometallic compounds have been synthesised for quite some time, such as Zeise’s salt, [Ni(CO)₄], alkyl zinc compounds and Grignard reagents, but the details of their structures and metal–carbon bonding had been largely unknown, as interest was focused on their application to organic reactions, until the epochal discovery of ferrocene. In 1951, S. A. Miller and P. L. Pauson et al. separately synthesised ferrocene, in which two cyclopentadienyl groups sandwich an iron atom, and R. B. Woodward, G. Wilkinson and E. O. Fischer determined its structure using the contemporary practical advances in X-ray crystallography, IR and NMR spectroscopy, and theoretical molecular orbital calculations. Wilkinson and Fischer were awarded the Nobel Prize for Chemistry in 1973. A theoretical interpretation of the metal–alkene bond (the Dewar–Chatt–Duncanson model) was also proposed in the 1950s, and since then the understanding of the transition metal–carbon bond has made great progress. At the same time, K. Ziegler and G. Natta (Nobel Prize in Chemistry 1963) discovered low-pressure polymerisation of ethylene and propylene using Ti as a catalyst, which greatly influenced the development of practical polymer-related industries. Thus, research on organometallic compounds has not only revealed the identity of the hitherto unknown transition metal–carbon bond and its interesting reactivity, but has also been the driving force behind the introduction of a number of industrially useful substances through organometallic-catalysed reactions, many of which have been awarded the Nobel Prize in Chemistry. Hideki Shirakawa, who discovered the Ziegler–Natta catalysed synthesis of polyacetylene, shared the 2000 Nobel Prize in Chemistry with A. J. Heeger and A. G. MacDiarmid for the discovery and development of conductive polymers based on this method. In the following year, 2001, the Nobel Prize in Chemistry was awarded to W. S. Knowles and R. Noyori for the asymmetric hydrogenation of alkenes using chiral bidentate phosphines such as DIPAMP and BINAP (see Section 2.2.2), and to K. B. Sharpless. In 2005, Y. Chauvin, R. H. Grubbs and R. R. Schrock were awarded the Nobel Prize in Chemistry for the development of olefin metathesis polymerisation using transition metal complexes as catalysts, and in 2010, the Nobel Prize in Chemistry was awarded to R. F. Heck, E. Negishi and A. Suzuki for their development of Pd-catalysed cross-coupling reactions through carbon–carbon bond formations. In all of these studies, organometallic complexes containing transition metals play an important role as catalysts. Since 1950, the development of organometallic chemistry has been aided by advances in theoretical and computational chemistry, particularly in molecular orbital methods. Among these, R. Hoffmann (who shared the 1981 Nobel Prize in Chemistry with K. Fukui) made a particularly important contribution in the early days of organometallic chemistry when he developed the extended Hückel molecular orbital (EHMO) method and discovered the so-called isolobal analogy, the similarity of the frontier orbitals between organic groups and inorganic fragments of metal complexes (see Box 1.2). This intuitively accessible theory has enabled many synthetic chemists to systematically understand the structures and electronic states of organometallic complexes. Density functional theory (DFT), developed by W. Kohn, has also made it possible to calculate the electronic structures of relatively large metal complexes in a short time, and now parallel computers at PC level can calculate even reaction pathways (Nobel Prize in Chemistry 1998, shared with J. A. Pople).

In Japan, organometallic chemistry has developed in connection with the development of homogeneous catalytic reactions of organic synthesis, where the organometallic compounds themselves are so-called metal complexes with typical metals, transition metals and in some cases metalloids at their centre. Recently, the development and application of such organometallic complexes (especially d-block organometallic complexes) has become more diverse, extending not only to the synthesis of organic substances but also to the conversion reactions of inert small molecules such as carbon dioxide and nitrogen. In other words, metal complexes that do not contain metal–carbon bonds are now also used in organometallic chemistry, broadening the definition of organometallic compounds and blurring its boundaries. While it is of course important to study coordination and organometallic compounds separately and in depth, it is also important to integrate the two into a comprehensive understanding of metal complex chemistry for the future. In order to understand coordination and organometallic compounds containing transition metals as a whole, as in metal complex chemistry, an understanding based on molecular orbitals is necessary, but in this chapter, the HSAB principle is studied to firstly provide the most basic understanding. Metal–carbon bonds are often considered as covalent bonds in organometallic chemistry, denoted M–C, but if they are considered as M⁺ ← C⁻ (Figure 1.2), it can be understood as a combination of a Lewis acid and a Lewis base, similar to a coordination bond. If we look at metal–carbon bonds as Lewis acid–base adducts in this way, how we can characterise the difference between coordination compounds and organometallic compounds will be learned in the following section.

Based on acid–base coupling constants (equilibrium constants), R. G. Pearson classified Lewis acids and Lewis bases into two categories, hard and soft, respectively.¹  The hard acids (HA) have a higher affinity for halide ions in the order of F⁻ ≫ Cl⁻ > Br⁻ > I⁻, which is the same trend as with the hydrogen ion H⁺. The affinity of HA for donor atoms of Groups 15 and 16 is generally N ≫ P > As > Sb > Bi and O ≫ S > Se > Te. In contrast, for soft acids (SA) such as Hg²⁺, Au⁺ and Pt²⁺, the affinity increases in the order F⁻ < Cl⁻ < Br⁻ ≪ I⁻ and for donor atoms of Groups 15 and 16, N ≪ P > As > Sb > Bi and O ≪ S ∼ Se ∼ Te. The order is generally reversed for hard and soft acids. Lewis bases that bind strongly to hard acids are classified as hard bases (HB), while those with high affinity to soft acids are classified as soft bases (SB). This classification is qualitative and only relative rather than quantitative, and there are acids and bases of intermediate hardness between the two. The classification of metal ions (Lewis acids) and ligands (Lewis bases) associated with metal complexes is given in Table 1.1. In such a classification, a useful rule of thumb has been found that hard acids bind strongly to hard bases and soft acids bind strongly to soft bases, known as the HSAB principle. There is no clear reason that can be explained in a few words as to why this tendency exists, and it is thought to be the result of a complicated combination of not only electrostatic (ionic) interactions but also σ- and π-bonding interactions, dispersion forces and solvent effects. Table 1.1 shows a simple way of organising these.

Looking at the hard acids in Table 1.1, many of them have a small ionic radius and a large positive charge, and these can be considered to have a high charge density and are not easily polarised. Polarisation refers to the induction of an electric dipole moment by the surrounding electric field. In contrast, soft acids often have a large ionic radius and a small positive charge, which means that they have a low charge density and are easily polarised. Specifically, hard acids, including H⁺, are dominated by alkali metal ions, alkaline earth metal ions, typical metal ions, 3d transition metals with high oxidation numbers, early transition metal ions (early transition metals are those in the first half of each transition series), and rare earth metal ions. In contrast, soft acids are dominated by 4d and 5d late transition metal ions in low oxidation states (late transition metals are those located in the second half of each transition series). Intermediate acids are relatively positioned between the two. Hard bases also have a donor atom with a small atomic radius, a high charge density and a low polarisation property. Soft bases, on the other hand, have a donor atom with a large atomic radius, a low charge density and are easily polarised. Specifically, hard bases include those with donor atoms of the second row elements in the periodic table, such as N and O, and halide anions F⁻ and Cl⁻, and the electronegativity of the donor atom is high. On the other hand, soft bases include those with donor atoms of P or S in the third period and I⁻, and of note are those with donor atoms of carbon such as CO, RN≡C (isocyanide), CN⁻, R⁻ (alkyl, aryl, etc.), ethylene and benzene along with H⁻ (hydride). The electronegativity of the donor atom is lower than that of the hard bases. In this way, it can be seen that adducts of hard acids and hard bases (HA–HB) are often typical coordination compounds (Werner-type complexes), while adducts formed from soft acids and soft bases (SA–SB) include organometallic compounds containing metal–carbon bonds as well as those in the broader sense. Thus, by considering the HSAB rule, coordination and organometallic compounds can be understood in the same terms.

For HSAB adducts between a Lewis acid (two-electron acceptor) and a Lewis base (two-electron donor), we consider the interaction between the acceptor orbital (lowest unoccupied molecular orbital, LUMO) of the Lewis acid and the donor orbital (highest occupied molecular orbital, HOMO) of the Lewis base. In general, the energy levels of the acceptor orbitals of hard acids (HA) are relatively higher than those of soft acids (SA). In addition, the donor orbital levels of hard bases (HB) are relatively lower than those of soft bases (SB). With this in mind, the donor–acceptor orbital interactions of hard acids and hard bases (HA–HB) and soft acids and soft bases (SA–SB) are shown in Figure 1.3. The energy difference between the acceptor orbital of HA and the donor orbital of HB is large with little overlap between them, so the interaction in HA–HB combination is predominantly ionic (electrostatic) (Figure 1.3a). In contrast, the energy difference between the acceptor orbital of SA and the donor orbital of SB is small with sufficient orbital overlap to form a covalent bonding interaction in the SA–SB combination (Figure 1.3b). In addition to the σ interaction, some effects of π-backdonation (from d_π metal orbital to π* ligand orbital) are also included in the SA–SB interaction and will be studied in the following chapters (see Chapters 2, 3 and 8).

From the above, it can be seen that coordination compounds contain coordination bonds with high ionic character formed by combinations of HA and HB, whereas organometallic compounds contain covalent bonds formed by combinations of SA and SB. However, this is only a relative view, and the stability and reactivity of the metal complexes can vary greatly depending on the nature of the metal centre and ligands. Even a metal ion with the same oxidation number tends to behave as a hard acid when bound to a hard base ligand and as a soft acid when bound to a soft base ligand. In addition, studies have recently been developed to target the synthesis of metal complexes combining hard metal ions and soft ligands (and vice versa) to achieve high reactivity derived from the instability of frustrated metal complexes.

Metal complexes are inorganic metal ions or metal atoms surrounded by various ligands, including organic ones, and thus modern metal complex chemistry is characterised as a molecular science that has developed in the interdisciplinary field of inorganic and organic chemistry. As can be seen from the HSAB rule, a wide variety of metal complexes have been synthesised and studied, ranging from coordination compounds with M ← L ionic bonds to organometallic compounds with M–L covalent bonds. In particular, transition metal complexes have been found to possess diverse properties based on d- and f-electrons and have continued to evolve, creating new interdisciplinary fields as their science has deepened and their applications have expanded. As a result, metal complexes have been used in various applied fields in recent years, and it is essential to learn not only the metal complexes themselves, but also the fundamentals of a wide range of peripheral disciplines such as electrochemistry, photochemistry, magnetism, electronic physics, analytical chemistry, quantum chemistry, crystallography, polymer science, catalysis, supramolecular science, biochemistry and molecular biology. From this point of view, in the basic chapters (Chapters 1–7), important fundamental knowledge has been introduced as much as possible, even if it is not directly related to metal complexes. In the advanced chapters (Chapters 8–13), we have tried to explain some of the recent interdisciplinary developments in an easy-to-understand manner.

• Q1.1

  Explain the HSAB principle.

• Q1.2

  Thiocyanate (SCN⁻) can coordinate as a ligand by the S or N atom (see Section 2.2.2). For the Co(iii) complexes of [Co(SCN)(NH₃)₅]²⁺ and [Co(SCN)(CN)₅]³⁻, state which donor atom of SCN⁻ coordinates to the Co(iii) centre, and why.

• Q1.3

  CH₃HgCl is thought to be a Lewis acid CH₃Hg⁺ combined with a Lewis base Cl⁻. If CH₃HgCl were to enter the human body, the CH₃Hg⁺ would bind to what sites on the amino acids in proteins? Note that methylmercury is highly toxic and causes Minamata disease.