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Highlights in inorganic chemistry over the last 100 years School of Chemistry, University of Bristol, Bristol, UK BS8 1TS The science summarised in this Highlight, which must be selective, is impressive.
The developments in inorganic chemistry over the last 100 years have beenawe-inspiring.
To decide on what are the highlights of inorganic chemistry for the last 100 years,essentially the 20th century, was quite a difficult task. To begin with I had to decidewhat qualifies as ‘inorganic’ chemistry. Second, I quickly realised that discoveriesoften owe their origins to timely developments in other fields—as became extremelyclear when considering the fantastic growth of instrumentation capabilities during thesecond half. Third, I soon came to appreciate the limits of my own knowledge of thehistory of inorganic chemistry over the last 100 years. Fourth, I was determined thatthe choice of highlights would be guided primarily by discoveries, events or ideaswhich have changed the way inorganic chemists have thought. In a short article suchas this, one cannot provide a comprehensive summary: just a few morsels which haveenlivened my appetite, and those of a few friends and colleagues, for inorganicchemistry.
My first practical thought was to look up the list of Nobel Prize winners in Chemistrysince 1900, identify those who were clearly ‘inorganic’ chemists and see how they hadchanged the ‘inorganic world’. This was interesting and informative, but I quicklyrecognised that it was impossible, and indeed undesirable, to ignore the Nobel Prizewinners in other areas of chemistry, but also in Physics, many of whom have hadprofound influence on the development not just of inorganic, but of all chemistry.
From the Nobel Prize winners list it is easy to spot the trends. In the first half of the century, the awards were mainly for the development of the nuclear model of theatom, the discovery of radioactivity and recognition of its implications, the isolationof radioactive elements (Ra, Po) and synthetic elements (trans-uranium) coupled, Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 albeit indirectly, to the development of nuclear energy and weaponry. All of thisreflects the underlying trends at the interface of chemistry and physics and thedevelopment of nuclear theory that began to blossom at the end of the 19th century.
The giants of that era included the Curies, Rutherford, Joliot and Joliot-Curie,Pauling, Mulliken, Fermi, McMillan, Seaborg, and Herzberg. The later parts of thecentury chart the rise of synthetic chemistry and its exploitation, including thedevelopment of organometallic chemistry and its influence on homogeneous catalysis,the deepening understanding of the physical processes involved in chemical reactions,with a concomitant influence on the thinking concerning biological processes andmaterials science.
But, for me, arguably the most important discovery of the 20th century was how to determine crystal and molecular structure by X-ray crystallography (von Laue; theBraggs, 1914–1915). Without X-ray crystallography, chemists’ understanding andcreativity would have been so very much slower to develop; their view of thestructural relationships of atoms within molecules and solids of simple and highlycomplex compositions, of implications for bonding theories, of coordinationgeometry and potential reactivity and physical behaviour, would have been extremelyhazy. Indeed, most instrumental techniques which are now regarded as essential formodern research, for the establishment of the ‘well-found laboratory’, such as nuclearmagnetic resonance (Nobel Prizes: Bloch, 1952; Ernst, 1991), vibrational andelectronic spectroscopy, electrochemistry, surface and microscopic analysis methods,were developed in the 20th century, many of them arising directly from the exigenciesand the spin-offs of global warfare.
There are two other seminal events, commemorated by Nobel Prizes, in the history of inorganic chemistry since 1900: the recognition of structural and geometricalrelationships in what later became known as coordination chemistry (Werner, 1913);and the discovery and elucidation of the nature of ferrocene (reported in 1951; NobelPrizes: Wilkinson and Fischer, 1973): but more of these later.
Nobel Prizes, however inspiring and well-deserved, do not inform and illuminate allof the highlights of inorganic chemistry during the last 100 years. Below, I havecompiled a selection of some of the most significant, intriguing, challenging,stimulating and thought-provoking discoveries in a century of inorganic chemistry.
Boron and silicon hydrides, organo-silicon compounds and polyhedra Stock’s and Kipping’s discoveries of boron and silicon hydrides and of organo-siliconcompounds, especially the ‘silicones’, led to amazing developments later in thecentury (not least in the soles of Neil Armstrong’s boots!). A major highlight must bethe explosion of structural types encountered in polyhedral boranes, the invention ofcarboranes and metallaboranes. These systems provoked intense thinking regardingthree-centre B–H…B and other forms of unexpected bonding, culminating inLipscomb’s award of a Nobel Prize in 1976, and a re-evaluation of the nature of the Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 so-called electron deficient compounds (for example, Li, Be and Al alkyls). A furtherspin-off of this chemistry was the creation of usable rules for structure prediction inpolyhedral cluster compounds (Wade, Mingos, Williams).
Metal hydride chemistry, although important academically and technically, was notmuch understood in the early part of the 20th century. The first molecular hydridocomplex of a transition metal, the unstable and reactive [Fe(CO)4H2], was preparedin 1931 (Hieber), the forerunner of many more stable hydrido species such as [M(g5-C5H5)2Hx] (M ~ Re, Mo, W, Ta, x ~ 1, 2 or 3; Wilkinson), [Pt(PR3)2HX] (Chatt), and [ReH9]22 (Ginsberg), following the explosion of organometallic andcoordination chemistry in the late 1950s. Many other poly-hydrido complexes weresubsequently discovered, culminating in the identification of stable transition metalcomplexes of dihydrogen, e.g. mer,trans-[W(CO)3(k2-H2)(PPri3)2] (Kubas, 1984), thedevelopment of ideas concerning agostic bonding and other forms of ‘unconven-tional’ hydrogen bonding. The study of metal hydrides and hydrogen complexesproved extremely important in the development of homogeneous hydrogenationcatalysis. Metal borohydrides were first prepared in the early 1940s, and LiAlH4, thatmost versatile of reducing agents, was synthesised in 1947 (Schlesinger). Although notmetal-based chemistry, the developments in non-aqueous chemistry led to thediscovery of superacids, 107–1019 times stronger than H2SO4 (Gillespie, Olah, 1960s)and ultimately to the isolation of stable carbocations (Nobel Prize: Olah, 1994).
The acquisition of crystal field theory from the physicists, and its use in theexplanation of structure, bonding, physical properties and reactivity of coordinationcompounds was one of the major events in the renaissance of coordination chemistryin the mid-20th century. This theory modified into ligand field theory as ideas ofcovalency in metal–ligand bonding matured, and with the development of computingpower in the 1970s and 1980s, molecular orbital calculations on relatively simplemolecules became more sophisticated and relevant to more complex systems. This hasculminated in the development of density functional theory (Nobel Prize to Kohn,1998) which is currently the most accessible of theoretical treatments for ‘real’molecules.
Two economically significant processes, the catalytic synthesis of ammonia from itsconstituent elements, and of the catalytic oxidation of ammonia to nitric acid, wereinvented at the beginning of the 20th century (Haber and Bosch, 1909; Ostwald,1908). However, it was only by the mid-century that ideas concerning the enzymaticfixation of N2 stimulated research on the coordination of dinitrogen to transitionmetals. The isolation of the first N2 complex (Allen and Senoff, 1965) and the Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 identification of N2 as a bridging ligand in [{Ru(NH3)5}2(m-N2)]41 (Taube, 1968)were key events. This was followed by an extended study of the reactions ofcoordinated N2, particularly involving acid at Mo and W centres stabilisedby chelating oligo-tertiary phosphines, resulting in the isolation of stable k1 andk2-hydrazine, hydrazido (12 and 22) (as MLNNH2 and MNHLNHM species)(Chatt, Schrock, 1970s–1990s). An even more remarkable discovery was the ability oflow-coordination molybdenum complexes, e.g. [Mo(NRAr)3] (R ~ alkyl, Ar ~ aryl)to break the N–N triple bond, giving [MoN(NRAr)3] (Cummins, 1999). Howevermuch of the most recent efforts in modeling dinitrogen fixation are related to theidentification, by X-ray crystallography (1993), of the structure of the nitrogenaseenzyme cofactor as a sulfido cluster of iron and molybdenum, without obviousinvolvement of coordinated N2.
The great oxidising strength of fluorine and many of its compounds with nitrogen andoxygen attracted the attention of rocket engineers in mid-century. From 1940, UF6was used in gaseous diffusion plants for the separation of uranium isotopes fornuclear reactor technology. Arguably, the properties of this highly reactivecompound led to the realisation that other very high oxidation state transitionmetal fluorides be prepared, and that they were highly effective fluorinating agents fororganic compounds, and this stimulated much work on metal and non-metalfluorides and their chemistry. Many exotic anions, cations and polyhalogenderivatives were discovered as a result of the growth of fluorine chemistry, and ledalso to the development of rules governing the structures of relatively simple non-metal and metal compounds (Gillespie, Nyholm, 1950s, 1960s; VSEPR principles).
The extremely powerful oxidising nature of PtF6 led accidentally to the isolation of[O2][PtF6]. Since Rn and Xe, until that time regarded as ‘inert’, had ionisationpotentials either less than or comparable to O2, it was realised that [PtF6] shouldoxidise these elements and this was achieved (Bartlett, 1962). Subsequently XeF4 andXeF2 were synthesised, and later, fluorides of Kr (Selig and Malm, 1962). This led inturn to oxides, oxyanions, complexes with electronegative N-based and alkyl ligands,and even Xe–Xe bonds. The elements of Group 18 may be rare, but they are certainlynot generally ‘inert’! The discovery of ferrocene in 1951 (Kealy, Pauson, Tebboth and Tremayne) and therecognition of its sandwich’ structure, confirmed by X-ray crystallography, led to thedevelopment of analogous metallocene chemistry, the synthesis of dibenzenechromium and the development of arene, cyclic and non-cyclic alkene and alkynecomplexes, carbene/alkylidene, carbyne/alkylidyne complexes, bis(cyclooctatetraene)uranium and similar compounds of other f-block elements, and the synthesis of manyhydrocarbon metal carbonyl complexes. Pioneers of this chemistry, which developedthrough the 1960s to the present day, include Wilkinson, Fischer, Stone, Cotton, Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 Lappert, Wilke, Maitlis, Green, Schrock and their many coworkers. Their discoverieshave had the most profound influence on the development of synthetic inorganicchemistry, such as the invention of metal-atom vapour techniques (Skell, Timms,Green), the routine use of Schlenk tube/steel catheter transfer, and inert-atmosphereglove-box working, and on the understanding and exploitation of reactions ofhomogeneously catalysed by metal compounds. The award of Nobel Prizes to Zeiglerand Natta (1963), coupled to the rise of transition metal organometallic chemistry,provided an extremely strong impulse for research on ‘industrially relevant’ chemicalprocesses. This has led to a variety of discoveries, to the so-called ‘metallocene’catalysts and to new types of coordination compounds which can effect poly-merisation with extremely high steric control (Kaminsky, Schrock, Grubbs,Brookhart, Gibson, 1990s onwards).
Ferrocene’s discovery and the rise of transition metal organometallic chemistrycoincided with a renaissance of interest in transition metal coordination chemistry,largely inspired and promoted by Nyholm (1950s) and partly related to the ease ofsynthesis of bulky and chelating and/or polydentate P- and As-containing ligands.
The discovery that low-oxidation state complexes of the noble metals (MII for Pd andPt, MI for Rh and Ir, M0 for Ru and Os) could readily undergo oxidative additionreactions (M0 A MII; MI A MIII, MII A MIV), particularly the synthesis andreactivity of Vaska’s compound [IrCl(CO)(PPh3)2] (1961), had an extremely powerfulinfluence on the development of homogeneous hydrogenation catalysis, particularlyvia rhodium complexes such as [RhCl(PPh3)3] (Wilkinson, 1965). So also had theideas engendered by studies of the bonding between transition metals and CO,alkenes (Chatt, Dewar, Duncanson, 1960s) and tertiary phosphines (Tolman coneangles, 1977).
Developments in the coordination chemistry of the s-block elements begansignificantly in the 1970s. This was greatly assisted by the use of acyclic and cyclicpolyethers as ligands, and by the synthesis of other macrocyclic polydentate ligandscontaining O and N donor atoms, such as the cryptands, cavitands, podands, etc.
This led ultimately to exploitation of not only clever ligand construction but also ofhydrogen-bonding and other weak bonding forces in directing self-assembly pro-cesses, and hence to the burgeoning field of ‘supramolecular chemistry’ (Nobel Prizes:Cram, Lehn, Pedersen, 1987).
With the development of relatively routine X-ray crystallography and NMRtechniques in the 1960s, structural studies of inorganic compounds rose to pro-minence. Important developments included the use of bulky amido ligands, such as Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 [N(SiMe3)2]2 (Bradley, Chisholm) to stabilise transition metals with low coordina-tion numbers (2, 3 or 4), in dramatic contrast to the characterisation of ever morecomplexes with very high coordination numbers (8–12). The chemistry of thedialkylamides, and their isoelectronic alkoxides, led ultimately to their developmentas volatile precursors for the preparation of films of pure metals and binarycompounds by MOCVD, MBE, MOVPE (Manesevit) and other controlleddecomposition techniques important for electronic device construction.
Stereochemical non-rigidity in phosphorus fluorides and other non-metal derivativeswas detected by variable temperature NMR spectroscopy and explained by theconcept of ‘pseudo-rotation’ (Berry, 1960); an idea which was later extended totransition metal complexes (Muetterties, 1970s). Exchange of carbonyl ligandsbetween metal centres in dinuclear and cluster compounds, and of metal fragmentsaround unsaturated cyclic hydrocarbon ligands also developed (Cotton, 1970s),facilitated by developments in NMR spectrometer instrumentation. Other physicaland synthetic studies involved the entrapment and study of unstable metal carbonylfragments, frequently in low-temperature matrices (Turner, Burdett, Hoffmann,1970s).
Interest in coordination chemistry was enlivened in the 1960s by the development ofdithiolene chemistry: metal complexes containing chelating unsaturated disulfurligands which engaged in extensive redox reactions, which had highly delocalisedground states and for which formal oxidation state assignment to the metals wasmeaningless (Gray, Holm, Davison, Schrauzer). Jørgensen (1963) described thisbehaviour as ‘non-innocent’, an appellation now applied to a wide variety ofmetal–ligand complexes, e.g. dipyridyls, catecholates, nitrosyls. The redox behaviourin this group of complexes reactivated interest in the use of electrochemicaltechniques in the study of the properties of coordination and organometalliccompounds (Heath, McCleverty, 1970s onwards). Among the biggest surprises inthis area was the discovery that the most oxidised six-coordinate speciesadopted trigonal prismatic geometries, not unprecedented in binary metal sulfidesystems, but unknown in coordination chemistry, and in dramatic contrast to theideas of Werner concerning the dominance of octahedral stereochemistry forsix-coordination.
The recognition that oligonuclear metal compounds could contain metal–metalbonds was a consequence of developments in organometallic and coordination,particular metal carbonyl, chemistry. However, the seminal event in the area ofmetal–metal bonds was the discovery of bond-multiplicity in transition metal Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 chemistry. The structure of salts of [Re2Cl8]22 revealed the presence of a Re–Requadruple bond (Cotton, 1960s onwards), a discovery leading to the successful searchof other systems containing double and triple bonds, with enormous implications formetal atom cluster chemistry. Examples of multiple bonding between the heavier non-metals analogous to that in carbon and nitrogen chemistry are much less common,the first compounds containing the SiLSi, PLP and AsLAs linkages being identified inthe early 1980s (West, Cowley, Power).
The chemistry of small metal atom cluster compounds grew naturally out of metalcarbonyl chemistry (Lewis, Johnson, 1970s onwards), but the discovery thathigh nuclearity metal carbonyl clusters not only existed but could be isolatedand characterised caused an explosion in this field. Possibly the key discoveries inthis area were made by the Chini group (late 1970s) with the noble metals, particularlyRh and Pt. Other remarkable developments in cluster synthesis involving all theGroup 8, 9 and 10 metal atoms were aided by development of theories of clusterbonding (Wade, Mingos) and by the application of the isolobal principle (Hoffmann,1982). Other types of cluster compounds, including the Chevrel phases and low-oxidation state metal halides, developed with the indispensable aid of X-raycrystallography, and were carried on the wave of enthusiasm for high-nuclearitycompounds.
The crystallographic identification of the vitamin B12 cofactor as a s-bonded alkylcompound of cobalt (Hodgkin, 1965) revealed that metals other than iron were highlysignificant in biologically important molecules. It also revealed that transition metalalkyls could be kinetically stable. The synthesis of b-elimination stabilised alkylcomplexes (Wilkinson 1970) opened the door to the systematic study of transitionmetal alkyls hitherto deemed either thermally or oxidatively too unstable to haveindependent existence. Studies of metal alkyls finally led to the metal-promotedactivation of C–H bonds in alkanes in mechanistically distinct steps (Shilov, 1960s;Bergman, Graham, 1983).
Concepts such as oxidative addition, reductive elimination, migratory insertion,inner- and outer-sphere electron transfer reactions, the trans effect, ‘soft’ and ‘hard’acids and bases, the collection and interpretation of stability (formation) constantdata all arose from ideas accumulated as a result of the growth of activity incoordination and organometallic chemistry in the mid- to late-20th century. Indeed,the understanding of mechanism in ‘inorganic’ chemistry flourished in the mid- tolate-20th century (Basolo, Pearson).
Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 The concepts described above form the basis of contemporary understanding of manymechanisms in biological processes and the influence of many of these ideas on thedevelopment of homogeneous catalysis (hydrogenation, hydroformylation, poly-merisation, alkene metathesis, etc.) has been crucial. The latter half of the 20thcentury has been largely dominated by the use of catalytic cycles exploiting theprinciples of oxidative addition/reductive addition, but with an emphasis on chiralcatalysis, particularly with respect to hydrogenation and oxygenation (Nobel Prizes:Knowles, Noyori, Sharpless, 2001). Organometallic chemistry has also had aprofound effect on stoichiometric organic synthetic chemistry (e.g. Heck, Stille,Grubbs, Sonogashira, Hagihara, Nozaki, Sharpless, 1960–1980s).
Although it was recognised early in the 20th century that metals played an importantrole in biology, e.g. haemin (Nobel Prize to Fischer, 1930), interest in enzymaticoxygenation, oxidation and reduction was greatly stimulated by the study ofmetalloproteins caused by definitive X-ray crystallographic studies (Lipscomb,Nobel Prizes to Kendrew, Perutz, 1962). Subsequently, inorganic chemists playedmajor roles in the spectroscopic studies of biomolecules (Williams, Thomson, Lippard,Gray, 1960s to present), in the exploitation of EXAFS and XANES when crystals forX-ray work were unavailable or unsuitable (Hodgson, George, Garner, Hasnain, 1970sonwards), and in the interpretation of the results in terms of contemporary inorganicchemistry. This has led to the recognition of the pervasive role of iron in storage andtransport of oxygen, and in electron transfer, to an understanding of the roles ofmolybdenum and tungsten in oxidases and of copper and zinc. Notable developmentsin bioinorganic chemistry have been the ingenuity of inorganic chemists in preparingmodels for biological inorganic centres which not only mimic the reactivity of the site,but also are structurally indistinguishable from the ‘real thing’, e.g. picket-fenceporphyrins and electron transfer tetranuclear iron sulfur clusters (Collman, Holm,1970s). That iron-nickel clusters are at the core of some hydrogenases was one surprisein the late 1990s; that the supporting ligands appear to be, inter alia, CO and CN2, longregarded as highly toxic, was another! The realisation that it is the whole of the activesite cavity that is involved in a biochemical process, and not just the metal centre, is oneof the most important recent developments in bioinorganic chemistry. This has led tothe construction of ever more sophisticated models which are extremely difficult toconceive and construct. The recent development of so-called maquettes, where a smallpolypeptide is designed and built around a metal centre, facilitates not only theprovision of the ‘correct’ donor atoms to coordinate the metal(s) but also residueswhich correctly position hydrogen bonds, etc., to the active site.
Closely related to developments in bioinorganic chemistry has been the discovery ofthe utility of a variety of inorganic compounds in chemotherapy and in non-invasive Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 diagnostics. This sparked a growth of work on metals in medicine. Key eventsincluded the discovery that cis-[PtCl2(NH3)2] was an effective agent in cancerchemotherapy (1969), and an understanding how this species interacts with DNA(Rosenberg, Lippard, Reedijk, 1980s onwards). The development of coordinationchemistry of technetium (Perrier and Segre´, 1937) led to remarkable development inclinical diagnosis (Davison, Jones, Bandoli, Deutsch, 1980s onwards). More recentwork has involved the study of radicals coordinated by metals (Wieghardt, late 1990s,2000s). One further remarkable event in medically-related bioinorganic chemistrydeserves mention: nitric oxide, lauded as molecule of the year in 1992, was recognisedas one of the most important physiological regulators, playing a key role in signaltransduction and cytotoxicity (Viagra was the noted outcome!). Haem and non-haem iron nitrosyl complexes have enjoyed a revival of interest as a result ofthis amazing discovery. In a similar vein, CO also appears to be an importantregulator, stimulating activity in the use of metal carbonyl compounds in diagnosisand chemotherapy.
In the development of solid state inorganic chemistry the role of X-ray crystal-lography, and later electron microscopy (Anderson, 1970s), cannot be understated. Arelated milestone which deserves mention is Pfeiffer’s suggestion (1915) that crystalsbe regarded as extremely high-molecular-weight coordination compounds, in whichatoms act as coordination centres, about which further atoms group themselves indefinite symmetrical relationships. This still reflects the way in which chemists thinkabout solid compounds. The understanding of the structure/function relationships incomplex metal oxides such as spinels, ferrites, aluminosilicates, solid electrolytes(Hagenmuller, 1970s) and other exotic materials, primarily delineated by crystal-lography, has led to incredible technical applications. The understanding of theimportance of imperfection in solids led to advances in theoretical treatments and thedevelopment of new materials (Magneli, Wadsley). The discovery of effectivesynthetic routes to zeolites and mesoporous solids has had a major impact onheterogeneous catalysis (Barrer, several industrial laboratories, e.g. du Pont, Mobil,Union Carbide, 1960s onwards). Developments in solid state batteries have beendriven partly by the need to find environmentally acceptable energy sources (Murphy,Tarascon, Bruce, Bell Labs, 1980s onwards). Perhaps the single most importantrecent event has been the discovery of high temperature superconductivity inmixed metal oxides, e.g. La2–xBaxCuO4 (Tc 30 K) and YBa2Cu3O7 (Chu and Wum,1986–87; Nobel Prizes to Bednorz and Mu¨ller, 1987).
There is a strong link between the chemistry and physics of solids, particularly in thetransition between molecular and extended array structures. Polymeric {SN}x wasfirst made in 1910, but it was only in 1975 that it was found to exhibit metallicbehaviour, thermal conductivity and superconductivity (below 0.26 K). Other such Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 species, such as the polymeric acetylenes, have been shown to behave similarly (NobelPrizes to Heeger, MacDiarmid, Shirakawa, 2000). The understanding of thebehaviour of one-dimensional solids such as the partially oxidised salts of planar[Pt(CN)4]22 was greatly advanced by the application of band theory (early 1980s).
The iso- and hetero-polyanions of vanadium, molybdenum and tungsten (1960sonwards) and related ‘bronzes’ are important catalytically and in other technicalapplications. This field has yielded truly remarkable mesoscopic anions suchas [Mo72Fe30O252(OAc)12{Mo2O7(H2O)}{H2MoO8(H2O)}(H2O)91], which containsthirty FeIII S ~ 5/2 centres, and the ‘hedgehog’ cluster anion, [HnMo368O1032-(H2O)240(SO4)48]482 (Mu¨ller, late 1990s, 2000s). The discovery of the fullerenes,C60 and C70 (Kroto, Smalley, Huffman, Kra¨tschmer, 1985, 1989), has spawnedthe growing field of carbon and other element nanotubes, clearly important newmaterials.
The study of electron transfer was not only important for the understanding of redoxreactions in inorganic chemistry, but also had a profound influence on the study ofelectron transfer in biology (Nobel Prizes: Taube, 1983; Marcus, 1992). One outcomeof this work was the Creutz–Taube ion [{Ru(NH3)5}2pz]n1 (n ~ 4, 5 or 6; pz ~pyrazine) and its analogues, which provoked much theoretical interest (Hush, Day,1970s onwards). The discovery of the photochemical behaviour of [Ru(bipy)3]21, inparticular the suggestion that the excited state could photolyse water, has stimulatedenormous activity in photochemically-driven electron transfer reactions (Balzani,Meyer, Gra¨tzel, 1980s onwards). This, together with a growth in the design andsynthesis of coordination compounds having manipulable magnetic properties(Goodenough, Kanamori, Kahn, 1980s), has laid the basis for the development ofnew inorganic materials and devices having useful optical, electronic and magneticproperties.
This extensive list of achievements, which I do emphasise cannot be exhaustive, isimpressive. The developments in inorganic chemistry, as in the rest of chemistry, allother sciences and technologies, and in medicine, over the last 100 years have beenawe-inspiring. A few of the key people have been mentioned, but it must be said thatthese tremendous advances could not have happened without the dedication of youngscientists studying occasionally for their first degrees but most often for theirdoctorates. These are the patient, careful and scrupulous researchers whom Sidgwickreferred to as the ‘private soldiers of chemistry’. That so much research led to so manyfantastic discoveries is due in no small part to the foresight of government agencies,aided and advised by active researchers, to ensure funding for research without toomuch tactical diversion. Also, many industrial organisations not only carried outtheir own relatively ‘blue sky’ research, but generously funded university laboratorieswithout significant interference in their directions of research. Our successors may Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13 look back on the 20th century as a ‘golden age’ for inorganic chemistry: it certainlywas a period of intense activity unfettered by short-term planning and narrowedperspectives! Whether the chemists of the 21st century will take that view is for thefuture, but for the present, let the discoveries described above stand as a testament tocuriosity-driven research which has delivered so many benefits to society.
Annu. Rep. Prog. Chem., Sect. A, 2004, 100, 3–13

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