Multiple Metal-Metal Bonds F. A. Cotton Department of Chemistry and Laboratory for Molecular Structure and Bonding, Texas A8M University, College Station, TX 77843 Today, the existence of multiple bonds, even quadruple ones, between metal atoms in stable, isolable compounds is well known, and the phenomenon is routinely covered in textbooks ( I ) . The establishment of this new realm of chemistry occurred rapidly and recently. Less than two decades ago no such bonds were known, and the very concept of any bond order higher than three was absent from chemical theory (2). Within the space of a few years, 1963-1966, double, ( 3 )triple (4) and quadruple (5) bonds between metal atoms were all discovered, and the field has since undergone vigorous growth as indicated in Figure 1, and documented in a recent monograph, (6) as well as several recent reviews (7-12). There are upwards of 900 known compounds containing metal-metal bonds of orders two to four, and more than 300 of these have heen s t r ~ ~ ~ r ~ charxterized ~rally h!. X-ray cryslallu~r~ph?. The suhiect id multiple bonds hetwrtn metal ntumc inheres in an interesting histoical setting, wherein the consciousness, by chemists, of the widespread existence and importance of even single bonds between metal atoms developed only slowly and, until recently, not very systematically. The earliest origins of the story go back about a century and a half. In this lecture I should like first to review this earlier history and then give detailed attention to some of the recent developments having to do specifically with multiple M-M bonds. From the Earliest Observations to the Concepts of Werner The two earliest descriptions of compounds we now know to contain metal-to-metal (M-M) bonds go back to the middle of the nineteenth century. In 1844, in the course of ;tudie> un the, chemistry ddivalcnt chrurn~um.E.-M. Peligot 1 /:$I d ~ w r i b r dthr isdatim ~~~~little red tranilwtmt crvitnli . . . which decompose upon exposure to air for afew moments." Peligot definitively characterized these crystals as a compound with the empirical formula C T ( C H ~ C O ~ ) ~ . Over H ~ Othe . years a number of other "Cr(RC02)2" type compounds were reported, but it was only in 1950 that someone, namely King and Garner, (14) made the interesting suggestion that the orange or red colors and low solubilities in water of these compounds "suggest a different type of bonding of the chromium from that in the typical blue and very soluhle salts of dipositive chromium." These workers then made magnetic measurements and showed that the carhoxylate compounds did not have unpaired electrons, in contrast to all the blue compounds that have four unpaired electrons per metal atom. These workers did not, however, propose the existence of a C-Cr bond, but instead suggested tetrahedral d:!s hybridization, whence the four d electrons were forced to pair up in the two remaining d orbitals. Only later, in 1970, in view of their structural relationship to Moz(02CR)4 and Re2(OzCR)4C12 compounds, was the true character of the C ~ Z ( O & Rmole)~ cules as species with multiple C y C r honds appreciated. (15) In the period 1857-1861, the Swedish chemist Christian Wilhelm Blomstrand (16) and coworkers prepared and investigated the dichloride of molybdenum, and later the di-
Dy
This paper was presented as tne Breakthrough Lecture sponsored the D vwon of Chemcal Educauon at the Spr ng 1983 Amer can
Cnem ca Soc ety Meeung hela
n
Sealile. Wasnlngton
YEAR
Figure 1. A histogram since 1964.
of publications dealing with
metal-metal mulliple bonds
bromide. I t was concluded that in the molecular formula, (MoC12),, n had to be a t least 3, because one-third of the halogen atoms were far more labile than the others. The formula MoaCls was thus proposed and a very stable "MosCl4" core, remarkably resistent to chemical change, including oxidation, was postulated. In subsequent decades only a little more in the way of experimental study appeared (171, and in the various editions of Werner's Neuere Anschauungen auf dem Gebiete der Anorganischen Chemie (18) as well as in a widely used textbook by Weinland (18) these compounds were mentioned briefly. They were invariably given the trinuclear formulation and beginning with the 3rd edition of Werner (as well as in Weinland) one finds the specific suggestion of halogen bridging, as for example:
It will he noted, of course, that there is no suggestion of metal-to-metal bonding. By this time the existence of tungsten analogs had also been discovered. During the mid-1920's Kurt Lindner and his students a t Berlin University carried out a number of studies of these compounds of divalent molybdenum and tungsten, as well as some work on what they considered to he divalent tantalum. Throughout this work, Lindner continued to accept the trinuclear formulas originally proposed by Blomstrand and in a final paper (19) entitled "Spatial Configuration of the Halogen Derivatives of Divalent Molybdenum, Tungsten and Tantalum" he attempted to assign geometrical structures to these species, proceeding from the Wernerian idea that polynuclear complexes could he obtained simply by joining octahedra andlor tetrahedra on common triangular faces. The structures proposed and discussed by Lindner, in none of Volume 60 Number 9
September 1983
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between metal atoms had entered anyone's mind. On the contrary, it was being argued that the Wernerian idealogy could be made to explain all known transition metal chemistry, by dint of ingeniously combining prototypal coordination onlvhedra. ~~------As the power and range of application of X-ray crystallography grew during the 1930'8, the possibility of discovering that non-Wemerian compounds exist became an experimental reality. However, for some years the pertinent facts accumulated only slowly, because relatively few persons were trained and expeiimentally equipped to determine the crystal structures of substances complex enough Le., a t least dinuclear) to be likely to exhibit metal-metal bonding. Also, by present day standards the number of crystal structures-even simple ones-being solved was extremely small and only a tiny fraction of that small number were for compounds of the heavier transition elements where M-M bonds (as we now know) were mainly to be expected. One of the most interestine fieures in the neriod under discussion was a Swedish chenktnarned ~ ~ r i l f ~ r o s Figset, ure 3, who published the structure of K3W2C19 in 1935 (20) and the structures of several Mo&ls'+ compounds in the period 1945-1949 (21,22). Since 1971Professor Brosset has been working in atmospheric research a t the Swedish Water and Air Pollution Research Institute in Stockholm, but prior to that he was a structural inorganic chemist. Last year I wrote to him to ask about the circumstances that led to his doina work that can properly be called classic on metal-metal bonding, and he replied as follows. L~
F gwe 2 Structures procme€ Dy K Lnndner in an
anempt to ratmna ze the chammy ofOw nnhlbdenuml I halides am rebled mmwm No matamla1 bonds are used. This is an assembly of several illushations given In Ref.
which did he propwe any direct bonding between metal atoms, are shown in Figure 2. His own concluding remarks (in my translation and with italics added) were as follows: The concept of the halides of divalent molybdenum, tungsten and tantalum as polynuclear spatial structures allows a complete interpretation of the coordination number of a considerable quantity of well-defined complex compounds in a way that is consistent with Werner's coordination theory. In particular,for about 50 trimolecular compounds, five spatial arrangements were established. which are distineuished bv a simole buildinx-UD . Dat. tern and admit of ionclus~oniruncrrnmp. the courdmauon num he*, of tnd~tdduolrenrrnl mrral aromr The chem~mlreactuns of rh!s gnnup of complerrs can he fully interpreted h\, meam of these structures. The indiuidual central atoms, in accord with the behavior of the same metals in their mononuclear complexes show coordination numbers of four and six.
When I was a young student and had to do my first Thesis (in Sweden we had to do twoTheses to get a doctor's degree, one preliminary and one final), I asked my tutor, Professor Arne Westgren, if he could suggest a subject to me. As you already know the Westgren Institution solely worked with crystal structure investigations. Evidently, Professor Westgren asked his colleague, Oskar Collenberg, who was Professor of inorganic chemistry at the Royal Institute of Technology, if he knew some interesting complex compounds to suggest. Obviously, Professor Westgren thought that one ought to widen the research field which earlier comprised only metal structures. Professor Collenberg sent me 5-6 small boxes containing crystals ~- in different colors. 1 started to oulverize these crvstals and had some powder photugrdphs token. 310~1 oith~m ~ h w e da trrrlble lot uf lines indorating lriy u n ~ cclla. t Hut one wrnpound gave a puuder photograph uhlrh sremad possihlr 10 walustc. 'I'he compound was K3W2Cls. As you know, I managed to establish the structure whereby the W-W bonding wasconfirmed. When I had todomy final Thesis, I studied the literature for some structural analogous compounds.
With the benefit of that marvelous faculty called hindsight, we can view Lindner's detailed efforts to force these compounds into the Wernerian framework as similar to those of the Ptolemaic astronomers in devising ever more elaborate patterns of epicycles in order to account for all astronomical observations within the framework of a geocentric cos~
~
l t z o u l d be noted that through approximately this same neriod of time there was also a series of investigations into the chemistry of what were a t first thought to bk tantalum dichloride and dibromide, but subsequently formulated as T-X7 (or occasionally, Ta&,e). Lindner's proposals (19) for t h e MosXs and W3Xs compounds were also intended by him to apply to the lower tantalum halides. Converglnp Strands, 1930-1960
As outlined above, a t t h e end of the decade of the 1920's there was no indication that the concept of bonds directly 714
Journal of Chemical Education
Flgure 3. A
recent photograph of ProfessorCyril1 Brasset.
When doing this I found a description of the compound T13AlzFg. I examined this compound and found that it consisted of two complex structures, i.e., TIAIFI and T12A1Fs. This coincidence was the reason why I devoted myself to complex aluminum fluorides for some wars. Very much later on, I again took up the problems about metal bonding and complex compounds. And it was in that time my works on molybdenum were created. Later on, I was appointed professor of inorganic chemistry at Chalmers University of Technology in Gothenburg and dedicated myself to other things than X-ray crystallography. In his studv of KqWCIo Brosset not onlv showed that a " binuclear anion was present (a fact already anticipated from studies of aqueous solutions) but that "the tungsten atoms lie painvise very close to each other. They are, apparently, within these nairs. . . in some wav bound toeether. The central triangle of chlorine atoms of thiwecl9 group has a considerably lager edee-3.83 A-than the outer trianeles-3.47 A-which can heiaken as a sign of an attractive force between the tungsten atoms." (The auotation is mv translation of the German original.) It was, however, especiallv in his work on the halo complexes of molybdenum(I1) ihat ~ i o s s eprovided t a real landmark in the development of transition metal chemistry. From the technical &ystallographic point of view alone, the work is a remarkable tour de force. These very large, complex structures demanded the utmost in cr.vstall&raphic rxpertise that was possible at that time. Their corret,tnesr and accuracy (to the stated limits ni error) remain unchallenged and they constitute a remarkable testament to the eminence of the Swedish schwl uf structural chemistry in those pioneering days. In hi3 first two paprrs (211 Hrosser showed that the compounds r~rt:viouslvcunonsed to have iormulrw Mo.CldOHI.,.7H9O and ~
~~
.~
0
-h--c'4)
h..1.
w Figure 4. The structural proposals made by Cyril1 Brasset to account for the various compounds based on the Mo&'+ core unit. Reproduced horn Ref.
(pa.
12H20 and I[MO~C~~]CI~(HZO)~~~~HZO, respectively, with the critical structural feature being the Mo6Cls4+ unit, Figure 4 (a). In his 1949 paper, (22) Brossett addressed himself to the complete reconciliation of the structures he had found, and their implications, with the results of chemical and solution studies by Lindner and earlier workers. In this brilliant paper Brossett masterfully deduced the structures of all principal compounds, as shown in Figure 4, which is reproduced directly from his paper. All of his prooosals have oroven to be correct. He even ;r&gnized that ' ' ~ s i h e[octahedron of molyhdenum atoms1 has an edge of only 2.6 kX,' the distance between the molybbenum atoms within this group is a little shorter than in molybdenum metal, which indicates the presence of strong bonds." Finally, by using (for the first time, I believe) a technique of X-ray scattering from a solution Brosset demonstrated that, contrary to e d i e r , unconvincing investigations purporting to show that Mo3C14 species exist in solution, the Mo6C1~unit persists in solution (22). Another interesting observation made a t about the same time. and also in Sweden. was Arne Maenkli's discoverv (23) that MOO* and WOz have'a rutile-like st&ture, but dis&&d in such a way that pairs of metal atoms approach each other to a distance of about 2.5A. This was interpreted as evidence for direct M-M bonding. In 1947, Linus Pauling received the Theodore William Richards Medal from the Northeastern Section of the American Chemical Society, and he presented an award address entitled Unsoloed Problems of Structural Chemistry; this address was published in full (24). In this wide-ranging and entertaining survey Paulina devoted considerable attention to the emerging body of experimental evidence for the existence of direct (and strong) M-M honds. He introduced this part of his discussion with the statement that, "Another question. . . about which little is known a t present is that of the extent to which covalent honds between metals atoms occur in nonmetallic compounds." After pointing out that the existence of the mercurous ion, Hgz2+,and molecules such as CI-Hg-HeCI, had already been known for many years he discussed in some detail the work of Brosset and MagnBli. He suggested that the W-W bond in W&s- is a multiple hond (hond number, 1.70) and that in Moo2 and W02 "there is an effort by each quadrivalent molybdenum or tungsten atom to use its two remainine valence electrons for the formation of a double hond with another atom of molybdenum or tungsten." He assiened a "hond number" of 1.47 to these honds. He pointed out that in addition to Brosset's studies of the Mo6 species, there was work on these and the tantalum halides T%Clla and T a , j B r ~underway in his laboratory; publications subsequently appeared (25,26). Pauling also referred to the structure (27) of Fee(C0)g in which an F e F e distance of 2.46 A (later corrected to 2.523(1) A (28)) had been found and he suggested that a covalent bond exists between the two iron atoms. None of these structural facts, nor Pauling's incisive and cogent comments, appears to have caught any one else's attention in such a way as to provoke active resehch interest in the subject of "covalent bonds hetween mrtnl atoms . . . in nonmetallic compounds." Perhaps the fact that Pauling's discussion was recorded only in Chemical and Engineering .VPU:Yrather than in t h e r e 4 r c h I~terarurecontrib~ted toits being little notired and, prrsumahlv, soon forgotten. 1 myself came across this article only by sheer accident about a y e a r ago. This section can be concluded bv notine one other development that occurred in the late 1950'8, &ely, the discovery of several M-M honds that are unsunnorted bv anv hrideine M atoms or groups. Whenever a putaiive M ~ bondWh&
' effectively,kX = A Volume 60 Number 9
September 1983
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btidging groups associated with it, as in all of the cases so far cited, there is necessarily some uncertainty as to the order of that bond, and perhaps, in some cases, there can even be some Jouht whether a direct M-M bond exists. Thus the obser\,ation dunhridyed, nl1)eit long and weak. 11 M bonds in the h l , l C O ~ In i o l e c ~ ~1291 l r ~ (11 = MIL Re1 and ln.,-(:-,tl-,hh( c o ) ~ ](30) ~ is of considerable significance. In thdse cases the existence of M-M sinele honds seemed assured althoueh subsequently, in view ortheir great lengths (2.9-3.3 A in t i e above cases), there have been sueeestions that the two halves .. of the mdrcule are held tugethrr I)?. an a r c ~ ~ m u l n t idml w s e attracti\v forces hetween CO -rruuvi - and perhnw hetwren CO groups and metal atoms. Even recently the issue has still been debated since experimental electron density studies of Mnz(CO)lo and some other, similar molecules, have failed to show a builduo of electron density between the metal atoms employing theconventional method of interpretation. However, this puzzle seems to have been recently resolved by my colleague M. B. Hall, (31) who shows that there is M-M bonding density present. Thus. -~over~a neriod ~ ~of some . three decades various strands of experimental evidence accumulated and converged, hut never actually coalesced to an articulated perception that in addition to their vast Wernerian chemistry, the transition elements also have an important chemistrv that lies outside the conceptual framework of Werner, namely, a chemistry featuring metal-to-metal bonds. ~
~~
.
~~~
An Irruption of New Facts, 1863-1965 In a brief period of about three years enough new experimental observations, and interpretations thereof, appeared in the literature to lay the groundwork for the present rich (and still rapidly growing) field of M-M multiple bonds. In 1962 in both my laboratory a t MIT and Christchurch University in New Zealand, X-ray crystallographic studies of "CsReC4" were carried out and the results submitted for publication independently and almost simultaneously in early 1963 (3,32,33). It was shown that this compound contains a triangular [ResCl1a]J- anion, Figure 5, in which the mean R e R e distance is 2.47 A. The significance of this structure was immediately evident to me and in our first report we gave not only a very precise description of the molecular structure but a discussion of the electronic structure that lead to the following conclusion:
. . .the d,,, d,, and d,, orbitals of each rhenium atom combine to give banding and antibonding 3-center MO's. There are just six bonding orbitals, which are filled by the twelve electrons. . . .
Figure 5. Thestructure otthe [Re~Cl,~l~ionas originally depicted in Ref. (3). Note mat in those times (1962-631 we have no "compufer-drawn"illustrations. This was drawn by a human hand (Aaron Bewand's) using pen and India ink. 716
Journal of Chemical Education
Thus, the existence of double bonds (i.e., six bonding electron oairs used for three Re-Re bonds) between the metal atoms was proposed and the following year the analysis leading to this conclusion was published in more detail, along with similar analyses of the M-M honding in M ~ ~ X L and I~+ Nh6Clla2+type systems (34). Much more elaborate and accurate treatments (35, 36) hy the SW-Xa-SCF or DV-Xa methods together with photoelectron spectroscopy studies have since confirmed the essential correctness of my original bonding proposal. In fact, d l of the simple, early proposals for M-M honding in clusters and M-M multiple bonds via d-orbital overlaps have subsequently been supported in their essentials (though, naturally, not in all details) by later rigorous calculations (6,371. It was in coniunction with the detailed discussion of the hmdiny in thr Re,. Yo,,.and S h n compounds as well 11ss l n e d term "metal trinuclrar M d ' ~necios\:MI that I ~ n t r ~ d u c ethe atom cluster compounds" for such substances. This term is, of course, now the one universallv used rather than the more esoteric term staphylonuclear ori&ally used by Cyrill Brosset or the term "cage", sometimes used by others but really quite inappropriate since the clusters seldom can and virtually never do contain anything, except, in rare instances, an H or C atom. Following the discovery of the [Re,7C1~2]3cluster anion, I and my coworkers rapidly established (39) that this type of cluster structure accounts for the greater part of all of the halide chemistry of rhenium(III), e.g., such compounds as Re"' chloride, bromide, and iodide, and a variety of adducts of the general type ResXsLs, some of which had been known but not understood since the early work of I. and W. Noddack. It was thus established for the first time that multiple bonds, specifically double bonds, between metal atoms could play a pervasive role in a large range of the chemistry of an element. Moreover, it was our work on the Res"' cluster chemistry that led us to the discovery of quadruple bonds since our first synthesis of the [Re2C18]2- ion occurred while we were attempting to generate clusters in aqueous solution by reduction of Re04-. Discovery of the Quadruple Bond The dominant event during the period in question was, of course, the recognition of the existence of a quadruple bond in the [Re2C18]2- ion. The events and the roles of various persons connected with this discovery have already been recounted in considerable detail in Chanter 1of Reference ( 6 ) and I shall not repeat all of that her; Suffice is to say that durine the last months of 1963 it was discovered bv one of mv colla&rators at MIT, Neil Curtis, that the reduction of R ~ o ~ '
Figwe 6. The slructwe of the [Re2C1.]2- ion as originally Owicted in Ref. (4D). Again, the drawing was done by hand (C. 8. Harris's).
by hypophosphorous acid in concentrated aqueous HCI gives a royal blue anion which he was able to isolate by precipitating it with various large cations (5). These blue substances had the same empirical composition, M1ReCI4, as the red Cs3Re3CIl2compound we had previously characterized, so we were, naturally, intensely curious to discover how the two "isomers" differed structurally. For technical reasons, a potassium salt, "KReC14.H20," seemed a good candidate for X-ray crystallographic study and Charles B. Harris determined its structure. (5,40) The result as far as the rheniumcontaining unit, [Re2Cls]2-, is concerned, is shown in Figure 6. Prior to the completion of our structure in about April of 1964, a paper (41) appeared in the MIT library giving the structure of a compound said to have the empirical formula (C5H6NH)HReCL,which was described as containing pyridinium ions, [Re2CIs]4- ions with a structure essentially the same as that of the [Re2Cls12- we found, as well as some "detached free hydrogen ion(s) . . . identified as situated on a fourfold position which is electrostatically stable." As has long since become clear, the compound so described is actually a Ref1' compound; it contains no "detached hydrogen ions," electrostatically stable or otherwise, and it does contain the [Re~Cls]~ ion. This [Re2ClsI2-structure presented me with an immediate and tantalizing theoretical challenge: How to explain both the extremely close approach (and without the aid of any bridges!) of the two rhenium atoms to each other and the fact that the rotational configuration is eclipsed rather than staggered. In 1964 (5a) (and shortly after in more detail (5b)) I proposed
the correct explanation for these facts, namely, the existence of a quadruple hond between the metal atoms, consisting of one a,two a,and one 6 components. The essence of this pronasal. hv now verv familiar. is shown in Fieure 7. It is the hieh ;nultiplkty I,!' the hmd that "ccounti fo'i its h n g 1 1 short. and it is the 6 comoonent that iavori the 1v4i~st.dn,rati10.,CH1~ - . . . cwnpound to a memhrr 111the group when I rece~veda letter from mv iriend HonnId Mason. who was at that tlmr a lecturer in the chemistry ~ e p a r t m e n at t Imperial College, London. He told me that to put an end to the speculation in Wilkinson's group as to the nature of the molybdenum(I1) carboxylates, one of his students had undertaken the crystallographic study of the acetate, the structure of which they had just solved. They had found exactly the structure I was expecting, based on our work with the Re2(O2CRI4X2compounds, which I had submitted for publication (42) a month or so earlier. I l telling h ~ m%,hotI thought h i s s t r u c t ~ m umte hark t ~ .\lason, imolie
