Allan E. Crease' University College London London, W C I HOAJ, Endand and Peter Legzdins The University o f British Columbia Vancouver, B. C. V6T 1W5, Canodo
I
I
A wide range of chemical reactions can be understood on a qualitative basis by considering the interactina species as hard or soft acids and bases and by applying the criterion that "hard acids prefer to associate with hard bases, and soft acids prefer to associate with soft bases" (I). This empirical rule occasionally can be used predictively, hut its importance derives primarily from its usefulness in systematizing a large amount of data concerning the relative stabilities of chemical compounds. This paper is concerned with the bonding modes of carbon monoxide to metals in the light of the above generalization. Pearson (I) has defined a soft base as one in which the donor atom is of high polarizability and of low electronegativity and is easily oxidized or is associated with empty, low-lying orbitals (e.g. (CeHs)aAs). A hard base has the opposite properties (e.g. HzO). Soft acids are characterized by low or zero oxidation states, large size, and several easily excited outer electrons (e.g. elemental metals). Conversely, the major features of hard acids are small size, a high positive oxidation state, and the absence of outer electrons which can be readily excited to higher states, (e.g. Mg2+). The general, but not inviolable, rule that hard-hard and soft-soft interactions are preferred is well exemplified by the stability of transition metal complexes and compounds. Species of the type ML, where M has a high oxidation number are favored by hard ligands (e.g. O F 6 ) , whereas for soft ligands the oxidation state of the metal is low or zero (e.g. (n6-C6H6)~Cr). The Lewis electron pair bonding diagram for carbon monoxide reveals that both the carbon and oxygen are potential base sites since both ends of the molecule possess a lone pair of electrons. However, carbon monoxide normally bonds to transition metals by attachment of its carbon atom, therehy leaving the oxygen atom coordinatively unsaturated. The transition metals in carhonyl compounds are generally considered to be soft acids because of their low formal oxidation states and their loosely held outer d-orbital electrons. Consequently, in accord with the general criterion above, the carbon end of CO is apparently the softer Lewis base site with the oxygen being much harder. Furthermore, it should be noted that in addition to the donation of electrons by CO to the metal in the a-bonding system, electrons are also back donated from the metal d-orbitals into the empty antibonding a*-orhitals of CO. In the metal-C0 linkage, therefore, the ligand also functions as a soft acid and the metal as a soft base. Formally isoelectronic with CO is C N which is known to he bidentate towards Lewis acids in various environments (e.g. CH3CNBFa (2) and K4Fe(CNBF& (3)). In a theoretical study of this type of coordination for CN-, Purcell ( 4 ) also considered HCO+ and COHC. In terms of valence bond theory, he concluded that (I) makes an enhanced contribution to COH+, while (11) is preferred by HCO+. This increased polarization of electrons by oxygen :c=o: (1)
cr
The Carbonyl Ligand as a Hard and Soft Base . .. the CO o-bond order of COH+ is also appreciably reduced and leads us to suspect that, should attempts to prepare Lewis acid adducts of metal carbonyl complexes in which the acid coordinates to the oxygen of carbon coordinated CO be successful, the infrared spectra of the adduct will be characterized by a pronounced lowering, relative to the carhonyl complex, of the CO stretching frequency. Furthermore, enhanced back honding from the metal to the CO would complement polarization of the CO r density, particularly in view of the anticipated increase in CO distance. The decrease of CO frequency should, in any event, amount to at least a few hundred wave numbers. Recently, it has been shown that the oxygen end of a coordinated carbonyl ligand can indeed function as a Lewis base towards sufficiently hard Lewis acids fcf. (5-9)). Thus, while soft acceptors such as BzHe fail to add to carhonyl compounds which exhibit low CO stretching frequencies (a property which is indicative of a high electron density on the carhonyl ligands), electron acceptors containing hard acid sites such as AI(III), Ga(III), and Eu(III) add easily under similar circumstances. The evidence for the existence of adducts in which both ends of the CO are bonded to metals has been primarily obtained from infrared data (cf (5, 6, and 7)), although several crystal structures have been determined fcf (8, 9. and IOj) and one electronic spectral study has been carried out (11). The formation of these adducts is certainly in accord with numerous other experimental observations which indicate the special stability of hard-hard and soft-soft interactions. As expected, the formation of M-CO-M' linkages results in a considerable lowering of the infrared stretching frequency (uco) of the carbonyl ligand bridging to the hard Lewis acid, M'. The infrared absorptions of the remaining uncomplexed carbonvl increase slightlv in fie. lieands ., quenc; if they change at all. This latter obiervhion can be rationalized in the following manner. As electrons are removed towards the Lewis acid, the electron density a t the transition metal, M, is lowered. This, in turn, results in less a hack donation into the uncomplexed carbonyls' **-orbitals, therehy causing a higher stretching frequency to be observed for these ligands. It can he readily seen that this additional effect is secondary in terms of the size of Auco because of the distance of the uncomplexed ligands from the site of adduct formation. The structural consequences of this bonding mode for CO are particularly interesting. The first reported example of a compound exhibiting C- and O-coordinated carbon monoxide was [(n5-C~H~)Fe(C0)~lz.2[A1(C~H~)3] (10). The sites of adduct formation are the bridging carbonyls of the iron-containing compound, i.e.
:c*: (11)
when coordination occurs at that atom led Purcell to speculate Volume 52, Number 8. August 1975 / 499
Indeed, the greater hasicity of the hridging CO group in the donor compound can he utilized to force structural rearrangements. For example, [(n5-C5H5)R~(C0)z]2 exists in solution as both hridged and non-bridged isomers in almost eaual amounts
Shriver (6) has shown that the equilibrium can be shifted entirely to the bridged form by the addition of aluminum alkylsin heptane. Just as in the iron case above, cis and trans isomers of the hridged adduct are obtained (with the illustrated cis form being favored), and the adduct can he readily dissociated by the addition of a stronger base such as triethylamine. Polynuclear carbonyls such as Fez(C0)9 and Ru~(CO)IZappear to undergo similar structural rearrangements in solution in the presence of A1Br.q (5) as again the affinity of the Lewis acid for the hridging carhonyl is a dominant factor. An explanation of the enhanced hasicity of a hridging carhonyl group relative to a terminal one is that the hybridization of oxygen in the former is su2 as opoosed to su in the latter. If increased basicity can be correked with lower percentage of s character of the electrons, then triply hndging c&honyls with an oxygen atom in an sp3 hybridization state should exhibit an even greater tendency to form adducts with hard Lewis acids. This expectation has been verified by utilizing [(n5-C5H5)Ni(CO)]z and (n5-CaH5)3Ni3(CO)z(6). The former compound possesses only doubly bridging carbonyl groups while the latter has only triply hridging carhonyl groups. (n5-C5H5)3Ni3(C0)z forms both 1:l and 1 2 conlplexes with aluminum alkyls in hydrocarbon solutions, whereas [(n5-C5H5)Ni(CO)]z formsonly a 1:l adduct. In solution, dicobalt octacarhonyl exists in two isomeric forms which are related by a temperature dependent equilibrium
a
A \c/
lOC),Co-
dCO1,
=
0
lOC),Co-CdCob
(IV)
(111)
Isomer (III) corresponds to the structure found in the solid state and has a formally bent metal-metal hond, whereas isomer (IV) has a linear Co-Co bond. Isomer (III) possesses three types of potential hase sites, namely the oxygen atoms of the terminal carhonyl ligands, the oxygen atoms of the bridging carbonyl ligands, and the electron pair in the bent metal-metal hond. Upon addition of a Lewis acid there exists the possibility, therefore, of not only shifting the isomer equilibrium hut also of forming a three-center two-electron hond involving both cobalt atoms and the central metal of the Lewis acid. In 1958, before the discovery of C- and 0-bonded carhonyls, Chini and Ercoli (12) isolated the adduct Coz(CO)s.AlBra which they described in terms of the three-center two-electron hond model. Support for this structural proposal was the ohservation (13) that the infrared spectrum of the adduct was virtually identical with that of the parent carbonyl in the CO-stretchine reeion. This result is sumrisine since it would he expectid that direct electron withdrawal from the cobalt atoms should be reflected in an increase in frequency of CO stretching modes. Indeed, later work reveals that when C O ~ ( C Ois) ~complexed with Lewis acids (e.g. AlBr3 (5), (14) or (C5H5)3Ln where Ln = a lanthanide 500 / Jourml of ChemicalEducatbn
(7)), lower frequency hands appear in the vco region of the infrared spectra and these hands are consistent with the acid being attached at the hridging carhonyl oxygen of isomer (III). The hard acids thus prefer to associate with the hard oxygen hase site rather than with the relatively softer site provided by the electrons in the cobalt-cobalt hond, a result which is completely in accord with the principle of hard and soft acids and hases. This ability of Coz(C0)a to use the hridging carhonyls as hard hase sites has an interesting consequence in its chemistry. Reaction of C02(C0)8 with different substrates under varying conditions leads to the cluster compounds Co3(C0)9X where X is a liaand attached to the three cobalt atoms via a sulphur, selenium, germanium, or, more commonly, a carbon atom. However, attempts to incomorate boron or silicon into analogous cobalt ciusters by similar reactions yield only compounds of the type C O ~ ( C O ) ~ (COX') where X' is the boron- or silicon-containing group fcf. (15)). Similarly, when the COZ(CO)~.AIBI~ adduct discussed ahove is heated in benzene a t 60°C, the pmduct ultimately obtained is C O ~ ( C O ) Q ( C O A I B ~ ~ A I B ~ ~ ) which is believed to have the structure shown below BI
Br.
CdCO),
Such results seem to indicate (15) that the harder Lewis acids initially engage in =,M,
bonding with Coz(CO)s and that this linkage persists throughout the chemical transformation thereby precluding incorporation of the acidic metal center into-the cobalt cluster. Terminal carbonyl ligands can also function as bases if sufficient electron density is localized on the oxygen atoms. For example, the anions [Co(CO)r]-, [Mn(CO)&, [(n5-C5Hs)Mo(C0)31-, and [(n5-CaHs)Fe(CO)z] (which customarily form metal-metal honds with transition and Group IV metals) hond to hard acids such as AP+, MgZ+, or MnZ+ by M-CO-M' linkages (cf. (8) and (9)). Thus, a single crystal X-ray study of [(n51C5H5)Mo(C0)312Mg(C5H5N)4(9) reveals octahedral coordination of magnesium by four normal pyridine ligands and the trans molybdenum-containing groups each hound via an oxygen atom. Furthermore, the complexed C - 0 bonds are longer than the non-complexed terminal C - 0 honds, a result that is in accord with the predictions of Purcell (4). The metal carhonyl moiety in these mzMg(py)4 compounds (m = metal carhonyl group; py = pyridine) behaves as a strong nucleophile, and the Co(C0)4 and M ~ ( C O ) E derivatives have the added advantage that they are appreciably soluhle in aromatic hydrocarbons. This solubility permits [Mn(CO)&Mg(py)4 in toluene to convert (C6H&SiCI into (CeH5)aSiMn(CO)s, a compound that is inaccessible if Na[Mn(CO)sl in tetrahydrofuran is employed as the reagent. The NO molecule is closely related to CO except that it contains one more electron which occupies a r*-orbital. The species NO+, CO and C N are thus isoelectronic. Although the hondine of the nitrosvl lieand to transition " m e t a l s k the subject of intensive investigation a t present, NO can he regarded in an analogous fashion to that of CO in terms of hard and soft interaitions with metals. Consequently, preliminary studies to ascertain the Lewis hasicity of coordinated nitrosyls have been carried out (16). The evidence for the existence of M-NO-M' linkages is not as extensive as in the carhonyl case, hut it has been shown ( N= O )CI, that in the compounds ( ~ ~ - C ~ H ~ ) M ( C O ) Z(M Mo, W) the NO ligand is a better base than the CO lig~~
ands towards the hard acids (C5H&Ln. The lanthanide acceptor sites are hard because their partially filled f shell of electrons does not sianificantlv enter into the bonding of these elements. As a & d l , the lanthanides are less larirahle than the transition elements and their f orbitals are not available to accept electrons from ligands by hack donation. In summary, it is clear that this recently observed bonding mode of coordinated carhonyl and nitrosyl ligands can be readily understood in terms of hard and soft acid-base theory. The harder oxygen end of the ligands prefers the hard acid sites provided by the elements of Groups IIa, IIIa, and IIIb of the periodic table. This preference can, on occasion, result in structural rearrangements of the parent donor compound in the presence of a hard Lewis acid. Finally, some of the adducts containing M-CO-M' bonds exhibit considerable pmmise as versatile synthetic reagents.
Literature Cited (1) Pesmn, R. G..J Amer. C h ~ m 8oe.. . RS, 3533 (1963); Seimee. 151, 172 (19661: J. CHEM. EUUC.,45.581 (1966); J C H E M . EDUC.,45.M3(1968). (2) Hoard. J. L., O w n , T. B.. Buzzell. 2.. and Salmon. 0.N., Aefo Crymllogr., 3. 121 119.Wl
(7) Cream, A . E., andlcgzdina. P.. J. Chsm. Sac., Dollon Tmns.. 1501 (1913). (8) Petemen. R. B.. Sfezowski. J. J.. Wan. C.. Burlitch. J. M., and Hughes. R. E.. J. A m r r Cham. Soe, 93.3532 (1971). (9) Ulmer. S. W.. Skantad, P. M.. Burlitch. J. M.. and Hughes, R. E.. J Amer Chsm Soc., 95,4469 (1973). 110) Nelson, N. J., Kime, N. E., and Shriver, D. F., J Amer Chom. Soe.. 91, 5173 (1969); Kim. N. E.. Nelson. N. J.. and Shtiuer, U. F., lnorg. Chim. Acio. 7. 393 (1973). (11) A1ich.A.. andShtiuer,D. F.,lnag. Chrm.. 11.2984 (1972). (121 Chini,P., andEreoli, R., G a n z Chim. Itol., RR, L170i1958). (13) Cotton. F.A., andMonehamp,R.R., J. Chem S o r . 1882(19MI). (141 Whyman. R..Nofure (PhysicolScience) (London). 230.139 11971). (151 Schmid,G.. andBatze1.V.J. Org~name~ol. Chem., d6.149(1972). (161 Clpase.A.E, sndLeerdins.P.,J. Chem. Soe.. Chom. Cornmun.. 268 (1972).
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