David W. Thompson
College of William and Mary Williamsburg, Virginio 23185
Inorganic Derivatives of Acetylacetone
Linkage isomerism, arising when a ligand coordinates t o a metal center in more than one way, is one of several types of isomerism which can bc identified among coordination compounds. The classic examples cited to illustrate linkage isomerism are a limited number of N and O bonded -NOz complexes although a few other examples are known as well (1). However, recent structural studies of inorganic derivatives of acetylacetone render these compounds cogent and fascinating examples of linkage isomerism and related structural phenomena arising from variable metal-ligand interactions. Several distinct bonding and structural types involving acetylacetone and its enolate anion, structure I, are known. By far the most frequently occurring acetylacetonate derivatives are those in which the enolate anion is coordinated to a central metal atom through both oxygen atoms. A classic example of this is the wellknown tris(acetylacetonato)iron(III) complex, structure 11. Crystallographic data show that the two chelate ring C-C bond distances are equal as are the two C-O bond distances and verify the CZrsymmetry of the acetylacetonate ligand. The ubiquity of oxygenchelated acetylacetonate complexes can be appreciated from the fact that such complexes have been reported for all the main group transition elements (except technetium) and lanthanide elements (except promethium) as well as numerous main group and acetinide elements. A second type of oxygen-chelated complex can be formed when acetylacetone does not lose its acidic proton to form an enolate anion. Rather as the neutral keto tautomer acetylacetone donates electrons from the oxygens of each carbonyl to an acceptor or acidic species. An illustrative example is structure 111. Although the number of donor-acceptor complexes that have been isolated is small, nevertheless, they probably form as intermediates in the formation of many of the chelated enolate derivatives. Exemplificative of a third type of oxygen-bonded acetylacetonate derivative is 2-trimethylsiloxy-2-pentene-4-one formed from the reaction trimethylchlorosilane and acetylacetone (8)
e*
acet,ylacet,onates of silicon are kno\vn. Dangling complexation of potential chelating ligands is rare in coordination chemistry. Far less numerous t.han oxygen-chelated acet,ylacetonate derivat,ives are those in which the met,al atom is bonded directly to the unique (3) carbon atom rather than t o the enolate oxygens. Although mebal complexes of this type have all been characterized since 1962, the first carbon bonded acet,ylacetonate complex, pot,assium chlorobis(acetylacetonato)plat~inate(II), IC[l't(a~ac)~Cl], was reported by Alfred Werner (5)in 1901. The [Pt,(acn~)~Cl] anion has been t,horoughly characterized (4-G), and is square planar having one 3-carbon-bonded and one oxygcn-chelated acet,ylacetonate ligand in coordination sphere (see structure IV). A second t,ype of 3-carbon-bonded acetylacetonat,e complex is illustrated by the trimcthyl(acet~ylacetonato)platinum(IV) dimer. Although the X-ray crystal structure analysis has not been carried out for this specific complex, both X-ray and nent,ron diffraction studies have been done for the analogous trimethyl(nonane-4,6-dionato)plat,inum(IV) complex (7, 8). This and other similarit,ies suggest that the acetylacetonate complcx has the same structure. The
anion
I
trans
It is to be noticed that this silicon compound contains a "dangling" ligand even though several chelated Volume 48, Number 1,
January 1971 / 79
structure of the nonane-4,6-dionate complex in which the enolate anion behaves as a tridentate ligand is illustrated in structure V. Compounds of this kind have been designated as "bridge-bonding" complexes by Gibson (9). Recently, Allen, et al. (10) have isolated the first ?r-bonded metal complex of a neutral acetylacetone molecule. Potassium chlorohis(acetylacetonato)platinate(II), K[Pt(acac)2CI], reacts with strong acids in aqueous solution t o give a complex wbkh analyzes as H P t ( a ~ a c ) ~ C lA . detailed infrared study (11) of H P t ( a c a ~ ) ~ as C lwell as other data are consistent mith the structure illustrated inVI. A final and unusual type of carbon-bonded a~et~ylacetonate is that in which the metal atom forms a derivative a t a terminal or 1-carbon. A tellurium compound illustrating this is shown in structure VII (12). The third and final class of acetylacetonat,e derivatives are those containing bridging enolate ligands giving rise to oligomeric or polymeric complexes. Within this class three types of polymeric complexes may he distinguished. These are: 1) oxygen-chelated oxygen-bridged complexes as she\\-11 in structure VIII; 2) oxygen-bonded two-center 6-diketonato complexessee structure I X ; and 3) bridge-bonded complexes as trimethyl(acetylacet.o~~at~o)platinum(IV) which has already beeu discussed above in t,he 3-carbon-bonded category.
A prime example of an oxygen-chelated oxygenbridged complex is his(acetylacetonato)nickel(II), Ni( a c a ~ ) which ~, has been shown to have the trimeric structure (structure VIII) (15). It can be seen that the oxygens of the ligand act as bridging atoms. Other complexes which polymerize via chelated oxygenbridges are Z n ( a c a ~ )(14), ~ and Co(acac)? (15), which are trimeric and tetrameric in the solid state, respect,ively. A second type of polymerization may occur through 0-diket,onate ligands which span two metal centers via oxygen coordination. Hou~ever,only one compound has been suggested as an example of this t,ype of bonding. I n 1965 Willtinson, et al. (16) reported bhe preparnt,ion of a rhenium complex, RepCl.t(acac)n, which they "considered to have acetylacetonato-groups hridging the metal atoms" (see structure IX). However, little evidence was cited to support this structure and a fiual decision must await further studies. The presentation of various structural patterns found in 8-ketoenolat,e compounds was divided somewhat, arbitrarily into three part,s: 1) oxygen-bonded 6-ketoenolate derivat,ives; 2) carbon-bonded enolates; and 3) 6-lietoenolate interaction givingrise t,o oligomeric or polymeric complexes. Since interest in the inorganic derivat,ive chemistry of 0-dilietones shorn no sign of diminishing at present,, it is not unlikely t,hat additional unusual bonding and structural phenomena mill be uncovered. Literature Cited
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