STRUCTURAL CHARACTERIZATION OF A TETRANUCLEAR RHODIUM COMPLEX, [RH2CL2(CO) (C2H5C2C2H5)2]2, CONTAINING A COORDINATE-COVALENT ELECTRON-PAIR RHODIUM(I)-RHODIUM(III) BOND

The rhodacyclopentadiene complex [Rh2Cl2(CO)(C2H5C2C2H5)2]2, produced in the reaction of 3-hexyne with chlorodicarbonyl rhodium dimer, crystallizes with four molecules arranged in a triclinic unit cell of symmetry B1 such that two half-molecules comprise the crystallographically asymmetric unit. The dimensions of this centrosymmetric B-centered cell are a = 16.78 Å,b = 20.45 Å, c = 9.60 Å, α = 96.4°, β = 89.5°, and γ = 102.7˚. The structure consists of discrete [Rh2Cl2(CO)(C2H5C2C2H5)2]2 molecules in which two cis-tetraethylbutadiene fragments (each arising from the polymerization of two diethylacetylene molecules) are each coordinated to a Rh(III) by two σ bonds to give a rhodacyclopentadiene ring system and to a Rh(I) by two μ-type bonds. Two symmetrically bridging chlorine atoms interconnect each Rh(III) of a rhodacyclopentadiene ring to the Rh(I) which is not μ-bonded to that ring. Each Rh(I) has a localized square-planar environment through linkage with the midpoints of the two olefinic groups and the two chlorine atoms, while each Rh(III) has a localized square-pyramidal environment through bonding with the apical carbonyl group and with the two basal terminal diene-carbon and two chlorine atoms. The resulting molecular configuration, required to have a crystallographic center of symmetry, ideally conforms to C2h symmetry. The unprecedented structural feature found in this complex is a coordinate-covalent Rh(I)-Rh(III) bond which is necessitated in order for each of the two centrosymmetrically related Rh(III) atoms per molecule to attain a coordinatively saturated electronic configuration. This proposed electron-pair metal-metal bond is in accord not only with the short Rh(I)-Rh(III) distance of 2.70 Å being similar to rhodium-rhodium distances in other organorhodium complexes (where molecular stabilization by Rh-Rh bonds is obvious), but also with the stereochemical similarity of this complex to ferracyclopentadiene complexes which also possess coordinate-covalent metal-metal bonds. The chemical implications of the two Rh(I)-Rh(III) bonds in the [Rh2Cl2(CO)(C2H5C2C2H5)2]2 molecule are discussed. © 1969, American Chemical Society. All rights reserved.