Olefin Complexes of Rhodium

STABLE olefin complexes are formed only by metals near the end of each transition series in the Periodic Table, and these metals form a triangle in the Table, Cu(I) ; Pd(II), Ag(I) ; Pt(II), {Au(I) }, Hg(II). It is characteristic of the complex chemistry of metals inside the triangle that they tend to form stronger complexes with ligands containing donor atoms from the third or subsequent rows of the Periodic Table than with the corresponding ligands containing donor atoms from the second row (counting hydrogen and helium as the first row)¹. This rule of co-ordination chemistry also extends to metals bordering on the `olefin-complex triangle', especially to the metals to the left of the triangle. It seems reasonable to expect, therefore, that the triangle might eventually be extended to the left in the Periodic Table. With this object in view, we are attempting to obtain olefin complexes of cobalt, rhodium, iridium, iron, ruthenium and osmium, using the strongly chelating di-olefin, 1 : 5-cyclo-octadiene, and are now reporting briefly a series of rhodium complexes. When an ethanolic solution of rhodium chloride (RhCl₃) to which 1 : 5-Cyclo-octadiene (C₈H₁₂) has been added is boiled under reflux, a very stable yellow crystalline solid of the composition C₈H₁₂RhCl is deposited. It is diamagnetic and dimeric, and so must have the chloro-bridged structure (I), in which the metal will have square planar (dsp2) co-ordination. When it is treated with an amine, am, the bridge is split to give mononuclear complexes of the type (II). Derivatives of (C₈H₁₂)₂Rh₂Cl₂ (I) in which the chlorine atom is replaced by other univalent acid radicals, for example, bromine, iodine and SEt, were prepared. The stability of the derivatives falls rapidly in the order Cl > Br > I > SEt, as would be expected, since this is the order of increasing trans-effect of these acid radicals.