Transition metal reactivity clarified First-row transition metals may be more reactive in addition reactions than previously predicted, when com pared with second- and third-row metals of the same periodic group in complexes with identical ligands. Comparisons of second- and third-row metals have generally indicated that the third-row metal is more reactive. Until recently, however, there has been no series of compounds with identical structures and electronic va lence shells for all three rows to per mit a comparison of activities of three transition metals in the same group. Work by Dr. Lauri Vaska and co workers at Clarkson College of Tech nology, Potsdam, N.Y., indicates that for at least the cobalt-rhodium-iridium group, cobalt (first row) is more re active in addition reactions than is iridium (third row), which is in turn more reactive than is rhodium (sec ond row). Specifically, Dr. Vaska, Loomis S. Chen, and Dr. Warren V. Miller have synthesized and studied univalent cationic complexes of the three transition metals with c/s-l,2-bis( diphenylphosphino ) ethylene [cis(CeH5)2PCH=^CHP(CeH5)2] [/. Amer. Chem. Soc, 93, 6671 (1971)]. Comparisons. Reactions of the three complexes with diatomic molecules permit the first comparison of reac tivities of planar d8 complexes of three different transitions in the same group. The complexes bind molecular oxygen and hydrogen, among others. Studies of the complexes are thus of interest for homogeneous catalysis and for biological reactions of metal complexes with gases. The Clarkson chemists have meas ured rates for the addition of molecu lar oxygen and hydrogen to complexes
Cobalt binds both oxygens
|*fiNmy! groupe not shown for reasons of simplicity
of Co(I), R h ( I ) , or I r ( I ) with two bidentate cis-l ,2-bis ( diphenylphos phino) ethylene ligands. At 25° C , the cobalt complex reacts with molecular oxygen about 3 Χ 104 times more rap idly than does the iridium complex, and about 105 times more rapidly than does the rhodium complex. The rates of the cobalt and iridium reactions with hydrogen do not differ quite as dramatically, but the rhodium com plex does not react with hydrogen at all. The binding of oxygen and hydro gen by the cobalt and iridium com plexes is irreversible at 25° C , where as oxygen dissociates from the rhod ium complex at a measurable rate. The surprisingly rapid reaction of cobalt is at present of uncertain sig nificance for the use of this type of co balt complex as a catalyst in reactions such as hydrogénation and oxygenation, Dr. Vaska says. Cobalt may react rapidly but bind reactants or products too strongly. Rhodium reacts more slowly but may be a better catalyst in some cases, because its binding is reversible. Dr. Vaska and his coworkers have also determined activation energies for the addition reactions and find that the energies are related to ligand field stabilization energies derived from measurements of electronic spectra. The ligand field stabilization energy is the energy difference between the complex's highest filled and lowest unfilled molecular orbitals. These molecular orbitals may involve the metal's dxy and dx2-y2 orbitals in the highest filled and lowest unfilled molecular orbitals, respectively. Activation energies for the addition of oxygen to the three complexes are proportional to the complexes' ligand field stabilization energies, with the two energies being lowest for the highly reactive cobalt complex. Structures. The structures, as well as the reactivities, of the three metal complexes can be compared. Dr. Vaska, working with Dr. Elmer L. Amma and graduate student Newton W. Terry, III, at the University of South Carolina, Columbia, has determined the crystal structure of the cobalt complex with bound molecular oxygen [/. Amer. Chem. Soc, 94, 653 (1972)]. This represents the first determination of the structure of a monomeric cobalt complex with oxygen. (Structures also have been determined for oxygenated cobalt complexes that are dimers with oxygen bridges.) Presently work is underway on the structures of the monomeric rhodium and iridium complexes. The axis of the oxygen molecule in
the cobalt complex is perpendicular to a line from the cobalt through the center of the oxygen molecule. This structure is analogous to those of several other oxygenated iridium and rhodium complexes, including a number of structures determined by Dr. James A. Ibers and coworkers at Northwestern University, Evanston, 111. The two cobalt-oxygen distances in the Clarkson cobalt complex are about equal: 1.871 A. and 1.902 A. The oxygen-oxygen bond distance is 1.420 Α., compared to 1.21 A. in the free oxygen molecule. A preliminary analysis of data from the oxygenated rhodium complex indicates the same orienta tion of the oxygen molecule, with rhodium-oxygen distances of 2.023 A. and 2.053 A. and an oxygen-oxygen dis tance of 1.433 A. The observed oxygen-oxygen bond distances in these complexes show that previously found correlations of reactivity and stability with oxygenoxygen bond lengths do not hold for the cobalt complex, Dr. Vaska points out. The cobalt complex, with its high rate of oxygen addition, would be ex pected to give an adduct with a longer oxygen-oxygen bond than observed. Clues. Dr. Vaska believes that the reactivity and structure of the cobalt complex give some important clues for oxygen binding by hemoglobin and other biological complexes of iron, a first-row transition metal adjacent to cobalt in the periodic table. The great difference in the rates of deoxygenation of the oxygenated cobalt complex and oxyhemoglobin, together with evi dence from other studies, supports Dr. Linus Pauling's proposal of a bent ar rangement for iron and oxygen in oxy hemoglobin: One atom of the oxygen molecule is bound to iron, with the other oxygen atom bent at an angle away from the iron-oxygen bond axis. Other cobalt complexes may indeed bind oxygen in the bent structure. Monomeric Schiff base complexes of cobalt(II) bind oxygen reversibility in a bent structure, according to evi dence from infrared and electron para magnetic resonance studies by Dr. Fred Basolo, Dr. Brian M. Hoffman, and graduate students Alvin L. Crumbliss and Damon L. Diemente at North western [/. Amer. Chem Soc, 92, 55, 61 (1970)]. EPR studies by Dr. Hoff man and other colleagues also indicate bent binding for oxygen with coboglobin, a complex of the globin protein from hemoglobin with cobalt in a protoporphyrin ring (C&EN, Sept. 28, 1970, page 40). The different oxidation states of cobalt—\-2 in these studies and + 1 in Dr. Vaska's work—may be a factor in the different geometries for oxygen binding.
