RESEARCH
Guidelines set for polymetallic synthesis Study includes reaction rates, alternate reaction routes, stability and redistribution behavior of products Some steps toward forming a systematic approach to polymetallic synthesis have been made at Virginia Polytechnic Institute by Dr. Raymond Dessy and his coworkers, Paul Weissman, Rudolph Pohl, Richard Kornmann, and Clifford Smith [/. Am. Chem. Soc, 88, 5117, 5121, 5124, 5129 (1966)]. Dr. Dessy believes their studies will aid inorganic coordination chemists interested in synthesizing compounds containing several metal-metal bonds. The simplest compounds containing metal-metal bonds are those with only two metals bonded to one another. One method of forming such a bond is to react a complex metal halide with a complex metallic anion. Dr. Dessy uses m to represent a metal, M, and one of its coordination or valence positions. Then m:~ represents a complex metallic anion and m'—x represents a complex metal halide. An S N reaction of these two species may result in m—mr and x - . Other possible reactions: an electron transfer resulting in m% m ,# , and x - ; a metal-halogen interchange forming m—x and m V ; and an S N reaction involving a ligand displacement other than x, resulting in m-m'-x. The electron transfer path may yield a metal-metal bond if a caged process occurs where the two radicals unite to form m—mr. Of importance to chemists involved in such syntheses, Dr. Dessy points out, is whether the interaction of m r and m'-x leads to the same products as does the interaction of m / :- and m—x. The question involves the concepts of commutation and noncommutation, the mathematical terms suggesting that A-B = B-A and that A-B ^ B-A. The metallic anions that Dr. Dessy and his group used for the study were prepared by electrochemical reduction of the m—m compound in dimethoxyethane at millimolar concentrations, with tetrabutylammonium perchlorate as the supporting electrolyte. This produces the metallic anion with a tetrabutylammonium counter ion. When metallic anions of iron, tin, lead, manganese, and molybdenum were reacted with complex metal chlorides containing the same ele62 C&EN DEC. 5, 1966
Commutation is shown when m r + m'—x leads to the same products as m':~ + m—x. Such cases are the matching sets 1 and 2. Other sets show noncommutation. Above, N.R. = no reaction; " m n " represents a complex radical other than Mn(CO)5+
ments, at least five different types of reactions took place. In his attempt to synthesize metal-metal bonds in homodimetallic compounds, Dr. Dessy succeeded with iron and tin. But the lead compounds did not form a pb-pb bond (pb represents Pb and one of its valence positions; the complex metallic anion is represented as pb:~, to differentiate from any common ions). The manganese and molybdenum compounds did not react at all under these conditions. He attributes the lack of reaction of the latter two to the drop in nucleophilic character in the sequence f e : - ^ sn: _ >pb:->mn:~>mo:-. Four systems (the iron and tin sets, and the iron and lead sets) gave commutation. Lack of commutation was evident in several combinations of the metallic anions and the metal chlorides. In several cases this was due to a low specific reaction rate constant for one of the members of the set, Dr. Dessy suggests. An example is the manganese and iron system. Another reason for lack of commutation may
have been the alternative reactions, in addition to the possibility of redistribution between m—m' and m:~. In choosing various reactions which may produce metal-metal bonds, prediction of relative reaction rates would definitely be useful. The VPI chemists measured, as model systems, the reaction rates of metallic anions with electrophilic substances (m/—x, in this case, are alkyl halides). Since reaction rate is a measure of nucleophilic activity, they were able to establish the nucleophilic activity of 19 anions derived from transition and main group elements. Dr. Dessy emphasizes the care that had to be observed in choosing a substrate to probe such nucleophilic activity. Some of his previous work strongly suggests that one-electron transfer processes are prevalent in metallic anion-substrate interactions. His results show that nucleophilic activity of the anions from Groups VIB (Cr, Mo, W ) , VIIB (Mn, Re), VA (As, Sb, Bi), and VIA (S, Se) increases with the atomic weight
within a given group. Anions derived from Group VIII (Fe and Ru) and IVA (Ge, Sn, Pb) showed decreasing nucleophilic activity with increasing atomic weight within a given group. He observed an overall span in relative rates of about 1 X 10 12 , with (C 6 H 5 ) 2 Sb:~ the most reactive and ( O C ) 5 ( N C ) C r : - the least reactive. Dr. Dessy says that his data agree well with qualitative data of other workers, such as Dr. George Parshall at Du Pont and Dr. F. G. A. Stone at Bristol University, England. This suggests that the observations are not isolated pieces of information. Once a chemist has synthesized a metal-metal bond, the stability of the bond becomes important. Dr. Dessy and his coworkers found that a possible parallel exists between the reduction potential and bond strength of compounds with metal-metal bonds. They measured reduction potentials for breaking metal-metal bonds in more than 50 compounds. They also determined the number of electrons involved in each case. They found two common routes for the electrochemical reduction of dimetallic compounds:
The first involves a two-electron addition to the compound, leading to two metallic anions via cleavage of the parent metal-metal bond. The second route is a one-electron reduction, leading to one metallic anion and a radical. In Dr. Dessy's experiments, homodimetallic (m—m) compounds followed only the route involving a twoelectron addition. In Groups VIB (Cr, Mo), VIIB (Mn, Re), and VIII (Fe, Ru), the reduction-scission of the metal-metal link required more cathodic potentials with increasing atomic weight within a given group. In Group IVA (Si, Ge, Sn, P b ) , VA (As, Sb, Bi), and VIA (S, Se), reduction-scission required less cathodic potential with increasing atomic weight within each group. These reductions are electrochemically irreversible processes, both on mercury and platinum microelectrodes. Therefore, the potentials cannot be correlated directly to a thermodynamic function. However, Dr. Dessy does point out one correlation between bond strengths and reduction potentials. The bond strengths in the catenated main group elements are fairly well established. These, in decreasing order, are: S - S > S e - S e ; S i - S i > G e -
Ge>Sn-Sn>Pb-Pb; As-As>Sb-Sb > B i - B i . Although the actual bond strengths among the transition series are not known, he notes, workers in the field tend to feel that stability increases with increasing atomic weight within a given group, for example, Mn—Mn
