The element tin does what carbon will not

New bonding suggests scientists may need to rethink heavy metal chemistry

Just because carbon jumps off a bridge, doesn’t mean that tin will too. Scientists have conducted a simple experiment that attaches a simple hydrocarbon to triple-bonded tin atoms, violating a well-established set of organic chemistry rules. The finding suggests that heavier elements don’t behave the same way as carbon, researchers report in the Sept. 25 Science.

STRANGE BONDFELLOWS | Triple-bonded tin in solution (green) breaks bonds to pick ethylene (solution turns yellow), violating organic chemistry rules. A slight change in pressure reverses the reaction (back to green). Yang Peng

In the new work, tin atoms bonded to ethylene, a small molecule consisting of two carbon and four hydrogen atoms. Tin should, in principle, be chemically similar to carbon, but carbon does not undergo the same reaction.

“I believe this is a reaction that could lead to further breakthroughs in fundamental science,” says organic chemist Lawrence Sita of the University of Maryland in College Park, who wrote a commentary on the research in the same issue of Science. “This could be a launching point for a number of experiments.”

In the mid-1960s, chemists Robert Woodward and Roald Hoffmann showed that certain reactions involving carbon-containing molecules are more likely than others. These reactions proceed in a predictable manner to predictable products based on the symmetry of orbitals, regions of space occupied by electrons as they whiz around the atoms involved.

Woodward and Hoffmann’s rules revealed order in a bunch of seemingly unrelated reactions. But the rules were based on carbon, and few considered how they applied to other elements. Because tin resides in the same column of the periodic table of the chemical elements as carbon, though, it should therefore exhibit similar chemical properties and presumably follow the same rules.

But now, Philip Power of the University of California, Davis and colleagues have demonstrated a reaction with tin that is forbidden by the Woodward-Hoffmann rules for carbon. The team added bulky hydrocarbon groups to tin in solution, creating a triple bond connecting two tin atoms plus the hydrocarbons. When Power and colleagues then dissolved ethylene into the solution, the tin atoms loosened their triple grip on each other and picked up ethylenes, forming a ringed structure. If tin followed the carbon rules, the triple bond would not have broken to form rings with ethylene.

“Everything is simple with carbon compounds—there is no in-between,” Power says. But this experiment shows that “the normal, beautifully simple rules you have for carbon no longer operate effectively for heavier elements.”

Tin, element 50, is the fourth element in the carbon column. With more protons — 50 to carbon’s 6 — tin’s nucleus is bigger and heavier, exerting different forces on the circling electrons. While the Woodward-Hoffmann rules are powerful and predictive, this research suggests that scientists need to think differently about carbon’s metallic neighbors, Sita says. “If we never had this reaction we would never know the current rules are deficient.”

The new reaction is also interesting because it takes place without the need for extreme temperature and pressure conditions, and it is reversible, Sita notes. When the scientists applied heat or reduced the pressure, the tin released the ethylene.

The research suggests that heavy metals in the carbon column, which also includes lead and germanium, are relatively underexplored. Carbon’s column sits at the heart of the transition between metals and nonmetals and merits more attention from researchers, Sita says.

Power says there are now a number of paths to pursue. Metals are a crucial part of many biological processes.

“Conventional wisdom says these elements aren’t interesting,” he says. But metals are important in such processes as photosynthesis and in carrying oxygen through the body. Power says thinking differently about metal chemistry could have implications for the development of solar cells and other energy-capturing devices.

Tin-Carbon bonding in action from Science News on Vimeo.

Triple-bonded tin in solution (green) breaks bonds to pick ethylene (solution turns yellow), violating organic chemistry rules. A slight change in pressure reverses the reaction (back to green).

Credit: Yang Peng

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