For instance, diethyl mercury is known to be one of the most dangerous neurotoxin known to mankind (http://en.wikipedia.org/wiki/Diethylmercury). The consensus on the mechanism of action for this compound is that it's the oxidation state II mercury itself that is neurotoxic, but it's the alkyl groups that help it go across the blood-brain barrier. Look up the structure of thiomersal. It is an organic mercury compound that has an alkyl group directly on mercury, thereby giving access to the brain directly.
Similar compound, dimethyl mercury is also very notorious for its neurotoxicity. I do research in organic chemistry for living and a fellow organic chemist one time accidentally dropped a drop of Dimethyl mercury on her hand. It went through the gloves that she was wearing and onto her skin. Within several hours she was dead from what the doctors described in layman terms as "her brain melted". It is scary stuff. Some things about chemicals you don't necessarily have to do human clinical trial to predict that it'd be dangerous. If you have trained eyes you can just look at the chemical strcuture and predict its toxicity. However, as the good Book says, the love of money is a root of all evil. Once a big pharma company starts liking using a compound, they'll push for research that says it's safe even if it's not. That's just the way it is sadly.
Math is like love -- a simple idea but it can get complicated. -- R. Drabek