PPD also forms a complex with some developing agents, the most common being Meritol which is a PPD/Pyrocatechin complex. It may well be less toxic as a complex although the commercial developers I used which cointained Meritol did have a warning label even in the late 1960s.
Another compound is between ppd and hydroquinone. This is a 1:1 addition compound where one ppd molecule sits on a hydroquinone molecule something like the two slices of bread in a sandwich. These compounds are held together by hydrogen bonds. While they are stable in solid form they dissociate back to ppd and hydroquinone or catechol when dissolved in water.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.