Compounds of formula I ##STR1## wherein R and R.sup.1 are identical or different and are a radical of the formula ##STR2## where R.sup.2, R.sup.3 and R.sup.4 are each independently C.sub.1 -C.sub.18 alkyl and together contain not more than 22 carbon atoms, and R.sup.3 and R.sup.4 are also hydrogen, or wherein R and R.sup.1 are C.sub.5 -C.sub.6 cycloalkyl, unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted phenyl, or naphthyl, C.sub.7 -C.sub.14 aralyl, furfuryl or thienyl, and wherein Y is --S--, --O--, --NH--, --N(C.sub.1 -C.sub.12 alkyl)--, --CH.sub.2 --, --CH(C.sub.1 -C.sub.12 alkyl)-- or --C(C.sub.1 -C.sub.12 alkyl).sub.2 -- and X.sup..sym. is 1 equivalent of a metallic cation selected from the group consisting of Li.sup..sym., Na.sup..sym., K.sup..sym., Mg.sup.2.sym., Ca.sup.2.sym., Ba.sup.2.sym., Zn.sup.2.sym., Mn.sup.2.sym., Ni.sup.2.sym., Fe.sup.3.sym., Fe.sup.2.sym., Co.sup.2.sym., Cu.sup.2.sym., Al.sup.3.sym., Cr.sup.3.sym., VO.sup. 3.sym., ZrO.sup.2.sym., MoO.sub.2.sup.2.sym. and TiO.sup.2.sym., are suitable as additives for lubricants and hydraulic oils and bring about an improvement in the extreme-pressure properties and in the anti-wear properties.