Complexes of lanthanides with heterocyclic carbenes

A complex of the formula

• ›L.sub.a Ln.sub.b X.sub.c !.sup.n (A).sub.n (I)
• wherein Ln is an ion in the oxidation state 2, 3 or 4 of the lanthanide elements having the atomic numbers 21, 39 and from 57 to 71 in the Periodic Table of the Elements, with the exception of europium and promethium, as central atom, X are monodentate or multidentate, charged or uncharged ligands bound to the central atom and L is selected from the group consisting of monocarbenes of the formulae ##STR3## and dicarbenes of the formulae ##STR4## likewise bound to the central atom, wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5 and R.sup.6 are individually selected from the group consisting of sulfonated or unsulfonated alkyl of 1 to 7 carbon atoms, sulfonated or unsulfonated aliphatic monocyclics or poly-cyclics of 5 to 18 carbon atoms, sulfonated or unsulfonated alkenyl of 2 to 5 carbon atoms, sulfonated or unsulfonated aryl of 6 to 14 carbon atoms and sulfonated or unsulfonated arylalkyl of 7 to 19 carbon atoms or R.sup.3, R.sup.4, R.sup.5 and R.sup.6 can be hydrogen, or R.sup.3 together with R.sup.4 and R.sup.5 together with R.sup.6 can in each case also be individually fused and sulfonated or unsulfonated groups of 3 to 7 carbon atoms, Y is an optionally unsaturated alkylidene of 1 to 4 carbon atoms or dialkyl-silylene or tetraalkyt-disilylene, A is a singly charged anion or the chemical equivalent of a multiply charged anion, b is an integer from 1 to 4, a is an integer from 1 to 4.b and c is zero or an integer from 1 to 3.b and n is zero or 1 to 3.b.