Patent ID: 6881830

Claim:
Metal complexes conforming to complexes of metals with an azo compound of the formula (I) or its tautomeric structures where R is OH, NH 2 , NH—CN, arylamino or acylamino, R′ is NH 2 , NH—CN, arylamino or acylamino and R 1 and R 1 ′ are independently —OH or —NH 2 , wherein the metal is selected from the group consisting of scandium (Sc), yttrium (Y), cerium (Ce), praseodymlum (Pr), neodymlum (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu), wherein the metal complexes of the scandium group include two azo units per metal atom (2:1 complexes).