A compound of the general formula 1, wherein, X and X' are halogen, preferably selected from the group consisting of Cl and Br; L is an uncharged ligand of the formula L1, L2, L3 or L4: wherein R10 and R11 are each, independently of one another a substituted or an unsubstituted side chain comprising 1 to 30 carbon atoms and optionally comprising one or more functional groups, and wherein R12 and R13 are each, independently of one another, H, C 1-6 -alkyl optionally substituted by a alkoxy radical OR15, or aryl optionally substituted by a alkoxy radical OR15, or form a 3- or 4-membered alkylene bridge, and wherein R15 is selected from the group consisting of C 1-20 -alkyl, aryl and C 7-18 -aralkyl, and wherein g and g' are each halogen z is a methylene or a carbonyl group; a, b and c are each H; R1 is H; R2 is selected from the group consisting of H, C1-12-alkyl, C5-12-cycloalkyl, C7-18-aralkyl or aryl; and R3 is selected from the group consisting of H, C 1-12 -alkyl, C 5-12 -cycloalkyl, C 7-18 -aralkyl, aryl, C 1-12 -halogeno-alkyl, C 1-12 -ammonium-alkyl, C 1-12 -pyridinum-alkyl, C 1-12 -aldehyde-alkyl, C 1-12 -nitro-alkyl, nitrile or a radical selected from the group consisting of ketones COR4, esters CO 2 R4, oxalates COCO 2 R4, sulfones SO 2 R4 or amides CONHR4, wherein R4 is selected from the group consisting of H, C 1-12 -alkyl, C 5-12 -cycloalkyl, C 7-18 -aralkyl, aryl, C 1-12 -halogeno-alkyl, C 1-12 -ammonium-alkyl, C 1-12 -pyridinum-alkyl, C 1-12 -aldehyde-alkyl, C 1-12 -nitro-alkyle and nitrile, or R3 is a side chain of the formula R3 c, R3 d, R3 e, R3 f, R3 g, R3 h, R3 i, R3 j, R3 k, R3 l, R3 m, R3 n, R3 o or R3 p: or, in the case where z is methylene, R3 is a side chain of the formula R3 a or R3 b: wherein A - is selected from the group consisting of F -, Cl -, Br -, I -, tetrafluoroborate BF 4 -, hexafluorophosphate PF 6 - and bis(trifluoromethylsulfonyl)amide NTf 2 -. X and X' are halogen, preferably selected from the group consisting of Cl and Br; L is an uncharged ligand of the formula L1, L2, L3 or L4: wherein R10 and R11 are each, independently of one another a substituted or an unsubstituted side chain comprising 1 to 30 carbon atoms and optionally comprising one or more functional groups, and wherein R12 and R13 are each, independently of one another, H, C 1-6 -alkyl optionally substituted by a alkoxy radical OR15, or aryl optionally substituted by a alkoxy radical OR15, or form a 3- or 4-membered alkylene bridge, and wherein R15 is selected from the group consisting of C 1-20 -alkyl, aryl and C 7-18 -aralkyl, and wherein g and g' are each halogen wherein R10 and R11 are each, independently of one another a substituted or an unsubstituted side chain comprising 1 to 30 carbon atoms and optionally comprising one or more functional groups, and wherein R12 and R13 are each, independently of one another, H, C 1-6 -alkyl optionally substituted by a alkoxy radical OR15, or aryl optionally substituted by a alkoxy radical OR15, or form a 3- or 4-membered alkylene bridge, and wherein R15 is selected from the group consisting of C 1-20 -alkyl, aryl and C 7-18 -aralkyl, and wherein g and g' are each halogen z is a methylene or a carbonyl group; a, b and c are each H; R1 is H; R2 is selected from the group consisting of H, C1-12-alkyl, C5-12-cycloalkyl, C7-18-aralkyl or aryl; and R3 is selected from the group consisting of H, C 1-12 -alkyl, C 5-12 -cycloalkyl, C 7-18 -aralkyl, aryl, C 1-12 -halogeno-alkyl, C 1-12 -ammonium-alkyl, C 1-12 -pyridinum-alkyl, C 1-12 -aldehyde-alkyl, C 1-12 -nitro-alkyl, nitrile or a radical selected from the group consisting of ketones COR4, esters CO 2 R4, oxalates COCO 2 R4, sulfones SO 2 R4 or amides CONHR4, wherein R4 is selected from the group consisting of H, C 1-12 -alkyl, C 5-12 -cycloalkyl, C 7-18 -aralkyl, aryl, C 1-12 -halogeno-alkyl, C 1-12 -ammonium-alkyl, C 1-12 -pyridinum-alkyl, C 1-12 -aldehyde-alkyl, C 1-12 -nitro-alkyle and nitrile, or R3 is a side chain of the formula R3 c, R3 d, R3 e, R3 f, R3 g, R3 h, R3 i, R3 j, R3 k, R3 l, R3 m, R3 n, R3 o or R3 p: or, in the case where z is methylene, R3 is a side chain of the formula R3 a or R3 b: wherein A - is selected from the group consisting of F -, Cl -, Br -, I -, tetrafluoroborate BF 4 -, hexafluorophosphate PF 6 - and bis(trifluoromethylsulfonyl)amide NTf 2 -.