A process of hydrogenating a nitrile rubber comprising a) contacting a complex catalyst with hydrogen in the absence of a nitrile rubber at a temperature in the range of from 75°C to 200°C to form a catalyst composition and thereafter b) hydrogenating the nitrile rubber in the presence of the catalyst composition formed in step a) and wherein the complex catalyst has the general formula where M is ruthenium or osmium, X 1 and X 2 are identical or different ligands, L is an electron donating ligand, which can be linked or not linked with X 1 to form a cyclic structure, R 1 is hydrogen, alkyl, cycloalkyl, alkenyl, alkynyl, aryl or heteroaryl and R 2, R 3, R 4 and R 5 are identical or different and are each hydrogen or an organic or inorganic substituent, R 6 is H, alkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl, -C(=O)R,-C(=O)OR, -C(=O)N 2, -C(=S)R, -C(=S)SR, -C(=S)OR, -C(=S)N 2, - S(=O) 2 N 2, -S(=O) 2 R, -S(=O)R or a group containing either a C=O or a C=S structural element adjacent to a carbon atom which is bound to Y, n is 0 or 1, wherein if n=1, then the element Y ---- n shall mean that Y and n are linked either by a single bond or by a double bond, wherein (i) if Y and n are linked by a single bond, then Y is oxygen, sulphur, N-R or P-R, and E is CH 2 or (ii) if Y and n are linked by a double bond, then Y is N or P, and E is CH, wherein if n=0, then Y is oxygen, sulphur, N-R or P-R and directly linked by a single bond to the phenyl moiety depicted above in formula and wherein in all above occurences of general formula R is hydrogen or alkyl, cycloalkyl, alkenyl, alkynyl, aryl or heteroaryl. a) contacting a complex catalyst with hydrogen in the absence of a nitrile rubber at a temperature in the range of from 75°C to 200°C to form a catalyst composition and thereafter b) hydrogenating the nitrile rubber in the presence of the catalyst composition formed in step a) M is ruthenium or osmium, X 1 and X 2 are identical or different ligands, L is an electron donating ligand, which can be linked or not linked with X 1 to form a cyclic structure, R 1 is hydrogen, alkyl, cycloalkyl, alkenyl, alkynyl, aryl or heteroaryl and R 2, R 3, R 4 and R 5 are identical or different and are each hydrogen or an organic or inorganic substituent, R 6 is H, alkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl, -C(=O)R,-C(=O)OR, -C(=O)N 2, -C(=S)R, -C(=S)SR, -C(=S)OR, -C(=S)N 2, - S(=O) 2 N 2, -S(=O) 2 R, -S(=O)R or a group containing either a C=O or a C=S structural element adjacent to a carbon atom which is bound to Y, n is 0 or 1, Y ---- n shall mean that Y and n are linked either by a single bond or by a double bond, wherein (i) if Y and n are linked by a single bond, then Y is oxygen, sulphur, N-R or P-R, and E is CH 2 or (ii) if Y and n are linked by a double bond, then Y is N or P, and E is CH, (i) if Y and n are linked by a single bond, then Y is oxygen, sulphur, N-R or P-R, and E is CH 2 or Y is oxygen, sulphur, N-R or P-R, and E is CH 2 (ii) if Y and n are linked by a double bond, then Y is N or P, and E is CH, Y is N or P, and E is CH, Y is oxygen, sulphur, N-R or P-R and directly linked by a single bond to the phenyl moiety depicted above in formula R is hydrogen or alkyl, cycloalkyl, alkenyl, alkynyl, aryl or heteroaryl.