NOVELTY - Transition metal complexes (I) are new.    USE - (I) are useful: in an oligomer, polymer or dendrimer, photocatalysis, oxygen sensors and electronic device such as organic integrated circuits, organic field effect transistors, organic thin-film transistors, organic light-emitting transistors, organic solar cells, organic optical detectors, organic photoreceptors, organic field-quench devices, light-emitting electrochemical cells or organic laser diodes; for producing singlet oxygen; and as light-emitting compound (in combination with a matrix material) in organic electroluminescent devices (all claimed).    ADVANTAGE - (I) exhibit high quantum-energy, long service life, high thermal stability and purity; and can be directly prepared in high yield.    DETAILED DESCRIPTION - Transition metal complexes of formula (M(L)n(L1)m) (I) are new.    M(L)n = a metal complex of formula (II);    M = transition metal;    A = O, S or NR;    X, Y1 = CR or N, where two adjacent groups of X is phenyl ring of formula (III) or fused phenyl ring of formula (IV);    E = C(R)2, NR, O or S;    R = H, D, F, Cl, Br, I, N(R1)2, P(R1)2, CN, NO2, Si(R1)3, B(OR1)2, C(=O)R1, P(=O)(R1)2, S(=O)R1, S(=O)2R1, OSO2R1, OH, SH, linear 1-40C-alkyl, linear 1-40C-alkoxy, linear 1-40C-thioalkoxy, linear 2-40C-alkenyl, linear 2-40C-alkynyl, branched or cyclic 3-40C-alkyl, branched or cyclic 3-40C-alkenyl, branched or cyclic 3-40C-alkynyl, branched or cyclic 3-40C-alkoxy, branched or cyclic 3-40C-thioalkoxy (where one or more non-adjacent CH2 group is optionally replaced by R1C=CR1, CC, Si(R1)2, Ge(R1)2, Sn(R1)2, C=O, C=S, C=Se, C=NR1, P(=O)(R1), SO, SO2, NR1, O, S or CONR1 and one or more H atoms are replaced by D, F, Cl, Br, I, CN or NO2), 5-60 membered (hetero)alkyl, 5-60 membered (hetero)alkoxy, 10-40 membered diarylamino, 10-40 membered diheteroarylamino or 10-40 membered arylheteroarylamino (all optionally substituted by R1), where two or more adjacent R forms a mono- or polycyclic, aliphatic, aromatic and/or benzoannelated ring system;    R1 = H, D, F, Cl, Br, I, N(R2)2, CN, NO2, Si(R2)3, B(OR2)2, C(=O)R2, P(=O)(R2)2, S(=O)R2, S(=O)2R2, OSO2R2, linear 1-40C-alkyl, linear 1-40C-alkoxy, linear 1-40C-thioalkoxyl, linear 2-40C-alkenyl, linear 2-40C-alkynyl, branched or cyclic 3-40C-alkyl, branched or cyclic 3-40C-alkenyl, branched or cyclic 3-40C-alkynyl, branched or cyclic 3-40C-alkoxy, branched or cyclic 3-40C-thioalkoxy (where one or more non-adjacent CH2 groups are optionally replaced by R2C=CR2, CC, Si(R2)2, Ge(R2)2, Sn(R2)2, C=O, C=S, C=Se, C=NR2, P(=O)(R2), SO, SO2, NR2, O, S or CONR2 and one or more H atoms are replaced by D, F, Cl, Br, I, CN or NO2), 5-60 memebred (hetero)aryl ring, 5-60 membered (hetero)aryloxy, 10-40 membered diarylamino, 10-40 membered diheteroarylamino or 10-40 membered arylheteroarylamino (all optionally substituted by R2), where two or more adjacent R1 together form a mono- or polycyclic, aliphatic or aromatic ring system;    R2 = H, D, F, 1-20C-aliphatic and/or (hetero)aryl (in which one or more H-atoms are replaced by D or F), where two or more of R2 together form a mono- or polycyclic, aliphatic or aromatic ring system;    L1 = co-ligand;    n = 1-3; and    m = 0-4.    Where: many ligands (L) can be linked together or with L1 over a bridge (V1) to form a tridentate, tetradentate, pentadentate or hexadentate ligand system. INDEPENDENT CLAIMS are included for:    (1) the preparation of (I);    (2) an oligomer, dendrimer or polymer comprising (I), where at least one of R represents a bond to the oligomer, polymer or dendrimer; and    (3) formulation comprising (I) or the oligomer, polymer or dendrimer and at least one further compound, preferably a solvent.