NOVELTY - Transition metal complexes (A) are new.    USE - (A) are useful in a layer or an electronic device, preferably organic electroluminescent devices or polymeric electroluminescent devices, 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. (A) are useful as an emitting compound in an emitting layer or charge transport compound in a charge transport or charge injection layer in the organic electroluminescent devices (all claimed).    ADVANTAGE - (A) exhibit high thermal stability and improve the operational lifetime of organic electroluminescent devices.    DETAILED DESCRIPTION - Transition metal complexes (A) of formulae (I) and (II) are new.    M = metal ion with an oxidation state of +2;    Ar1 = 5-60C-(hetero)aryl ring system (optionally substituted with one or more R);    Y1 = C, N or P, with the proviso that two carbon atoms and two N atoms, or two carbon atoms and two P atoms are bonded to the metal;    X = B(R1), C(R1)2, Si(R1)2, C=O, C=NR1, C=C(R1)2, O, S, S=O, SO2, N(R1), P(R1) and/or P(=O)R1, 5-60C-(hetero)aryl ring system, which is optionally substituted with one or more R; either    R = linear-1-40C-alkyl, -1-40C-alkoxy, -1-40C-thioalkoxy, branched or cyclic-3-40C-alkyl, -3-40C-alkoxy or -3-40C-thioalkoxy (all optionally substituted with one or more R2, where one or more non-adjacent CH2-groups are replaced by R2C=CR2, C?=C, 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 hydrogen atoms are replaced by D, F, Cl, Br, I, CN or NO2), D, F, Cl, Br, I, N(Ar2)2, N(R2)2, CN, NO2, Si(R2)3, B(OR2)2, C(=O)Ar2, C(=O)R2, P(=O)(Ar2)2, P(=O)(R2)2, S(=O)Ar2, S(=O)R2, S(=O)2Ar1, S(=O)2R2, -CR2=CR2Ar2, OSO2R2, or 5-60C-(hetero)aryl ring system and/or 5-60C-(hetero)aryloxy ring system (both optionally substituted with one or more R2); and    R1 = linear-1-40C-alkyl, -1-40C-alkoxy, -1-40C-thioalkoxy, or branched or cyclic-3-40C-alkyl, -3-40C-alkoxy or -3-40C-thioalkoxy (all optionally substituted with one or more R2, where one or more non-adjacent CH2-groups are replaced by R2C=CR2, C?=C, Si(R2)2, Ge(R2)2, Sn(R2)2, and one or more hydrogen atoms are replaced by D, F, Cl, Br, I, CN or NO2), H, D, F, CF3, or 5-60C-(hetero)aryl ring system and/or 5-60C-(hetero)aryloxy ring system (both optionally substituted with one or more R2); or    R1R, two or more R = mono- or poly-cyclic, aliphatic or aromatic ring system;    Ar2 = 5-40C-(hetero)aryl ring system (optionally substituted with one or more non-aromatic R2); either    R2 = H, D, F or 1-20C-aliphatic and/or 1-20C-(hetero)aromatic in which one or more H atoms are replaced by F; or    two or more R2 = mono- or poly-cyclic, aliphatic or aromatic ring system; and    n = 0 or 1, when n is equal to 0, then X is bounded by H, R or Ar1.    INDEPENDENT CLAIMS are included for:    (1) oligomers, polymers or dendrimers comprising (A), where one or more bonds are present in (A) to form the polymers, oligomers or dendrimers;    (2) preparing (A);    (3) an organic electronic device, preferably organic electroluminescent devices or polymeric electroluminescent devices, 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, comprising (A); and    (4) layer comprising one or more (A).