NOVELTY - Transition metal complexes (A) are new.    USE - (A) are useful in a layer or an electronic device, preferably organic electroluminescent devices, 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) is useful as an emitting compound in an emitting layer or a 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 ions with an oxidation state of +2, preferably Pt or Pd;    Ar1=5-60C-(hetero)aryl ring system (optionally substituted with one or more R), preferably benzene, naphthalene, pyridine, quinoline, isoquinoline, pyrimidine, pyrazine, pyridazine, furan, thiophene, pyrrole, benzofuran, benzothiophene or indole;    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; 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, and one or more non-adjacent CH 2-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, SO 2, NR2, O, S or CONR2, and one or more hydrogen atoms are replaced by D, F, Cl, Br, I, CN or NO 2), H, D, F, Cl, Br, I, N(Ar2) 2, N(R2) 2, CN, NO 2, 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, OSO 2R2, 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, and one or more non-adjacent CH 2-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 NO 2), H, D, F, CF 3, 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); and 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.    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) electronic devices, preferably organic electroluminescent devices, 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 (A).