NOVELTY - Tetra or hexa coordinated transition metal carborane complexes (I) are new.    USE - (II) is useful for producing (I). (I) are useful: as emitter complexes: in an oxygen permeable film of a device for measuring the concentration of oxygen in a medium, where the device is an optoelectronic device, preferably an outwardly non-encapsulated organic LED, and the optoelectronic device is organic LED, light emitting electrochemical cells, organic LED sensors, preferably non-hermetically sealed to the outside gas and vapor sensors, organic solar cells, organic photovoltaics, organic FET, organic lasers, organic diodes, organic photodiodes and down conversion systems for the conversion of UV-radiation into visible light or for converting light of a shorter wavelength to light of a longer wavelength; for generating singlet oxygen, preferably for synthetic chemical, antibacterial and therapeutic purposes; for the radiotherapeutic use in boron neutron capture therapy; and for the determination of the air pressure (all claimed).    ADVANTAGE - (I): have improved stability and oxygen sensor properties, and high emission quantum yields; and are capable of measuring the concentration of oxygen in a medium in a quantitative, simple and economical manner.    DETAILED DESCRIPTION - Tetra or hexa coordinated transition metal carborane complexes of formula (M(L)(L1)x(L2)y(L3)) (I) are new.    M = tetra or hexa coordinated transition metal ion comprising Re(I), Os(II), Ir(I), Ir(III), Pt(II), Pt(IV), Au(III), Ru(II), Rh(I), Rh(III) or Pd(II), where x is 1 and y and z are 0 for tetra coordinated metal ions, and x is 3 and y and z are 0, or x and y are 1 and z is 0, or x and y are 0 and z is 1 for hexa coordinated transition metal ion;    L = a tridentate ligand of the formula (II);    a1-a3 = metal ion coordinating position (C or N);    L1 = a monodentate ligand, which is a neutral or singly negatively charged ligand;    L2 = a bidentate ligand, which is a neutral or singly negatively charged ligand;    L3 = tridentate ligand, which is a neutral or negatively charged ligand having a structure to formula (IV), in which the bridges C9 and C10 contain 1-8C; either    R1-R3 = alkyl, aryl, heteroaryl (all optionally substituted or halogenated), H, halide (F, Cl, Br, I), -OR1a, -C(O)OR1a, -C(O)R1a, -S(O)2R1a, -P(O)(R1a)(R2a), -S(R1a) -P(R1a)(R2a), -OP(R1a)(R2a) or -N(R1a)(R2a); or    R1-R3 optionally linked together = fused fragments;    R1a, R2a = alkyl, aryl, heteroaryl (all optionally substituted) or H;    G1, G2 = C or N; and    D5-D7 = N, C, O, P or S.    INDEPENDENT CLAIMS are included for:    (1) preparing (I);    (2) a device for measuring the concentration of oxygen in a medium, comprising (I); and    (3) the ligand (II).