The present invention relates to organometallic transition metal compounds of the formula (I) M1 is a metal of group 3, 4, 5 or 6 of the Periodic Table of the Elements or the lanthanicles, X are identical or different and are each an organic or inorganic radical, where two radicals X can also be joined to one another, n is a natural number from 1 to 4, T1, T2 are identical or different and are each a divalent group selected from the group consisting of —O—, —S—, —Se—, —Te—, —N(R13)_′—P(R13)—, —As(R13)—, Sb(R13)—, —Si(R13)2-, —C(R13 R14)—C(R13 R15)— and —C(R14)═C(R15)—, where R13, R14 and R15 are identical or different and are each hydrogen or an organic radical having from 1 to 40 carbon atoms, R1, R7 are identical or different and are each hydrogen or an organic radical having from 1 to 40 carbon atoms, R2 R8 are identical or different and are each hydrogen or an organic radical having from 1 to 40 carbon atoms, R3 R9 are identical or different and are each halogen or an organic radical having from 1 to 40 carbon atoms, where R3 is not methyl when T1 is —C(H)═C(H)—, R4, R5 R6, R10, R11 and R12 are identical or different and are each hydrogen, halogen or an organic radical having from 1 to 40 carbon atoms, or two adjacent radicals R4 , R5, R6, R10, R11 and R12 together with the atoms connecting them form a monocyclic or polycyclic, substituted or unsubstituted ring system which has from 1 to 40 carbon atoms and may also contain heteroatoms selected from the group consisting of the elements O, S, Se, Te, N, P, As, Sb and Si, or, if T1 or T2 is —O—, —S—, —Se— or —Te—, the radical R3 together with R4 and/or the radical R9 together with R10 forms a monocyclic or polycyclic, substituted or unsubstituted ring system which has from 1 to 40 carbon atoms and may also contain heteroatoms selected from the group consisting of the elements O, S, Se, Te, N, P, As, Sb and Si, and A is a bridge consisting of a divalent atom or a divalent group, biscyclopentadienyl ligand systems having such a substitution pattern, catalyst systems comprising at least one of the organometallic transition metal compounds of the present invention, a process for preparing polyolefins by polymerization or copolymerization of at least one olefin in the presence of one of the catalyst systems of the present invention and the polyolefins obtainable in this way, the use of the biscyclopentaclienyl ligand systems of the present invention for preparing organometallic transition metal compounds and a process for preparing organometallic transition metal compounds using the biscyclopentaclienyl ligand systems.