Organometallic transition metal compounds of the formula (I) where M1 is an element of group 3, 4, 5 or 6 of the Periodic Table of the Elements or the lanthanides; X are identical or different and are each halogen, hydrogen,C¿1?-C20-alkyl, C2-C20-alkenyl, C6-C22-aryl, alkylaryl or arylalkyl having from 1 to 10 carbon atoms in the alkyl part and from 6 to 22 carbon atoms in the aryl part, -OR?5 or -NR5R6¿, where two radicals X may also be joined to one another or two radicals X may together form a substituted or unsubstituted diene ligand, in particular a 1,3-diene ligand; where R?5 and R6¿ are each C¿1?-C10-alkyl, C6-C15-aryl, alkylaryl, arylalkyl, fluoroalkyl or fluoroaryl each having from 1 to 10 carbon atoms in the alkyl radical and from 6 to 22 carbon atoms in the aryl radical; n is a natural number from 1 to 4 which is equal to the oxidation number of M?1¿ minus 2; R1 is a C¿2?-C40 radical which is branched in the α position; R?2¿ is hydrogen or a C¿1?-C40 radical which may be branched or unbranched in the α position; R?3, R3a and R3b¿ are identical or different and are each hydrogen or a C¿1?-C40 radical, where at least one of the radicals R?3, R3a, R3b¿ is not hydrogen; R4 is a substituted or unsubstituted C¿6?-C40-aryl radical or C2-C40-heteroaromatic radical containing at least one heteroatom selected from the group consisting of O, N, S and P; T is a divalent C1-C40 group which together with the cyclopen tadienyl ring forms a further saturated or unsaturated ring system having a ring size of from 5 to 12 atoms, where T may contain the heteroatoms Si, Ge, N, P, O or S withing the ring system fused onto the cyclopentadienyl ring; and Z is a bridge consisting of a divalent atom or a divalent group, can be used in catalyst systems for preparing polyolefins by polymerization or copolymerization of at least one olefin. Biscyclopentadienyl ligand systems which have such a substitution pattern and can be used for preparing these organometallic transition metal compounds are also provided.