Organometallic transition metal compounds of the formula (I) where M.sup.1 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.sub.1 C.sub.20-alkyl, C.sub.2 C.sub.20-alkenyl, C.sub.6 C.sub.22-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.sup.5 or --NR.sup.5R.sup.6, 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.sup.5 and R.sup.6 are each C.sub.1 C.sub.10-alkyl, C.sub.6 C.sub.15-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.sup.1 minus 2; R.sup.1 is a C.sub.2 C.sub.40 radical which is branched in the .alpha. position; R.sup.2 is hydrogen or a C.sub.1 C.sub.40 radical which may be branched or unbranched in the .alpha. position; R.sup.3, R.sup.3a and R.sup.3b are identical or different and are each hydrogen or a C.sub.1 C.sub.40 radical, where at least one of the radicals R.sup.3, R.sup.3a, R.sup.3b is not hydrogen; R.sup.4 is a substituted or unsubstituted C.sub.6 C.sub.40-aryl radical or C.sub.2 C.sub.40-heteroaromatic radical containing at least one heteroatom selected from the group consisting of O, N, S and P; T is a divalent C.sub.1 C.sub.40 group which together with the cyclopentadienyl 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 ##STR00001##