A mono-hydroindacenyl complex having the formula Hydroindac-Y m M A X A n, where the variables have the following meanings: Hydroindac is an hydroindacenyl system having an arylalkyl substituent having from 1 to 10 carbon atoms in the alkyl radical and 6-20 carbon atoms in the aryl radical, Y is a substituent which is bound to Hydroindac and comprises at least one uncharged donor containing at least one atom of group 15 or 16 of the Periodic Table, M A is chromium, molybdenum or tungsten and m is 1,2 or 3, the radicals X A are each, independently of one another, fluorine, chlorine, bromine, iodine, hydrogen, C 1 -C 10 -a)ky), C 2 -C 10 -alkenyl, C 6 -C 20 -aryl, arylalkyl having 1-10 carbon atoms in the alkyl radical and 6-20 carbon atoms in the aryl radical, NR 23A R 24A, OR 23A, SR 23A, SO 3 R 23A, OC R 23A, CN, SCN, β-diketonate, CO, BF4 -, PF 6 - or bulky noncoordinating anions or two radicals X A form a substituted or unsubstituted diene ligand, in particular a 1,3-diene ligand, and the radicals X A may be joined to one another, R 23 A-R 24 A are each, independently of one another, hydrogen, C 1 -C 20 -alkyl, C 2 -C 20 -alkenyl, C 6 -C 20 -aryl, arylalkyl having from 1 to 10 carbon atoms in the alkyl radical and 6-20 carbon atoms in the aryl radical, SiR 25a 3, where the organic radicals R 23 A-R 24 A may also be substituted by halogens or nitrogen- and oxygen-containing groups and two radicals R 23 A-R 24 A may also be joined to form a five- or six-membered ring, the radicals R 25A are each, independently of one another, hydrogen, C 1 -C 20 -alkyl, C 2 -C 20 -alkenyl, C 6 -C 20 -aryl, arylalkyl having from 1 to 10 carbon atoms in the alkyl radical and 6-20 carbon atoms in the aryl radical and two radicals R 25A may also be joined to form a five- or six-membered ring andd n is 1,2, or 3. Hydroindac is an hydroindacenyl system having an arylalkyl substituent having from 1 to 10 carbon atoms in the alkyl radical and 6-20 carbon atoms in the aryl radical, Y is a substituent which is bound to Hydroindac and comprises at least one uncharged donor containing at least one atom of group 15 or 16 of the Periodic Table, M A is chromium, molybdenum or tungsten and m is 1,2 or 3, the radicals X A are each, independently of one another, fluorine, chlorine, bromine, iodine, hydrogen, C 1 -C 10 -a)ky), C 2 -C 10 -alkenyl, C 6 -C 20 -aryl, arylalkyl having 1-10 carbon atoms in the alkyl radical and 6-20 carbon atoms in the aryl radical, NR 23A R 24A, OR 23A, SR 23A, SO 3 R 23A, OC R 23A, CN, SCN, β-diketonate, CO, BF4 -, PF 6 - or bulky noncoordinating anions or two radicals X A form a substituted or unsubstituted diene ligand, in particular a 1,3-diene ligand, and the radicals X A may be joined to one another, R 23 A-R 24 A are each, independently of one another, hydrogen, C 1 -C 20 -alkyl, C 2 -C 20 -alkenyl, C 6 -C 20 -aryl, arylalkyl having from 1 to 10 carbon atoms in the alkyl radical and 6-20 carbon atoms in the aryl radical, SiR 25a 3, where the organic radicals R 23 A-R 24 A may also be substituted by halogens or nitrogen- and oxygen-containing groups and two radicals R 23 A-R 24 A may also be joined to form a five- or six-membered ring, the radicals R 25A are each, independently of one another, hydrogen, C 1 -C 20 -alkyl, C 2 -C 20 -alkenyl, C 6 -C 20 -aryl, arylalkyl having from 1 to 10 carbon atoms in the alkyl radical and 6-20 carbon atoms in the aryl radical and two radicals R 25A may also be joined to form a five- or six-membered ring andd n is 1,2, or 3.