A compound of the formula I shown below: wherein: R 1 and R 2 are each independently alkyl; R 3 and R 4 are each independently hydrogen or alkyl, or R 3 and R 4 are linked such that, when taken in combination with the atoms to which they are attached, they form a 6-membered fused aromatic ring optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, aryl, heteroaryl, carbocyclic and heterocyclic, wherein each aryl, heteroaryl, carbocyclic and heterocyclic group is optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, halo, amino, nitro, cyano, alkylamino, [ alkyl] 2 amino and -S 2 alkyl; R 5 and R 6 are each independently hydrogen or alkyl, or R 5 and R 6 are linked such that, when taken in combination with the atoms to which they are attached, they form a 6-membered fused aromatic ring optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, aryl, heteroaryl, carbocyclic and heterocyclic, wherein each aryl, heteroaryl, carbocyclic and heterocyclic group is optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, halo, amino, nitro, cyano, alkylamino, [ alkyl] 2 amino and -S 2 alkyl; Q is a bridging group comprising 1, 2 or 3 bridging atoms selected from C, N, O, S, Ge, Sn, P, B, or Si, or a combination thereof, and is optionally substituted with one or more groups selected from hydroxyl, alkyl, alkenyl, alkynyl, alkoxy and aryl; X is selected from zirconium, titanium or hafnium; and each Y group is independently selected from halo, hydride, a phosphonated, sulfonated or borate anion, or a alkyl, alkenyl, alkynyl, alkoxy, aryl or aryloxy group which is optionally substituted with one or more groups selected from alkyl, halo, nitro, amino, phenyl, alkoxy, -C NR x R y, or Si[ alkyl] 3; wherein R x and R y are independently alkyl; with the proviso that: i) when R 3 and R 4 are hydrogen or alkyl, R 5 and R 6 are not linked to form a fused 6-membered aromatic ring that is substituted with four methyl groups; and ii) when R 5 and R 6 are hydrogen or alkyl, R 3 and R 4 are not linked to form a fused 6-membered aromatic ring that is substituted with four methyl groups. R 1 and R 2 are each independently alkyl; R 3 and R 4 are each independently hydrogen or alkyl, or R 3 and R 4 are linked such that, when taken in combination with the atoms to which they are attached, they form a 6-membered fused aromatic ring optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, aryl, heteroaryl, carbocyclic and heterocyclic, wherein each aryl, heteroaryl, carbocyclic and heterocyclic group is optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, halo, amino, nitro, cyano, alkylamino, [ alkyl] 2 amino and -S 2 alkyl; R 5 and R 6 are each independently hydrogen or alkyl, or R 5 and R 6 are linked such that, when taken in combination with the atoms to which they are attached, they form a 6-membered fused aromatic ring optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, aryl, heteroaryl, carbocyclic and heterocyclic, wherein each aryl, heteroaryl, carbocyclic and heterocyclic group is optionally substituted with one or more groups selected from alkyl, alkenyl, alkynyl, alkoxy, halo, amino, nitro, cyano, alkylamino, [ alkyl] 2 amino and -S 2 alkyl; Q is a bridging group comprising 1, 2 or 3 bridging atoms selected from C, N, O, S, Ge, Sn, P, B, or Si, or a combination thereof, and is optionally substituted with one or more groups selected from hydroxyl, alkyl, alkenyl, alkynyl, alkoxy and aryl; X is selected from zirconium, titanium or hafnium; and each Y group is independently selected from halo, hydride, a phosphonated, sulfonated or borate anion, or a alkyl, alkenyl, alkynyl, alkoxy, aryl or aryloxy group which is optionally substituted with one or more groups selected from alkyl, halo, nitro, amino, phenyl, alkoxy, -C NR x R y, or Si[ alkyl] 3; wherein R x and R y are independently alkyl; with the proviso that: i) when R 3 and R 4 are hydrogen or alkyl, R 5 and R 6 are not linked to form a fused 6-membered aromatic ring that is substituted with four methyl groups; and ii) when R 5 and R 6 are hydrogen or alkyl, R 3 and R 4 are not linked to form a fused 6-membered aromatic ring that is substituted with four methyl groups. i) when R 3 and R 4 are hydrogen or alkyl, R 5 and R 6 are not linked to form a fused 6-membered aromatic ring that is substituted with four methyl groups; and ii) when R 5 and R 6 are hydrogen or alkyl, R 3 and R 4 are not linked to form a fused 6-membered aromatic ring that is substituted with four methyl groups.