Abstract: Transition metal catalysts comprise (a) a source of a Group 3 to 10 transition metal, (b) a ligand having the formula: R1R2X—Y—XR3R4 wherein X is phosphorus, arsenic or antimony, Y is a bridging group having the formula: Z-(A)-D-Rm wherein Z is the moiety linking the X groups, A is a linear or cyclic hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl linking group wherein the number of atoms directly linking Z to D is 1, 2 or 3, D is N, P, As, O, S or Se, R is hydrogen, alkyl, hydrocarbyl, substituted hydrocarbyl, heteroalkyl, heterohydrocarbyl or substituted heterohydrocarbyl, and m is 1 or 2, R1, R2, R3, and R4 are the same or different and represent hydrocarbyl or functionalized hydrocarbyl moieties with the proviso that if D is nitrogen, R is not a cyclic ether, and optionally an activator. The transition metal catalysts are suitable for the selective trimerisation or tetramerisation of olefins in particular ethylene.