The present invention relates to chromium(III) complexes defined by following formula: 1 wherein: each R1 is independently selected from hydrogen, C1-C20 alkyl, C3-C20 cycloalkyl, C6C20 aryl, C2-C20 alkenyl, optionally comprising 1 to 5 heteroatoms such as Si, N, P, O, F, Cl, B; each R2 is independently selected from hydrogen, C1-C20 alkyl, C3-C20 cycloalkyl, silyl, C6-C20 aryl, C2-C20 alkenyl, optionally comprising 1 to 5 heteroatoms such as Si, N, P, O, F, Cl, B; R1 and R2 can join together forming one or more aromatic or aliphatic rings optionally containing heteroatoms; each A is independently selected from nitrogen, phosphorous, As, Sb; each E is independently selected from carbon, phosphorus, nitrogen; Z is a group comprising at least a nitrogen, phosphorous, oxygen or sulfur atom which is able to coordinate to the chromium atom; Z can join one or more R2 groups to form one or more rings; each p is independently 1 or 2; each n is independently 0, 1 or 2; each X is independently an atom or group covalently or ionically attached to the chromium atom. This invention further relates to olefin polymerization catalysts obtainable by treating a chromium complex with an organoaluminium compound. The catalyst preparation and its use in polymerization are described.