The present invention relates to chromium(III) complexes defined by following formula: wherein: each R<SUB>1 </SUB>is independently selected from hydrogen, C<SUB>1</SUB>-C<SUB>20 </SUB>alkyl, C<SUB>3</SUB>-C<SUB>20 </SUB>cycloalkyl, C<SUB>6</SUB>-C<SUB>20 </SUB>aryl, C<SUB>2</SUB>-C<SUB>20 </SUB>alkenyl, optionally comprising 1 to 5 heteroatoms such as Si, N, P, O, F, Cl, B; each R<SUB>2 </SUB>is independently selected from hydrogen, C<SUB>1</SUB>-C<SUB>20 </SUB>alkyl, C<SUB>3</SUB>-C<SUB>20 </SUB>cycloalkyl, silyl, C<SUB>6</SUB>-C<SUB>20 </SUB>aryl, C<SUB>2</SUB>-C<SUB>20 </SUB>alkenyl, optionally comprising 1 to 5 heteroatoms such as Si, N, P, O, F, Cl, B; R<SUB>1 </SUB>and R<SUB>2 </SUB>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 R<SUB>2 </SUB>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.