Reivindicaciones

1. A chromium (III) tridentate complex of formula I <img class="EMIRef" id="449869956-ib0048" /> wherein: Z is a group comprising at least a nitrogen, phosphorous, oxygen or sulfur atom which is able to coordinate to the chromium atom; each R<1> is independently selected from the group consisting of H and a monovalent radical comprising from 1 to 30 carbon atoms and from 0 to 5 heteroatoms independently selected from Si, N, P, O, F, Cl and B atoms; each R<2> is independently selected from the group consisting of H and a monovalent radical comprising from 1 to 30 carbon atoms and from 0 to 5 heteroatoms independently selected from Si, N, P, O, F, Cl and B atoms; each R<2> can join together with either R<1> or Z forming one or more aromatic or aliphatic rings optionally containing heteroatoms; each A is independently selected from nitrogen, phosphorous, As and Sb; each E is independently selected from carbon, phosphorus and nitrogen; each p is independently 1 or 2; each n is independently 0, 1 or 2; and each X is independently an atom or group covalently or ionically attached to the chromium atom. 2. Chromium complex according to claim 1, wherein Z is selected from the group consisting of: <img class="EMIRef" id="449869956-ib0049" /> <img class="EMIRef" id="449869956-ib0050" /> wherein each R<3> is independently selected from linear or branched C<1> -C<20> alkyl, C<6> -C<30> aryl, hydroxyl, amino, nitro, tri(linear or branched C<1> -C<20> alkyl)silyl, linear or branched C<1> -C<20> alkoxy, chlorine, bromine, fluorine and trifluoromethyl; m is 0, 1, 2 or 3; A is nitrogen; and E is carbon. 3. Chromium complex according to anyone of claims 1 or 2, wherein each R<1> is independently s phenyl radical optionally substituted in the positions 2 and/or 6. 4. Chromium complexes according to anyone of claims 1 or 2, wherein each R<2> is independently selected from hydrogen, linear or branched C<1> -C<20> alkyl, C<3> -C<20> cycloalkyl, silyl, C<6> -C<30> aryl, linear or branched C<2> -C<20> alkenyl, optionally comprising 1 to 5 heteroatoms independently selected from Si, N, P, O, F, Cl and B atoms. 5. An olefin polymerisation catalyst comprising a chromium complex according to anyone of claims 1 to 4 and one or more activators. 6. Olefin polymerisation catalyst according to claim 5, further comprising an inorganic or organic support. 7. Olefin polymerisation catalysts according to anyone of claims 5 or 6, further comprising a Ziegler-Natta catalyst. 8. Olefin polymerisation catalyst according to anyone of claims 5 or 6, further comprising a metallocene catalyst. 9. A process for the (co)polymerisation of alpha olefins characterized by the use of a catalyst system according to anyone of claims 5 to 8. 10. Process according to claim 9, wherein the olefin used is ethylene optionally in combination with a C<3> -C<8> alpha olefin.