A catalyst of formula (I') wherein: M 1 and M 2 are independently selected from Zn, Cr, Co, Cu, Mn, Mg, Ni, Fe, Ti, V, Cr -X, Co -X, Mn -X, Ni -X, Fe -X, Ca, Ge, AI -X, Ti -X, V -X, Ge - 2 or Ti - 2; wherein at least one of M 1 or M 2 is selected from Ni(ll) and Ni -X; R 1 and R 2 are independently selected from hydrogen, halide, a nitro group, a nitrile group, an imine, an amine, an ether group, a silyl group, a silyl ether group, a sulfoxide group, a sulfonyl group, a sulfinate group or an acetylide group or an optionally substituted alkyl, alkenyl, alkynyl, haloalkyl, aryl, heteroaryl, alkoxy, aryloxy, alkylthio, arylthio, alicyclic or heteroalicyclic group; R 3 is independently selected from substituted alkylene, alkenylene, alkynylene, heteroalkylene, heteroalkenylene, heteroalkynylene, arylene, heteroarylene or cycloalkylene, wherein alkylene, alkenylene, alkynylene, heteroalkylene, heteroalkenylene and heteroalkynylene, may optionally be interrupted by aryl, heteroaryl, alicyclic or heteroalicyclic; R 5 is independently selected from H, or optionally substituted aliphatic, heteroaliphatic, alicyclic, heteroalicyclic, aryl, heteroaryl, alkylheteroaryl or alkylaryl; E 1 is C, E 2 is O; E3, E4, E5 and E6 are selected from N, NR 4, O and S, wherein when E3, E4, E5 or E6 are N, - - - - - - is, and wherein when E3, E4, E5 or E6 are NR 4, O or S, - - - - - - is; R 4 is independently selected from hydrogen, or optionally substituted aliphatic, heteroaliphatic, alicyclic, heteroalicyclic, aryl, heteroaryl, alkylheteroaryl or alkylaryl; X is independently selected from OC R x, OSO 2 R x, OSOR x, OSO(R x) 2, S R x, OR x, phosphinate, halide, nitrate, hydroxyl, carbonate, amino, nitro, amido or optionally substituted aliphatic, heteroaliphatic, alicyclic, heteroalicyclic, aryl or heteroaryl; R x is independently hydrogen, or optionally substituted aliphatic, haloaliphatic, heteroaliphatic, alicyclic, heteroalicyclic, aryl, alkylaryl or heteroaryl; and G is absent or independently selected from a neutral or anionic donor ligand which is a Lewis base. M 1 and M 2 are independently selected from Zn, Cr, Co, Cu, Mn, Mg, Ni, Fe, Ti, V, Cr -X, Co -X, Mn -X, Ni -X, Fe -X, Ca, Ge, AI -X, Ti -X, V -X, Ge - 2 or Ti - 2; wherein at least one of M 1 or M 2 is selected from Ni(ll) and Ni -X; R 1 and R 2 are independently selected from hydrogen, halide, a nitro group, a nitrile group, an imine, an amine, an ether group, a silyl group, a silyl ether group, a sulfoxide group, a sulfonyl group, a sulfinate group or an acetylide group or an optionally substituted alkyl, alkenyl, alkynyl, haloalkyl, aryl, heteroaryl, alkoxy, aryloxy, alkylthio, arylthio, alicyclic or heteroalicyclic group; R 3 is independently selected from substituted alkylene, alkenylene, alkynylene, heteroalkylene, heteroalkenylene, heteroalkynylene, arylene, heteroarylene or cycloalkylene, wherein alkylene, alkenylene, alkynylene, heteroalkylene, heteroalkenylene and heteroalkynylene, may optionally be interrupted by aryl, heteroaryl, alicyclic or heteroalicyclic; R 5 is independently selected from H, or optionally substituted aliphatic, heteroaliphatic, alicyclic, heteroalicyclic, aryl, heteroaryl, alkylheteroaryl or alkylaryl; E 1 is C, E 2 is O; E3, E4, E5 and E6 are selected from N, NR 4, O and S, wherein when E3, E4, E5 or E6 are N, - - - - - - is, and wherein when E3, E4, E5 or E6 are NR 4, O or S, - - - - - - is; R 4 is independently selected from hydrogen, or optionally substituted aliphatic, heteroaliphatic, alicyclic, heteroalicyclic, aryl, heteroaryl, alkylheteroaryl or alkylaryl; X is independently selected from OC R x, OSO 2 R x, OSOR x, OSO(R x) 2, S R x, OR x, phosphinate, halide, nitrate, hydroxyl, carbonate, amino, nitro, amido or optionally substituted aliphatic, heteroaliphatic, alicyclic, heteroalicyclic, aryl or heteroaryl; R x is independently hydrogen, or optionally substituted aliphatic, haloaliphatic, heteroaliphatic, alicyclic, heteroalicyclic, aryl, alkylaryl or heteroaryl; and G is absent or independently selected from a neutral or anionic donor ligand which is a Lewis base.