AbstractThe tripod ligands RCH2C(CH2X)(CH2Y)(CH2Z) form tetracoordinate compounds with NiHal2. If two of the donor functions X, Y, Z are phosphanes, square‐planar coordination is observed. In contrast, donor sets X,Y,Z = P,N,N or N,N,N form tetrahedral compounds, by P,N or N,N coordination, that are active in the catalysis of olefin oligomerization with MAO as a co‐catalyst. These catalyst systems are also active in the stereoselective heterodimerization of olefins. The co‐dimerization of styrene and propene by these systems leads to 4‐phenyl‐2‐pentene almost exclusively, with regioselectivities up to 98% and (E)/(Z) ratios of up to 95:5, surpassing other reported catalysts for this process. The products of these catalytic transformations have been fully characterized by GC/MS techniques as well by isolation, NMR and MS characterization of the pure compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)