Oligomerisation of olefinic compounds in an aliphatic medium

A process for producing an oligomeric product containing octene by the tetramerisation of an olefinic compound in the form of ethylene comprising contacting ethylene with a tetramerisation catalyst in an aliphatic liquid medium at a reaction temperature of at least 50° C., wherein the catalyst comprises the combination of

• i) a source of a transition metal;
• and
• ii) a ligating compound of the formula
• (R1)mX1(Y)X2(R2)n
• wherein:
• X1 and X2 are independently selected from the group consisting of
• N, P, As, Sb, Bi, O, S and Se;
• Y is a linking group between X1 and X2;
• m and n are independently 0, 1 or a larger integer;
• and
• R1 and R2 are independently hydrogen, a hydrocarbyl group or a heterohydrocarbyl group, and R1 is the same or different when m>1, and R2 is the same or different when n>1 and
• wherein the product produced includes more than 30% by mass of octene.