A catalyst composition comprising a bis-amide compound represented by the formula ##STR1## wherein M is zirconium, hafnium or titanium; each N is a three coordinate nitrogen atom; each e is Si, Ge or Sn; each Z is, independently, a hydride or R; each R is, independently, a hydrocarbyl one or more carbon atoms of which may be substituted by an element selected from Si, O, P, N and S; L is a neutral Lewis base and m is a number from 0 to 2; or a dimer thereof and a second component. Preferably the second component is a compound which is capable of providing a bulky and labile anion �A!.sup.-, which anion is substantially non-coordinating under the reaction conditions and contains at least one boron atom. Also provided is a process for oligomerizing or co-oligomerizing alpha olefins in the presence of this catalyst composition.