A This invention relates to a composition comprising a catecholate ligand, palladium or nickel, and an ancillary ligand with the following structure: ##STR00001## where Pn is a Group-15 element; H is hydrogen; R.sup.7 and R.sup.8 are independently hydrogen or C.sub.1-C.sub.30 hydrocarbyl radicals, or both are C.sub.1-C.sub.30 hydrocarbyl radicals that form a ring structure comprising one or more aromatic or non-aromatic rings; and R.sup.13-R.sup.18 are, independently, hydrogen or C.sub.1-C.sub.30 hydrocarbyl radicals. The composition can be used to oligomerize ethylene.