Ansa bis(.mu.-substituted) Group 4 metal and aluminum compounds comprising a single Group 4 metal atom and two aluminum metal atoms corresponding to the formula: ##STR1## wherein: L' is a .pi.-bonded group,M is a Group 4 metal,J is nitrogen or phosphorus;Z is a divalent bridging group causing the complex to have an ansa structure,R' is an inert monovalent ligand;r is one or two;X independently each occurrence is a Lewis basic ligand group able to form a .mu.-bridging ligand group, and optionally the two X groups may be joined together, andA' independently each occurrence is an aluminum containing Lewis acid compound that forms an adduct with the metal complex by means of the .mu.-bridging groups, and optionally two A' groups may be joined together thereby forming a single difunctional Lewis acid containing compound, and a method of preparation comprising contacting a charge-neutral Group IV metal coordination complex having at least two Lewis basic groups with at least two molar equivalents of charge-neutral aluminum coordination complexes having Lewis acidic aluminum atoms such that at least two of the aluminum atoms of the aluminum coordination complexes bond to at least two of the Lewis basic groups of the Group IV coordination complex.