Two new, potentially tridentate ligands, Ph2P(CH2CH2O)2Me (POO) and Me2As(CH2CH2O)2Me(AsOO), have been synthesized and characterized by multinuclear NMR spectroscopy. Mononuclear complexes of these ligands, cis-(CO)4Mo(EOO)2 (E = P, As), cis,cis,trans- and cis,trans,cis-(CO)2Cl2Ru(POO)2, [(1,5-cod)Rh(POO)2][ClO4] and Cl2M(POO)2, (M = Pd and Pt), have also been prepared and characterized by multinuclear NMR and IR spectroscopy. In all cases, these ligands are coordinated only through the group 15 donor atom. Different procedures have been developed to give either the cis,cis,trans or cis,trans,cis isomers of Cl2(CO)2Ru(POO)2 complex in high yields. The latter isomer is unusual and has not previously been reported with ligands of this type. The reactions of the cis-(CO)4Mo(EOO)2 complexes with methyllithium have been examined. The carbonyl ligands in these complexes do not react with methyllithium at room temperature. These results are in direct contrast to those of Powell and coworkers who reported facile reactions between methyllithium and the carbonyl ligands in similar cis-(CO)4Mo(Ph2PO(CH2CH2O)3PPh2 complexes. These results confirm Powell's conclusions that the number and type of the hard donor atoms in these complexes greatly affect the reactivity of methyllithium towards the carbonyl ligands. © 1989.