359 Journal of Organometallic Chemistry, 395 (1990) 359-373 Elsevier Sequoia S-A., Lamanne JOM 20944 Cyclometallated compounds V *. Double cyclopalladation of diphenyl pyrazines and related ligauds Peter J. Steel * and Graham B. Caygill Chemistry Department, ‘University of Canterbwy, Christchurch (New Zeeland) (Received March 13th, 1990) 2,3-Diphenylpyrazine and four structurally related ligands have each been singly and doubly cyclopalladated and the products character&d by ‘H and 13C NMR studies of their acetylacetonate complexes. The structure of a doubly cyclometal- lated Pd(acac) complex of 2,3diphenylpyrazine has been determined by an X-ray diffraction study (R = 0.039). A strong steric interaction between the two cyclopal- ladated phenyl rings is relieved by twisting; the two chelate ring mean-planes are mutually inclined at an angle of 19.6(5)“. Introduction The study of binuclear complexes containing bridging heterocyclic ligands has been the subject of much recent attention [l]. For example, 2,3-di(2-pyridyl)pyraz.ine forms both homo- and hetero-bimetallic complexes of type 1 in which the possibility exists for communication between the two metals via the r-system of the pyrazine ring [2,3]. By changing the ligand it is possible to vary the distance between the metals and accordingly the propensity for transfer of energy or electrons, or for magnetic interaction between the two metal centres [4]. We are currently studying doubly cyclometallated complexes in which chelating pyridine rings of binucleating ligands are replaced by phenyl rings that are capable of undergoing cyclometallation. Whereas cyclometallated complexes 2 of 2-phenyl- pyridine are well studied [5]. analogues of 2,2’-bipyridine complexes 3, relatively few examples of the double cyclomefallation of a single ligand are known [6,7]. Recently we described [8] the preparation of complexes of type 4 [M = Pd(acac)], which were * For part IV see ref. 8. 0022-328X/90/$03.50 Q 1990 - Elsevier Sequoia S.A.