The coordination behavior prior to C-M bond formation of the chelating aromatic PCP substrate DPPMH (3; DPPMH = 1,3-bis((diphenylphosphino)methylene)mesitylene) has been studied in order to determine the factors which control the complex formation of such ligands. Reacting 3 with (RCN)(2)MCl(2) (R = Me, Ph; M = Pd, Pt) and (COD)PtX(2) (X = Cl, Me; COD = 1,5-cyclooctadiene) resulted in the formation of several 8- and 16-membered mono- and binuclear palladium(II) and platinum(II) macrocycles : trans-[(DPPMH)PdCl2](2) (5), trans-[(DPPMH)-PtCl2](2) (6), cis-(DPPMH)PtCl2 (7), cis-(DPPMH)PtMe(2) (8), and cis-[(DPPMH)PtMe(2)](2) (9). Compounds 5-9 were fully characterized using NMR, FAB-MS, FD-MS, elemental analysis, and X-ray crystallography. Thermolysis of the bimetallic trans-[(DPPMH)PtCl2](2) (6) results in the formation of the monomeric cis-(DPPMH)PtCl2 (7). The product formation depends on the neutral-(nitriles or COD) and anionic ligands (Cl and CH3) of the metal precursor. The molecular structures of trans-[(DPPMH)PdCl2](2) (5) and cis-[(DPPMH)PtMe(2)](2) (9) have been determined by complete single-crystal diffraction studies. Crystal data for 5 : monoclinic, space group P2(1)/n with a = 14.547(3) Angstrom, b = 17.431(4) Angstrom, c = 27.839 (5) Angstrom, beta = 99.56(2)degrees, V = 6961(3) Angstrom(3), and Z = 4. The structure converged to R = 0.048 and R(w) = 0.049. Crystal data for 9 : monoclinic, space group P2(1)/n with a = 19.187(4) Angstrom, b = 19.189(4) Angstrom c = 20.705(2) Angstrom, beta = 103.41(3)degrees, V = 7415(3) Angstrom(3), and Z = 4. The structure refinement converged to R = 0.0977 and R(w) = 0.2212.