Diphenylphosphine-oxide-fused NiII porphyrin 8 was synthesized from 3,5,7-trichloroporphyrin 5 via a reaction sequence of nucleophilic aromatic substitution with lithium diphenylphosphide, oxidation with H2 O2 , and palladium-catalyzed intramolecular cyclization. Reduction of 8 with HSiCl3 gave diphenylphosphine-fused NiII porphyrin 9. The embedded P=O and P moieties serve as a strong electron-accepting and electron-donating group to perturb the optical and electrochemical properties of the NiII porphyrin. NiII porphyrin 9 is diamagnetic with a low-spin NiII center in solution but becomes paramagnetic with a five-coordinated NiII center with high-spin (S=1) state in the solid state. Diphenylphosphine-oxide-fused ZnII porphyrin 10 was also synthesized and shown to form a face-to-face dimer with mutual O-Zn bonds in the crystal and in nonpolar and moderately polar solvents. The dimerization of 10 in CDCl3 has been revealed to be an entropy-driven process with a large entropy gain (ΔSD =207 J K-1  mol-1 ).