Azetidine reacts in tetrahydrofuran (thf) at room temperature or below with the co-ordinated RNC ligand in each of the neutral cis-[MCl₂(PPh₃)(CNR)][M = Pd or Pt; R = C₆H₄OMe-p or Bu^t) and cationic trans-[MCl(PPh₃)₂(CNC₆H₄OMe-p)]BF₄(M = Pd or Pt) complexes to form the corresponding derivatives containing the acyclic diaminocarbene moiety M–C([graphic omitted]H₂)NHR. Azetidine reacts also with the bis(isocyanide) complexes cis-[PdCl₂(CNR)₂](R = Bu^t or C₆H₄OMe-p) converting one (R = Bu^t) or two (R = C₆H₄OMe-p) isocyanide ligands into acyclic carbenes. The isocyanide complexes cis-[MCl₂(PMe₂Ph)(CNC₆H₄OMe-p)](M = Pd or Pt) react in thf with 2 equivalents of azetidine to afford the cationic acyclic diaminocarbene complexes [MCl(PMe₂Ph)([graphic omitted]H){C([graphic omitted]H₂)NHC₆H₄OMe-p}]Cl containing a metal-co-ordinated azetidine ligand. In contrast to the reactions of the three-membered heterocycle aziridine with isocyanide ligands which lead to five-membered cyclic carbenes, the corresponding reactions of azetidine give no evidence of six-membered diaminocarbenes. The carbene complexes have been characterized by their elemental analyses, i.r., ¹H, and ³¹P n.m.r. spectra and in a few cases by their ¹³C n.m.r. and mass spectra. The structure of [PdCl(PMe₂Ph)([graphic omitted]H){C([graphic omitted]H₂)NHC₆H₄OMe-p}]Cl·CH₂Cl₂ was determined by X-ray diffraction: space group P2₁/n, a= 24.849(4), b= 10.174(2), c= 11.895(2)Å, β= 91.85(3)°, R= 0.054 (R′= 0.059) for 3 734 observed reflections. The geometry around Pd is a distorted square with the diaminocarbene ligand trans to chloride. The Pd–C(sp²) distance is 1.990(6)Å and the bond lengths within the carbenoid system indicate extensive electronic delocalization.