New neutral derivatives [Cu(L)(PR 3 ) 2 ] (LH=1-phenyl-3-methyl-4-R 1 (C O)-pyrazol-5-one, in detail L 1 H, R 1 =C 6 H 5 ; L 2 H, R 1 =CH 3 ; L 3 H, R 1 =CF 3 ; L 4 H, R 1 =C 6 H 1 1 ; L 5 H, R 1 =C 6 H 5 CH 2 ; L 6 H, R 1 =(C 6 H 5 ) 2 CH; R=C 6 H 5 , p-CH 3 C 6 H 4 , C 6 H 5 CH 2 or C 6 H 1 1 ) and [Cu(L) 2 ] (L=L 4 , L 5 and L 6 ) have been synthesised and characterised by analytical and spectroscopic techniques. The X-ray crystal and molecular structures of [Cu(L 1 )(PCy 3 ) 2 ], [Cu(L 5 )(PPh 3 ) 2 ] and [Cu(L 6 )(PPh 3 ) 2 ] have been resolved, in which the copper atom is found in a strongly distorted tetrahedral CuO 2 P 2 environment. The interbond angles P Cu P, P Cu O and O Cu O are strongly influenced by the steric properties of PR 3 and also to a lesser degree by the 4-acyl moiety of the L donor. [Cu(L)(PR 3 ) 2 ] derivatives undergo partial dissociation in chloroform solution through breaking of Cu P and, when R=Cy, also of Cu O bonds. [Cu(L 4 )(PPh 3 ) 2 ] reacts with 1-methylimidazolin-2-thione (Hmimt), with 1,10-phenanthroline (Phen), with 2,9-dimethyl-1,10-phenanthroline (Cupr) and with bis(diphenylphosphino)propane (DPPP), yielding the mixed-ligand compounds [Cu(PPh 3 )(Hmimt) 3 ](L 4 ), [Cu(L 4 )(PPh 3 )(Phen)], [Cu(Cupr) 2 ](L 4 ) and [Cu(DPPP) 2 ](L 4 ), respectively.