New neutral derivatives [Cu(L)(PR3)2] (LH=1-phenyl-3-methyl-4-R1(CO)-pyrazol-5-one, in detail L1H, R1=C6H5; L2H, R1=CH3; L3H, R1=CF3; L4H, R1=C6H11; L5H, R1=C6H5CH2; L6H, R1=(C6H5)2CH; R=C6H5, p-CH3C6H4, C6H5CH2 or C6H11) and [Cu(L)2] (L=L4, L5 and L6) have been synthesised and characterised by analytical and spectroscopic techniques. The X-ray crystal and molecular structures of [Cu(L1)(PCy3)2], [Cu(L5)(PPh3)2] and [Cu(L6)(PPh3)2] have been resolved, in which the copper atom is found in a strongly distorted tetrahedral CuO2P2 environment. The interbond angles PCuP, PCuO and OCuO are strongly influenced by the steric properties of PR3 and also to a lesser degree by the 4-acyl moiety of the L donor. [Cu(L)(PR3)2] derivatives undergo partial dissociation in chloroform solution through breaking of CuP and, when R=Cy, also of CuO bonds. [Cu(L4)(PPh3)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(PPh3)(Hmimt)3](L4), [Cu(L4)(PPh3)(Phen)], [Cu(Cupr)2](L4) and [Cu(DPPP)2](L4), respectively.