Copper(I) acylpyrazolonate derivatives of formula [Cu(Q)(PPh3)(2)] (HQ = HQ(C2F5) = 1-phenyl-3-methyl4-pentafluoropropanoyl-pyrazol-5-one, HQ(CF3,CF3) = 1-(4-trifluoromethyl)phenyl-3-methyl-4-trifluoroacetylpyrazol-5-one, HQ(naph) = 1-phenyl-3-methyl-4-(1-naphthoyl)pyrazol-5-one, HQ(fur) = 1-phenyl-3-methyl-4-furylpyrazol-5-one, HQ(thi) = 1-phenyl-3-methyl-4-thienoylpyrazol-5-one, HQ(CF3,py) = 1-(2-pyridyl)-3-methyl-4-trifluoromethylpyrazol-5-one) have been synthesized and characterized, both in the solid state and in solution. The isomorphous crystal structures of [Cu(Q(fur))(PPh3)(2)] and [Cu(Q(thi))(PPh3)(2)] and of [CU(Q(C2F5))(PPh3)(2)] show their copper atoms in distorted tetrahedral environments. Apart from the derivative [Cu(Q(naph))(PPh3)(2)] they are fluxional in chloroform solution, dissociating partially to the [Cu(Q)(PR3)] fragment and free PR3, or existing in solution as [Cu(O-2-Q)(PR3)(2)] or [Cu(O-Q)(PR3)(2)] species, with CuO2P2 or CuOP2, metal environments, in equilibrium with each other. By contrast, in [Cu(Q(CF3,py))(PPh3)(2)] the Q(CF3.py) ligand is coordinated to copper through the nitrogen atoms of the pyrazole and pyridine rings. The latter compound reacts with Zn(BF4)(2)center dot xH(2)O and [Ru(p-cymene)Cl-2](2) affording ionic heterobimetallic adducts. (c) 2005 Elsevier Ltd. All rights reserved.