Influence of catecholate bridges on the transmetalation of tetranuclear copper(II) complexes [NCuCl]4Y2 and N4Cu4(OH)2Cl4Y·3H2O (N = N,N-diethylnicotinamide; Y = 3,4,5,6-tetrachlorocatecholate) with Ni(NS)2 (NS = S-methyl isopropylidenehydrazinecarbodithioate) in methylene chloride