Two polymorphic products, [{Cu(tmeda)(mu-OH)}(2)Au(CN)(4)][Au(CN)(4)] (1) and [Cu(tmeda)(mu-OH)Au(CN)(4))(2) (2), were synthesized from {Cu(tmeda)(mu-OH)}(2)X-2 (tmeda = N,N,N',N'-tetramethylethylenediamine, X = CIO4-, BF4-) and 2 equiv of K[Au(CN)(4)], and their X-ray structures were determined. Both compounds have {Cu(tmeda)(u-OH)}(2)(2+) dimers with [Au(CN)(4)](-) units bound in the axial positions. However, in 1, two trans N-donor cyanides of each [Au(CN)(4)](-) unit bind to adjacent copper(II) dimers, forming a 1-D chain, whereas complex 2 is molecular, with two mono-coordinated [Au(CN)4]- units. The 1-D polymorph 1 is formed from aqueous solution, while the molecular polymorph 2 is obtained with X = BF4- in methanol. The polymorphs have slightly different Cu-O-Cu angles, a key magnetostructural parameter, such that the 1-D chain 1, with an angle of 96.6(2)degrees, shows ferromagnetic interactions with 2J = +57.5 cm(-1) and g = 2.097, whereas the molecular complex 2, with an angle of 98.92(17)degrees, shows antiferromagnetic interactions with 2J = -143.6 cm(-1) and g = 2.047. A similar Cu(II) complex, [{Cu(tmeda)(mu-OH)}(2)Au(CN)(4)][CIO4](MeOH)-Me-. (3), was synthesized in methanol when X = CIO4-, in which the [Au(CN)(4)]unit bridges the two Cu(II) centers within the dimer in an intramolecular fashion via cis N-donor cyanides. The average Cu-O-Cu angle of 98.4(2)degrees in 3 generates antiferromagnetic interactions with 2J = -64.8 cm(-1) and g = 2.214. Complexes 1-3 represent the first examples of [Cu(tmeda)(mu-OH)](2)(2+) dimers with Cu-O-Cu angles under 100, thereby extending the range of 2J coupling constants for this moiety from 149 to 566 cm(-1). The switch to ferromagnetic interactions in 1 as a result of the coordinating, bridging [Au(CN)4]- anion suggests that cationic, dinuclear moieties that are typically antiferromagnetically coupled may, with an appropriate coordinating counterion, become ferromagnetic units.