The digold(I) complexes Au-2(mu-C(6)H(4)PR(2))(2) [R = Ph (1a), Et (1b)] obtained by treatment of AuBr(PEt(3)) with o-LiC(6)H(4)PR(2) undergo addition with halogens or benzoyl peroxide to give metal-metal bonded digold(II) complexes Au(2)X(2)(mu-C(6)H(4)PR(2))(2) [R = Ph, Et; X = I (2a, 2b), Br (3a, 3b), Cl (4a, 4b), O(2)CPh (5a, 5b)], which are structurally similar to the bis(ylide) complexes Au(2)X(2){mu-(CH2)(2)PR(2)}(2). The benzoate ligands in 5b are monodentate and the gold-gold bond length [2.5243(7) Angstrom] is significantly less than that in the diiodide (2a) [2.5898(6) Angstrom, 2.5960 (A) for independent molecules], reflecting the trans influences of the axial anionic ligands. The corresponding complexes Au(2)X(2)(mu-C(6)H(4)PR(2))(2) [R = Ph, Et; X = O(2)CMe (6a, 6b), ONO2 (7a, 7b)] are made from 2-4 and the appropriate silver salt. The axial anionic ligands undergo immediate scrambling when solutions of Au(2)X(2)(mu-C6H4- PR(2))(2) and Au2Y2(mu-C(6)H(4)PR(2))(2) are mixed. The bridging C(6)H(4)PR(2) units also scramble rapidly on mixing solutions of Au(2)X(2)(mu-C(6)H(4)PPh(2))(2) [X = I (2a), Br (3a)] and Au(2)X(2)(mu-C(6)H(4)PEt(2))(2) [X = I (2b), Br (3b)], but this occurs only slowly for X = Cl and not at all for X = O(2)CPh, O(2)CMe, or ONO2. Solutions of the diiodo complexes 2a, Zb and the dibromo complexes 3a, 3b isomerize cleanly to the digold(I) complexes Au(2)X(2)(mu-R(2)PC(6)H(4)C(6)H(4)PR(2)) [R = Ph, Et; X = I (8a, 8b), Br (9a, 9b)] containing 2,2’-biphenylyl(diphenylphosphine) or 2,2’-biphenylyl(diethylphosphine), respectively, as a consequence of a reductive elimination in which a C-C bond is formed at the expense of two Au-C bonds. In 8b the Au-Au separation is 3.167(1) Angstrom and the phenyl rings of the biphenyl unit are almost orthogonal. Qualitatively, the rates of isomerization of Au(2)X(2)(mu-C(6)H(4)PR(2))(2) to Au(2)X(2)(mu-R(2)PC(6)H(4)C(6)H(4)PR(2)) are in the order R = Ph > Et; X = I > Br > > Cl; isomerization does not occur for X = O(2)CPh, O(2)CMe, or ONO2. The rates of thermal isomerization of 2a and 3a are first order in complex, only slightly sensitive to solvent polarity, and, for 2a, inhibited by iodide ion. It is suggested that reversible loss of halide ion initiates aryl group transfer between the gold atoms, thus allowing reductive elimination of Au-C bonds to take place at one center. Treatment of 2a or 3a with an excess of iodine or bromine gives initially digold(III) complexes cis,trans-Au(2)X(4)(mu-C(6)H(4)PPh(2))(2) [X = I (14), Br (15)], which are in equilibrium with monomers AuX(2)(C(6)H(4)PPh(2)) [X = I (16), Br (17)], as shown by P-31 NMR spectroscopy. These species isomerize at room temperature by internal electrophilic cleavage of their Au-C bonds to give stable gold(I) complexes of (2-halogenophenyl)diphenylphosphine, AuX(o-XC(6)H(4)PPh(2)) [X = I (12), Br (11)].?