Metal−Ligand Bonding in Coinage Metal−Phosphine Complexes: The Synthesis and Structure of Some Low-Coordinate Silver(I)−Phosphine Complexes

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Abstract
Reaction of AgBF4 with 2 equiv of Ph3P in acetonitrile followed by recrystallization from dichloromethane/hexane yields the mixed phosphine−nitrile complex [(Ph3P)2AgNCCH3]BF4 (I) as its dichloromethane solvate, I·0.5CH2Cl2. This solvate crystallizes in the monoclinic space group C2/c with a = 22.928(5), b = 12.700(3), c = 25.156(5) Å, β = 97.53(3)°, and Z = 8. The acetonitrile ligand in I is loosely bound to the metal center and easily lost, even in the solid state, without decomposition. Hence, recrystallization of dried samples of I·0.5CH2Cl2 from CH2Cl2/hexane results in isolation of the novel low-coordinate phosphine complex [(Ph3P)2Ag]BF4 (II). II crystallizes in the monoclinic space group C2/c with a = 21.733(9), b = 12.272(4), c = 24.356(9) Å, β = 95.01(3)°, and Z = 8. In both cases, a weak interaction is present between a fluorine atom of the BF4- anion and the silver cation. However, these interactions appear to be essentially electrostatic rather than dative in nature, implying that I is best considered a three-coordinate silver complex and that II is a rare, structurally characterized example of a two-coordinate silver−phosphine complex. These solid-state geometric assignments are supported by 31P NMR studies, which reveal a Ag−P coupling constant of 550 Hz for II, consistent with the presence of a linear two-coordinate complex in solution. The NMR data also indicate that the phosphine ligands are involved in exchange processes, which are accelerated by the presence of a donor solvent such as acetonitrile. Comparison of II with its gold analogue supports the previously stated concept that gold atoms are smaller than silver atoms. An analysis of 13 other isostructural pairs of silver and gold complexes culled from the crystallographic database lends further support to this concept.

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