THE HETERONUCLEAR CLUSTER CHEMISTRY OF THE GROUP IB METALS .17. SYNTHESIS AND X-RAY CRYSTAL-STRUCTURE OF THE HEXANUCLEAR MIXED-METAL CLUSTER AG2RU4(MU(3)-H)(2)(CO)(12)(P(C(6)H(4)ME-2)(3))(2)] - AN INVESTIGATION OF THE STERIC PROPERTIES OF THE P(C(6)H(4)ME-2)(3) LIGAND

Treatment of a dichloromethane solution of the salt [N(PPh(3))(2)](2)[ Ru-4(mu-H)(2)(CO)(12)] with two equivalents of the complex [Ag(NCMe)(4 )]PF6 at -30 degrees C, followed by the addition of two equivalents of the very bulky phosphine ligand P(C(6)H(4)Me-2)(3) (cone angle 194 de grees) affords the mixed-metal cluster Ag2Ru4(mu(3)-H)(2)(CO)(12){P(C( 6)H(4)Me-2)(3)}(2)] (I) in ca. 70% yield. A single-crystal X-ray diffr action study has revealed that I has a capped trigonal bipyramidal met al framework structure, with the two silver atoms in close contact [Ag -Ag 2.876(2) Angstrom]. This skeletal geometry is very surprising in v iew of a previous observation that P(t)Bu(3) (cone angle 182 degrees), which is supposedly a less sterically demanding phosphine ligand than P(C(6)H(4)Me-2)(3), is sufficiently bulky to prevent the two Ag(P(t)B u(3)) units being adjacent in the metal core of the closely related cl uster [Ag2Ru4(mu(3)-H)(2)(CO)(12)(P(t)Bu(3))(2)]. In the solid-state s tructure of I, it appears that two of the three C(6)H(4)Me-2 rings in each of the two P(C(6)H(4)Me-2)(3) ligands can adopt relative orientat ions which allow the phosphine to behave as a less sterically demandin g ligand than its large cone angle might otherwise suggest. Remarkably , the formal replacement of the two PPh(3) ligands attached to the sil ver atoms in the capped trigonal bipyramidal metal framework of the cl osely related cluster [Ag2Ru4(mu(3)-H)(2)(CO)(12)(PPh(3))(2)] by two P (C(6)H(4)Me-2)(3) groups in I causes very little change in most of the metal-metal distances, despite the fact that the cone angle of PPh(3) (145 degrees) is 49 degrees smaller than that of P(C(6)H(4)Me-2)(3). In solution, compound I undergoes two types of dynamic behaviour at am bient temperatures. One process involves an intramolecular rearrangeme nt of the metal core, which exchanges the two silver atoms between the two inequivalent sites, and the other process is an intermolecular ex change of P(C(6)H(4)Me-2)(3) ligands between clusters.