Formation of thallium(I) sandwich M3TlM3 clusters, [(μ6-Tl)M6(μ2-CO)6(PEt3)6]+(M = Pt, Pd), with two unconnected triangular M3(μ2-CO)3(PEt3)3 units: implications of comparative analysis of isostructural 5d106s2 Tl(I)–(M3)2 sandwiches (M = Pt, Pd) with known 5d10 Au(I)–(Pt3)2 sandwich

The preparation, isolation, and structural/spectroscopic IR, 31P{1H} NMR characterization of two new isostructural 5d106s2 Tl(I) sandwich clusters, [(μ6-Tl)Pt6(μ2-CO)6(PEt3)6]+ (1) and [(μ6-Tl)Pd6(μ2-CO)6(PEt3)6]+ (2) as [PF6]− salts are presented. Each of these closed-subshell M3TlM3 sandwiches (M = d10 Pt (1), Pd (2)) containing two unconnected triangular M3(μ2-CO)3(PR3)3 units is held together solely by delocalized/electrostatic M3–Tl–M3 bonding. 1 and 2 were synthesized in ca. 90% yields by reactions (under markedly different boundary conditions) of M4(μ2-CO)5(PEt3)4 (M = Pt (3), Pd (4)) with TlPF6. Their isostructural geometries and stoichiometries were unequivocally established from low-temperature CCD X-ray crystallographic determinations. Both 1 and 2 (without their P-attached ethyl substituents) closely conform to a centrosymmetric trigonal-antiprismatic architecture of trigonal D3d symmetry. A comparison of their well-refined isomorphous crystal structures reveals that the Tl–Pd mean of 2.91 A in 2 is 0.05 A smaller than the Tl–Pt mean of 2.96 A in 1. In solution, 2 is much more kinetically labile than 1 and (unlike 1) readily converts under N2 into the recently reported [Tl2Pd12(μ2-CO)6(μ3-CO)3(PEt3)9]2+ (5) as the [PF6]− salt, which was isolated in ca. 90% yield from the same reactants (viz., Pd4(μ2-CO)5(PEt3)4 and TlPF6). In fact, obtaining crystalline material of 2 from recrystallization procedures was greatly hampered by its facile transformation into large quantities of co-crystallized 5. A comparative analysis of the molecular parameters and relative stabilities of the closed-subshell 5d106s2 Tl(I)–(M3)2 sandwiches (M = d10 Pt (1), Pd (2)) with the corresponding known closed-subshell 5d10 Au(I)–(Pt3)2 sandwich together with an examination of relative shifts of corresponding bridging carbonyl IR frequencies for selected pairs of related clusters provide compelling evidence that the so-called “inert” 6s2 electron-pair on the Tl(I) exerts an overall destabilizing influence: namely, that the highly electrophilic 5d10 Au(I) forms considerably stronger delocalized sandwich Pt3–Au–Pt3 bonding (due to its empty, relativistically stabilized 6s acceptor AO) which is presumed to have considerable covalent character. The Tl(I)–Pt(0) distances in 1 are similar to the Tl(I)–Au(I) distances found for another recently reported class of two electronically equivalent closed-subshell Au3TlAu3 sandwich units (i.e., 5d10 Au(I) vs. 5d10 Pt(0)) formed by intercalation of Tl+ between two electron-rich intramolecular, weakly bonded (aurophilic) Au3 triangles in trinuclear cyclic gold(I) benzylimidazolate and carbeniate molecules; the Au3TlAu3 sandwich units stack into linear chains with intermolecular aurophilic Au(I)–Au(I) interactions between four of the six Au(I) atoms in adjacent units. Of particular interest is that the Tl(I)–Au(I) distances (means, 3.02 and 3.09 A) in the distorted trigonal-prismatic (μ6-Tl)Au6 sandwich units of the geometrically related Tl+-intercalated TR(bzim) and TR(carb) complexes are 0.2–0.3 A longer than the Ag(I)–Au(I) distances (mean, 2.81 A) in the initially known Au3AgAu3 sandwich unit of the Ag+-intercalated TR(bzim) analogue; it is similarly proposed that this parallel (M′–Au) bond-length difference may likewise be attributed to the considerably smaller electrophilic character of the central 5d106s2 Tl(I) vs. that of the 4d10 Ag(I) due to the overall destabilizing effect of the thallium(I) 6s2 electron-pair.