New carbide clusters in the cobalt subgroup. Part 12. Synthesis and X-ray characterization of the tetradecanuclear carbido-carbonyl cluster [Rh14C2(µ-CO)21(CO)12]2–

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Abstract
The compound [Rh14C2(µ-CO)21(CO)12]2– has been synthesized by reaction of [Rh6C(CO)15]2– with [Rh(CO)2(MeCN)2]+. Its molecular structure has been determined by single-crystal X-ray crystallography: monoclinic, space group C2/m, with a= 13.496(1), b= 37.175(2), c= 15.858(1)Å, β= 113.9(1)°, and Z= 4. The structure has been solved from 3 460 reflections collected by counter methods and refined by least-squares calculations to R= 0.042. The dianion has idealized D3h symmetry and contains two Rh7C moieties related by a crystallographic mirror plane. Each unit is a C3v monocapped trigonal prism. The two halves are linked by a Rh–Rh bond between the capping atoms and by three bridging carbonyl ligands. The remaining 30 CO groups are distributed 15 on each Rh6 prism, the geometry of the parent [Rh6C(CO)15]2– anion being retained. The Rh–Rh bond between the two Rh7 units is remarkably short, 2.570(2)Å. Other relevant (average) bond distances are as follows: Rh–Rh 2.778, Rh–C(carbide) 2.14; Rh–C and C–O for terminal and bridging ligands 1.89 and 1.14 and 2.07 and 1.17 Å, respectively. A comparison is made with the clusters [Rh8C(CO)19], [Cu2Rh6C(CO)15(MeCN)2], and [Ag{Rh6C(CO)15}2]3–.