The analogy between alkenes and the dinuclear species [MRh(µ-CO)2(η-C5Me5)2](M = Co or Rh); dynamic nuclear magnetic resonance, X-ray crystallographic, and extended-Hückel molecular-orbital studies of the bonding between unsaturated CoRh and Rh2 fragments and the pentacarbonylmolybdenum group

Abstract

Treatment of [Mo(CO)₅(thf)](thf = tetrahydrofuran) with [CoRh(µ-CO)₂(η-C₅Me₅)₂] affords the heterotrimetallic complex [MoCoRh(µ-CO)₂(CO)₅(η-C₅Me₅)₂]. Variable-temperature ¹³C-{¹H} n.m.r. studies on this species revealed that in solution rotation of the CoRh(µ-CO)₂(η-C₅Me₅)₂ fragment occurs about an axis through the Mo and the midpoint of the Co–Rh vector, and that the preferred orientation of the Co–Rh vector with respect to the four equatorial carbonyl ligands of the square-pyramidal Mo(CO)₅ moiety is staggered. The estimated barrier to rotation (ΔG‡₂₅₈= 52.3 ± 1.3 kJ mol ^–1) is in the range observed and calculated for the hindered rotation of alkenes in related d⁶ ML₅ alkene complexes where, however, the orientational preference of the alkene is to lie so as to eclipse an LML vector. The structure of the analogous trinuclear compound [MoRh₂(µ-CO)₂(CO)₅(η-C₅Me₅)₂] has been determined by a single-crystal X-ray diffraction study. The core of the molecule consists of an MoRh₂ isosceles triangle [Mo–Rh (mean) 3.009(1), Rh–Rh 2.623(1)Å]. The molybdenum atom carries five carbonyl groups, the rhodium atoms are each ligated by an η-C₅Me₅ group, and the Rh–Rh vector, staggered with respect to the four equatorial carbonyl groups of the Mo(CO)₅ fragment, is symmetrically bridged by two CO ligands. Relatively short non-bonded contacts between the oxygen atoms of the equatorial CO ligands and the hydrogen atoms of the η-C₅Me₅ groups suggest a significant steric barrier to the ‘alkene-like’ rotation of the Rh₂(µ-CO)₂(η-C₅Me₅)₂ fragment. Extended-Hückel molecular-orbital calculations on the model species [MoRh₂(µ-CO)₂(CO)₅(η-C₅H₅)₂] indicate that the dirhodium fragment and an alkene bond to an Mo(CO)₅ moiety in a similar fashion. However, whereas the barrier to alkene rotation in d⁶ ML₅ alkene complexes derives from electronic effects, no significant electronic contribution to the barrier of rotation of the trinuclear metal complex is indicated.