The analogy between alkenes and the dinuclear species [MRh(µ-CO) 2 (η-C 5 Me 5 ) 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 Rh 2 fragments and the pentacarbonylmolybdenum group

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.