A structural study of [CpM(CO)3H] (M = Cr, Mo and W) by single-crystal X-ray diffraction and DFT calculations: sterically crowded yet surprisingly flexible molecules

Abstract

The single-crystal X-ray structures of the complexes [CpCr(CO)₃H] 1, [CpMo(CO)₃H] 2 and [CpW(CO)₃H] 3 are reported. The results indicate that 1 adopts a structure close to a distorted three-legged piano stool geometry, whereas a conventional four-legged piano stool arrangement is observed for 2 and 3. Further insight into the equilibrium geometries and potential energy surfaces of all three complexes was obtained by DFT calculations. These show that in the gas phase complex 1 also prefers a geometry close to a four-legged piano stool in line with its heavier congeners, and implying strong packing forces at work for 1 in the solid state. Comparison with their isolelectronic group 7 tricarbonyl counterparts [CpM(CO)₃] (M = Mn 4 and Re 5) illustrates that 1, 2 and 3 are sterically crowded complexes. However, a surprisingly soft bending potential is evident for the M–H moiety, whose order (1≈2 < 3) correlates with the M–H bond strength rather than with the degree of congestion at the metal centre, indicating electronic rather than steric control of the potential. The calculations also reveal cooperative motions of the hydride and carbonyl ligands in the M(CO)₃H unit, which allow the M–H moiety to move freely, in spite of the closeness of the four basal ligands, helping to explain the surprising flexibility of the crowded coordination sphere observed for this family of high CN complexes.