Experimental Charge Density and Neutron Structural Study of cis-HMn(CO)₄PPh₃:  Comprehensive Analysis of Chemical Bonding and Evidence for a C−H···H−Mn Hydrogen Bond

The structure and bonding in cis-HMn(CO)₄PPh₃ have been studied by low-temperature neutron and high-resolution X-ray diffraction, the latter study using a charge-coupled device (CCD) area detector. A charge density analysis, including the deformation density, a full topological analysis of ρ, and selected topological analysis of −∇²ρ, has been conducted. cis-HMn(CO)₄PPh₃ adopts an approximately octahedral geometry, the largest deviation being the C(1)−Mn−C(3) angle of 160.0(1)°. The hydride ligand (Mn−H(1), 1.573(2) Å) is nucleophilic in nature (i.e., hydridic, with an effective atomic charge of −0.4e) and makes a short contact (2.101(3) Å) with an electrophilic (+0.3e) ortho phenyl hydrogen. The electrostatic component of the H^δ+···H^δ- interaction energy is calculated to be 5.7 kcal/mol from the experimental data. This electrostatic evidence coupled with the geometry (C−H···H 129.0(2)° and H···H−Mn 126.5(1)°) and the identification of an H···H bond path in the charge density distribution strongly supports the characterization of this interaction as an intramolecular C−H···H−Mn hydrogen bond. Both the deformation density and the topological study clearly illustrate the σ-donor nature of both the H−Mn and Ph₃P−Mn interactions and the σ-donor/π-acceptor nature of the manganese−carbonyl bonds. The topological study further confirms the decrease in C−O bond order upon coordination to the metal and demonstrates for the first time by this method that the metal−ligand bonds, although showing characteristics of a closed-shell interaction, do have a significant dative covalent component to the bond. The latter is reinforced by a study of the derived Mn d-orbital populations, in which populations of the d_z² and d_x²-y² orbitals are significantly higher than would be predicted by a simple crystal field theory model of metal−ligand bonding.