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)(4)PPh3 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 p, and selected topological analysis of -del(2)p, has been conducted, cis-HMn(CO)(4)PPh3 adopts an approximately octahedral geometry, the largest deviation being the C(1)-Mn-C(3) angle of 160.0(1)degrees. The hydride ligand (Mn-H(1), 1.573(2) Angstrom) is nucleophilic in nature (i.e., hydridic, with an effective atomic charge of -0.4e) and makes a short contact (2.101(3) Angstrom) with an electrophilic (+0.3e) ortho phenyl hydrogen. The electrostatic component of the Hdelta+... Hdelta- 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)degrees and H ... H-Mn 126.5(1)degrees) 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 sigma-donor nature of both the H-Mn and Ph3P-Mn interactions and the sigma-donor/pi-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.