see L.J. Farrugia et al (2003) Acta Cryst B59 234-247 and L.J. Farrugia & C. Evans (2005) J. Phys Chem A, 109, 8834-8848

As expected for a first row transition metal, the manganese atom in Mn₂(CO)₁₀ shows only three shells of charge concentrations in L(r). The 3d electrons are subsumed with the core 3s and 3p into the inner valence shell charge concentration (i-VSCC), which is distinctly non-spherical. The (3,-3), (3,-1) and (3,+1) critical points in L(r), in the region of ~0.33-0.36 Å from the nucleus, constitute the atomic graph.
The cuboidal form, with the eight charge concentrations maximally avoiding the ligand charge concentrations, is the one most commonly observed for transition metals The graph is consistent with the the qualitative expectations of ligand field theory, in that the core-like 3d electrons avoid the charge concentrations of the carbonyl ligands. It is interesting to note that the charge depletions do not exactly coincide with the Mn-Ligand vectors, but are more closely opposed to these vectors.

The graph is very similar to that observed for HMn(CO)₄(PPh₃), and provides further confirmation of the covalent nature of the Mn-Mn bond.

Figure 1. The atomic graph of the Mn atom in Mn2(CO)10. The pink spheres represent the (3,+1) critical points of charge depletion, the light blue spheres the (3,-3) critical points of charge concentration and the yellow spheres the saddle (3,-1) critical points in the Laplacian L(r)

For the carbonyls Fe(CO)₅ and Ni(CO)₄ with idealised D_3h and T_d symmetry respectively, we found that the experimentally atomic graphs of the metal atoms were quite model dependent. For instance, in order to obtain a graph with the expected D_3h symmetry, it was essential to restrict the multipole populations to this symmetry (the crystallographic site symmetry of the Fe atom is only C₂). For the almost spherically symmetric charge density around the Ni atom in Ni(CO)₄, it proved even mpre difficult to reproduce the theoretical atomic graph from multipole refinement either with experimental or theoretical structure factors.

Figure 2 Atomic graph of the Fe atom in Fe(CO)5 from (a) gas phase quantum density (b) experimental multipole pseudoatom with just the required C2 site symmetry. The colour coding is green (3,-3), yellow (3,-1), red (3,+1) and blue (3,+3).

The interatomic surfaces of the terminal C and O atoms in metal carbonyls are quite reproducible and are indicative of the strongly polarised bond. The surface lies much closer to the C atom, resulting in a large positive charge, generally ~ 0.9-1.0 at the C atom, with a larger negative charge ~ -1.1 for the O atom.

Figure 3. The interatomic surfaces of the C and O atoms in Ni(CO)4 truncated at 3.5 a.u.