[Ta(NMe₂)₅]

The five NMe₂ groups as a square-based pyramidal set are classified as X₅L₄ (neutral representation, 13 electrons, σ and π-bonding) for the d-block metal rather than X₅L₅ groups). The highest overall symmetry is C_2v rather than C_4v because of the nitrogen substituents. The apical σ and π orbitals are A₁ and B₂ while the basal σ and π orbitals are 2A₁ + 2A₂ + 2B₁ + 2B₂. One of the two A₂ orbitals is non-bonding as the metal orbitals have just one A₂ (the d_xy). This reduces the electrons donated by the five NMe₂ groups from 15 to 13, hence X₅L₄. Therefore the overall EN number is 18 (5 + 13) and not 20. See:

• K. Hagen, C.J. Holwill, D.A. Rice, and J.D. Runnacles, Inorg. Chem., 1992; 31, 4733-4736, https://pubs.acs.org/doi/pdf/10.1021/ic00049a004.