The lowest energy CpFeCHP(CH₃)B_n−3H_n−3 (n = 8 to 12) structures, including the experimentally known CpFeCHP(CH₃)B₈H₈, have been investigated by density functional theory. The central FeCPB_n−3 polyhedra in all of the lowest energy such structures are the most spherical closo deltahedra. The heteroatoms are so located to have adjacent iron and phosphorus atoms and non-adjacent phosphorus and carbon atoms. One of the Fe–B bonds from the degree 6 iron vertex in the 11-vertex CpFeCHP(CH₃)B₈H₈ structure appears to be fragile, readily elongating to ∼3.1 Å in one of the low-energy structures, consistent with experimental observation on this system.