Planar hepta-, octa-, nona-, and decacoordinate first row d-block metals enclosed by boron rings

Possible planar hypercoordinate molecules with first row d-block metal atoms in the centers of boron rings are explored comprehensively by density-functional theory (DFT) computations. Many optimized MB(n) (n = 7, 8, 9, and 10) neutral and charged clusters have local D(nh) minima, although these may not be the most stable isomers. The larger B(9) and B(10) rings are versatile in accommodating first row d-block metals, whereas the more compact B(8) ring only can enclose smaller transition metals (such as Mn, Fe, and Co) effectively. Delocalized pi and radial molecular orbitals involving boron are crucial in stabilizing these highly symmetrical planar hypercoordinate molecules. Early and middle transition metal d-orbitals participate in metal-boron covalent bonding, whereas partial ionic bonding is more important for the late d-block elements. Potential energy surface scans established several of these species to have planar hypercoordinate global minima: D(8h) FeB(8)(2-) was identified here, and D(8h) CoB(8)(-) and D(9h) FeB(9)(-) were identified in an earlier complementary study.