Adamantane (C10H16) is the smallest diamondoid, consisting of a
single diamond-like cage that carries two types of carbon atoms,
labeled C(1) and C(2). A C(1) is bound to three C(2)'s and one H atom,
while a C(2) is bound to two C(1)'s and two H's. C(1)–C(2)
interatomic distances are 1.538 Å and angles between two C(1)–C
(2) bonds are 109.5°; both results are close to the respective values in
crystalline diamond [14]. Since these C–C interatomic distances are
very small, one may define adamantane as a compact caged molecule,
and it could be anticipated that introducing a transition metal atom in
the tetrahedral site of the cage would be energetically costly and
cause large outward relaxations