Macropolyhedral boron-containing cluster chemistry: two-electron variations in intercluster bonding intimacy. Contrasting structures of 19-vertex [(η5-C5Me5)HIrB18H19(PHPh2)] and [(η5-C5Me5)IrB18H18(PH2Ph)]

Fused double-cluster [(η5-C5Me5)IrB18H18(PH2Ph)] (8), from syn-[(η5-C5Me5)IrB18H20] (1) and PH2Ph, retains the three-atoms-in-common cluster fusion intimacy of 1, in contrast to [(η5-C5Me5)HIrB18H19(PHPh2)] (6), from PHPh2 with 1, which exhibits an opening to a two atoms-in-common cluster fusion intimacy. Compound 8 forms via spontaneous dihydrogen loss from its precursor [(η5-C5Me5)HIrB18H19(PH2Ph)] (7), which has two-atoms-in-common cluster-fusion intimacy and is structurally analogous to 6.