In a single step, from [Cp*RuCl 2 ] 2 (Cp* ) η 5 -C 5 Me 5 ) and Li [BH 4 ], nido-1,2-(Cp*Ru) 2 (µ-H) 2 B 3 H 7 , 1, is produced in high yield. Addition of BH 3 ‚THF to 1 results in conversion to nido-1,2-(Cp*Ru) 2 (µ-H)B 4 H 9 , 2. Reaction of BH 3 ‚THF directly with [Cp*RuCl 2 ] 2 yields a mixture of 1 and 2. In two steps, a rhodium analogue, nido-2,3-(Cp*Rh) 2 B 3 H 7 , 9, is accessible by the reaction of [Cp*RhCl 2 ] 2 and Li [BH 4 ] to exclusively produce (Cp*Rh) 2 B 2 H 6 , 8, which adds BH 3 ‚THF to give 9 as the major product in a mixture. Reaction of BH 3 ‚THF directly with [Cp*RhCl 2 ] 2 yields the chloro derivative of 9, nido-1-Cl-2,3-(Cp*Rh) 2 B 3 H 6 , 11, in high yield via the intermediate positional isomer, nido-3-Cl-1,2-(Cp*Rh) 2 B 3 H 6 , 10. With high concentrations of Co 2 (CO) 8 , 1 reacts with Co 2 (CO) 8 to give nido-1-(Cp*Ru)-2-(Cp*RuCO)-3-Co(CO) 2 (µ 3 -CO)B 3 H 6 , 3, whereas low concentrations permit competitive degradation of 1 to yield arachno-(Cp*Ru)(CO)(µ-H)B 3 H 7 , 4. On the other hand, reaction of 11 with Co 2 (CO) 8 gives closo-1-Cl-6-{Co(CO) 2 }-2,3-(Cp*Rh) 2 (µ 3 -CO)B 3 H 3 , 12. Mild thermolysis of 3 results in loss of hydrogen and the formation of closo-6-Co(CO) 2 -2,3-(Cp*Ru) 2 (µ-CO)(µ 3 -CO)B 3 H 4 , 5, whereas thermolysis of 2 results in loss of hydrogen and formation of pileo-2,3-(Cp*Ru) 2 B 4 H 8 , 6, with a BH-capped square pyramidal structure. Finally, 6 reacts with Fe 2 (CO) 9 to yield pileo-6-Fe(CO) 3 -2,3-(Cp*Ru) 2 (µ 3 -CO)B 4 H 4 , 7, with a BH-capped octahedral cluster structure. The overall isolated yield of 7, formed in four steps from [Cp*RuCl 2 ] 2 , is ≈50% and evidences good control of reactivity.

The reaction of (CpReH(2))(2)B(4)H(4) with monoborane leads to the sequential formation of (CpRe)(2)B(n)()H(n)() (n = 7-10, 1-4). These species adopt closed deltahedra with the same total connectivities as the closo-borane anions [B(n)()H(n)()](2)(-), n = 9-12, but with flattened geometries rather than spherical shapes. These rhenaborane clusters are characterized by high metal coordination numbers, Re-Re cross-cluster distances within the Re-Re single bond range, and formal cluster electron counts three skeletal electron pairs short of that required for a canonical closo-structure of the same nuclearity. An open cluster, (CpReH)(2)B(7)H(9) (5), is isolated that bears the same structural relationship to arachno-B(9)H(15) as 1-4 bear to the closo-borane anions. Chloroborane permits the isolation of (CpReH)(2)B(5)Cl(5) (6), an isoelectronic chloro-analogue of known open (CpWH(2))(2)B(5)H(5) and (CpRe)(2)B(6)H(4)Cl(2) (7), a triple-decker complex containing a planar, six-membered 1,2-B(6)H(4)Cl(2) ring. Both are putative five- and six-boron intermediates in the formation of 1. Electronic structure calculations (extended Hückel and density functional theory) yield geometries in agreement with the structure determinations, large HOMO-LUMO gaps in accord with the high stabilities, and (11)B chemical shifts accurately reflecting the observed shifts. Analyses of the bonding in 1-4 reveal that the CpRe.CpRe interaction generates fragment orbitals that are able to contribute the "missing" three skeletal electron pairs required for skeletal bonding. The necessity of a Re.Re interaction for strong cluster bonding requires a borane fragment shape change to accommodate it, thereby explaining the noncanonical geometries. Application of the debor principle of borane chemistry to the shapes of 1-4 readily rationalizes the observed geometries of 5 and 6. This evidence of the scope of transition metal fragment control of borane geometry suggests the existence of a large class of metallaboranes with structures not found in known borane or metal clusters.