Sealed-tube pyrolysis of [nido-µ_4,5-{trans-(Et₃P)₂Pt(H)}-µ_5,6-H-2,3-Me₂-2,3-C₂B₄H₄] affords the closo-carbametallaborane [1,1-(Et₃P)₂-2,3-Me₂-1,2,3-PtC₂B₄H₄], structurally characterised by X-ray diffraction. Crystals are monoclinic, space group P2₁/a, Z= 4, in a unit cell with lattice parameters a= 15.691(6), b= 10.118(6), c= 15.949(6)Å, and β= 115.76(3)°. Using 7 437 data collected at 215 K on a four-circle diffractometer, refinement converged at R 0.067. The molecule has a highly distorted pentagonal-bipyramidal cage with a novel (C_2v) conformation of the Pt(PEt₃)₂ fragment. In contrast, pyrolysis of [nido-µ_4,5-{trans-(Et₃P)₂ Pt(H)}-µ_5,6-H-2,3-C₂B₄H₆] proceeds with separation of the cage carbon atoms, affording[closo-1,1-(Et₃p)₂-1,2,4-PtC₂B₄H₆], which may alternatively be prepared directly from 1,6-C₂B₄H₆ or 5-Me₃N-2,4-C₂B₄H₆. A crystallographic study of this closo-carbametallaborane confirms the cage carbon positions. Crystals are monoclinic, space group P2₁/n, with eight molecules in a unit cell of dimensions a= 16.045(7), b= 17.354(5), c= 16.918(6)Å, and β= 117.11(3)°. Refinement converged to R 0.043 for 8 275 data recorded at 215 K. The two crystallographically independent molecules in the asymmetric unit both have pentagonal-bipyramidal cages, but are related as rotational conformers about the metal–cage axis. Comparison of the molecular parameters of the three seven-vertex cages thus studied with those of analogous 12-vertex (icosahedral) systems suggests that the barrier to rotation of a metal fragment, about the axix linking it to the cage, should be less in the former case, and this has been verified by variable-temperature n.m.r. studies.