A new nido-5-vertex cluster, phosphacarba-nido-pentaborane, 2-tBu-1,2-PCB3H5Electronic supplementary information (ESI) available: rotatable 3-D molecular structure diagrams of MP2-optimised geometries for 2-tBu-1,2-PCB3H5 2, 1-tBu-2,1-PCB3H5 and P4 in CHIME format. See http://www.rsc.org/suppdata/cc/b2/b204409a/