Stabilisation of an ortho-deprotonated mesityl group within the unusual [{2,4,6-Me3C6H2P}{4,6-Me2C6H2(2-CH2)P}Sn]3? stannate ionElectronic supplementary information (ESI) available: synthesis of 1. See http://www.rsc.org/suppdata/cc/b3/b303390m/