Synthesis and characterisation of the mixed-sandwich complex closo-[3-(η-C5Me5)Fe-4-Me2S-1,2-C2B9H10] and its charge-transfer salts; crystal structure of [Fe(η-C5Me5)-(C2B9H10SMe2)]+[ddq]˙–·CH2Cl2(ddq = 2,3-dichloro-5,6-dicyano-p-benzoquinone)