Interstitial carbon, in contrast with substitutional carbon, forms defects with hydrogen which are both electrically active and stable to high temperatures. Ab initio cluster calculations show that the most primitive defect, CiH, diffuses very rapidly and is expected to complex with many other impurities notably C, H and O. We describe here the structure and properties of Ci,nHm defects where n and m are less than 3. The most stable defects of the type CiCsHm have very different structures when m = 0 and when m > 0. In the former case, the C-C bond is metastable and only C-Si-C bonds are formed. However in the second case, the presence of a H atom stabilises the C-C bond. These results are supported by experiment. We show that the vibrational modes of the Cs-CiH defect are close to those observed for the T-photoluminescent centre and this defect is passivated when complexed with a further H atom. This defect then provides the first example of a fully characterised carbon centre containing a C-C bond.