Wikipedia has a table with all the elements known to form a bond with carbon. Obvious members are hydrogen and oxygen, more obscure elements on this list are xenon and uranium (sub branch organoactinide chemistry). Thanks to the efforts of Goodwin et al. at the Los Alamos National Laboratory another element, californium (Cf), can now perhaps be added to the list. Their recent paper (DOI) describes the synthesis of a californium metallocene. In this compound a central Cf atom is sandwiched between two tetramethylcyclopentienyl rings and it also has a chlorine ligand. A close existing radioactive relative would be the uranium metallocene tetracyclopentadienyluranium
But californium is trickier than uranium because it is a synthetic element and can only be made inside a nuclear reactor. So it is not only radioactive (problematic) but also very scarce (problematic). As a precaution all work was scheduled to take place within a single day to prevent radiation damage to the ligands. In the first part of the synthesis, safety precautions surely were relaxed! "In an open front hood and with no attempt to exclude air" a milligram amount of californium(III) nitrate was dissolved in nitric acid and hydrofluoric acid, the resulting green solid was identified as californium trifluoride and redissolved in water with boric acid and nitric acid.
Fluoride was then replaced by chlorine with an ion exchange column (gamma spectroscopy detector!) and hydrochloric acid. To the californium(III) chloride solution in water was then added DME and then trimethylsilyl chloride, With the californium compound now coordinated to DME it was then reacted with tetramethylcyclopentadienyl potassium in diethyl ether to form the Cf metallocene as a dark orange compound.
The authors explain that based on single-crystal X-ray diffraction the central Cf atom in the bent metallocene is pseudo tetrahedral with the two Cp units, a chlorine atom and also a chlorine atom from coordinated potassium chloride. So do the rural carbon guys get along with the radiant california girls? They pretty much stick to themselves. That party never happens, the 10 guys watching from the 2p balcony are just ogling at the 9 girls on the 5f dance floor. As explained in the article a "mismatch between the contracted Cf3+ 5f orbitals and C 2p orbitals is too large for substantial" radioactive dating and the compound is ionic. But there is hope. The related dysprosium metallocene was also synthesized but this compound is not brightly colored at all. The Cf compound behaves differently owing to increased ligand to metal charge transfer. Not a date then but least one of the girls smiles back.
One word on the first Cf-C bond claim made in the article. It refers to another californium cyclopentadienyl compound that is also brightly colored which was synthesized in 1970 in the Oak Ridge National Laboratory (Laubereau & Burns DOI). However the authors argue that this compound was never "authenticated" which has me thinking "authentication"? Does an office exist someplace where chemists take their new compounds for authentication? Not to my knowledge, would also be quite a big office. Perhaps a nuclear chemists thing.