Contrasting the group 6 metal-metal bonding in sodium dichromate(ii) and sodium dimolybdate(ii) polymethyl complexes: synthetic, X-ray crystallographic and theoretical studies.

Extending the class of group 6 metal-metal bonded methylate compounds supported by alkali metal counter-ions, the first sodium octamethylmolybdate(ii) complex [(TMEDA)Na]4Mo2Me8 and heptamethylchromate(ii) relations [(donor)Na]3Cr2Me7 (donor is TMEDA or TMCDA) are reported. The former was made by treating [(Et2O)Li]4Mo2Me8 with four equivalents of NaOtBu/TMEDA in ether; whereas the latter resulted from introducing TMEDA or TMCDA to ether solutions of octamethyldichromate [(Et2O)Na]4Cr2Me8. X-ray crystallography revealed [(TMEDA)Na]4Mo2Me8 is dimeric with square pyramidal Mo centres [including a short Mo-Mo interaction of 2.1403(3) Å] each with four methyl groups in a mutually eclipsed conformation. In dinuclear [(TMCDA)Na]3Cr2Me7 trigonal bi-pyramidal Cr centres each bond to three terminal methyl groups and one common Me bridge, that produces a strikingly short Cr-Cr contact of 1.9136(4) Å. Broken symmetry density functional theoretical calculations expose the multiconfigurational metal-metal bonding in these compounds with a Mo-Mo bond order of 3 computed for octamethylmolybdate(ii). This is contrasted by the single Cr-Cr bond in heptamethylchromate(ii) where the singlet ground state is derived by strong antiferromagnetic coupling between adjacent metal ions.