Why can sulfur have an expanded octet but not oxygen? In my search to understand the bonding in structures like $\ce{SF6}$ I found many sources that said it was because sulfur has d orbitals to accommodate an expanded octet, which made sense to me. But I also found sources like the paper by Reed and Weinhold (1986) that say d orbitals contribute very little to the bonding in $\ce{SF6}$. An alternative explanation is that $\ce{SF6}$ has three-center/two-electron bonds. What I don't understand is why compounds like $\ce{SF6}$, $\ce{ClF5}$, $\ce{PCl5}$ exist for third row element centers, but not $\ce{OF6}$ if it has nothing to do with d orbitals? References would be great.
sulfur has 2 electrons in its 3s subshell , and 4 in 3p subshell. On absorbing energy the paired electrons get excited to to the next higher energy level which is the 3d subshell. So after exciting it's electrons six unpaired electrons are present which can take part in bonding via overlapping of orbitals. Other compounds can also be explained similarly. Also the d- orbitals contribute to only two bonds out of six bonds( which would have been meant in the paper by Reed and Weinhold (1986) ). Oxygen has no empty orbitals with it . It only possess orbitals upto 2p which contains 4 electrons. So it cannot expand its octet by exiting it's electrons.