text: *Group 2 elements calcium, strontium, and barium can all form octacarbonyl complexes M(CO) (M = Ca, Sr, Ba). The compounds were characterized in cryogenic matrices by vibrational spectroscopy and in gas phase by mass spectrometry.
  *Group 4 elements with 4 valence electrons are expected to form heptacarbonyls; while these are extremely rare, substituted derivatives of Ti(CO) are known.
  *Group 5 elements with 5 valence electrons, again are subject to steric effects that prevent the formation of M–M bonded species such as V(CO), which is unknown. The 17-VE V(CO) is however well known.
  *Group 6 elements with 6 valence electrons form hexacarbonyls Cr(CO), Mo(CO), W(CO), and Sg(CO). Group 6 elements (as well as group 7) are also well known for exhibiting the cis effect (the labilization of CO in the cis position) in organometallic synthesis.
  *Group 7 elements with 7 valence electrons form pentacarbonyl dimers Mn(CO), Tc(CO), and Re(CO).
  *Group 8 elements with 8 valence electrons form pentacarbonyls Fe(CO), Ru(CO) and Os(CO). The heavier two members are unstable, tending to decarbonylate to give Ru(CO), and Os(CO). The two other principal iron carbonyls are Fe(CO) and Fe(CO).
  *Group 9 elements with 9 valence electrons and are expected to form tetracarbonyl dimers M(CO). In fact the cobalt derivative of this octacarbonyl is the only stable member, but all three tetramers are well known: Co(CO), Rh(CO), Rh(CO), and Ir(CO). Co(CO) unlike the majority of the other 18 VE transition metal carbonyls is sensitive to oxygen.
  *Group 10 elements with 10 valence electrons form tetracarbonyls such as Ni(CO). Curiously Pd(CO) and Pt(CO) are not stable.