Carbonyl group bond length

All three structures have 0(, symmetry and are very similar. The bond length from the central atom to the carbonyl group is slightly different in each compound, and it is longest for the molybdenum substituent. The internal structure of the carbonyl groups is essentially unchanged by substitution. ... 

We have recently investigated X-ray structure of 3-methoxycarbonyl-[l,4,2] diazaphospholo[4,5-a]pyridine as first example of molecular structure determination for [l,4,2]diazaphosphole ring [84], The ester substituent lies strictly in the molecular plane with carbonyl group in the trans orientation with the formal C=P bond. Endocyclic P-N and P-C bonds are averaged between respective single and double bond lengths. 

Figure 4.101 displays the subtle variations in metal-carbonyl bond lengths in the group 6 M(CO) complexes. In each case one can clearly distinguish the coordinate omc bonds (solid lines) from the hypervalent toMc prebonds (dashed lines). The latter are about 0.1A longer, but exhibit a similar vertical variation within the group. 

---