Which of the following carbonyls will have the strongest C-O bond : -LSB- Mn -LRB- CO -RRB- 6 -RSB- ^ + , -LSB- Cr -LRB- CO -RRB- 6 -RSB- , -LSB- V -LRB- CO -RRB- 6 -RSB- ^ - , -LSB- Fe -LRB- CO -RRB- 6 -RSB- ?
Why is Co -LRB- CO -RRB- 4 tetrahedral and diamagnetic ?