Semi-polar double bond

Lowry and Sidgwick used for this bond between nitrogen and oxygen the names semi-polar double bond (covalent bond + ionic bond) or coordinate link and the symbol — The direction of the arrow indicates the direction of the electron transfer, thus... 

In phosphoryl compounds such as POCI3 the covalent bond to the oxygen atom is often regarded as formed by donation of the lone pair electrons from the P atom. Such donation confers semi-polar or part-ionic properties on the bond and it can be written as P+-0", P O or as P=0, the double bond completing the formal pentavalency of the central phosphorus atom. There is, however, controversy over the nature of this bond (see below). 

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