What is a #"coordinate covalent bond?"#

A ccordinate bond, or dative covalent bond, or donative coordinate bond is a bond where the 2 bonding electrons come from the 1 atom.

Explanation:

Consider #BF_3#; there are 3 covalent bonds, and the 2 electrons that form the bond come from each binding atom, one from the boron centre, and one from the halogen. #BF_3# is a good Lewis acid (electron acceptor), and electron donors (Lewis bases) will bind to the boron centre thru a coordinate or donative bond (i.e. DONATES) from a lone electron pair to the electron-poor centre. The complex it forms with trimethylamine, #F_3BlarrNMe_3# features a coordinate bond, in which the #B-N# electrons are conceived to come from the lone pair on the nitrogen.

Complex coordination chemistry of transition metals are very rich sources of examples of coordinate bonds. In a complex such as #ML_6# (or #[Co(NH_3)_6]^(3+)# or even #[M(OH_2)_6]^(n+)#) the ligand (ammonia or water) is conceived to donate its electron density (a pair of electrons) to #Co^(3+)#.