Role of bridging ligand .pi.-acidity in determining the nature of the lowest excited states of binuclear complexes of nickel(0). Ligand-centered (M2 .fwdarw. .mu.-LCT), metal-centered (.mu.-L .fwdarw. M2CT), and intraligand (IL) lowest excited states for the series of complexes Ni2(.mu.-L)(CNMe)2(PPh2CH2PPh2)2 (L = CNR, CNMe(R)+, NO+)