RDA5807SP FM Radio Receiver - i2C

The registers are 16 bit wide therefore you have to send them in two parts.
That data sheet says the high byte followed by the low byte. So to send a value in a variable called val use:-

Wire.write(val >> 8); // hi byte 
Wire.write(val & 0xff); // lo byte

Values come back from the chip with the hi byte first.